Ask Hackaday: How Can You Store Energy At Home?

Amidst the discussions about grid-level energy storage solutions, it is often easy to forget that energy storage can be done on the level of a single house or building as well. The advantages here are that no grid management is needed, with the storage (electrical, thermal, etc.) absorbing the energy as it becomes available, and discharging it when requested. This simplifies the scale of the problem and thus the associated costs significantly.

Perhaps the most common examples of such systems are solar thermal collectors with an associated hot water storage tank, and of course batteries. More recently, the idea of using a battery electric vehicle (BEV, ‘electric car’) as part of a home storage solution is also gaining traction, especially for emergencies where the grid connection has failed due to a storm or similar emergencies. But all-in-all, we don’t see many options for home-level energy storage.

The Grid Storage Problem

Bath County Pumped Storage Station (Credit: CHA)
Bath County Pumped Storage Station (Credit: CHA)

A while back, we looked at the motivations behind grid-level storage, including current and future technologies. The increasing focus on long-duration storage is driven by the increasing amount of intermittent, non-dispatchable energy sources on the grid, including PV solar and wind turbines. As these produce highly fluctuating levels of energy, storing excess power for later use is useful and arguably essential.

Unfortunately, the conclusion there is that grid-level storage at a scale sufficient to store and time-shift such amounts of energy on the level of an entire nation is not feasible. Of note here is that virtually all of newly produced battery capacity today and for the future will go into BEVs, which is where the idea of Vehicle2Grid (V2G) was pitched as a potential grid-level storage, This we looked at too, and found it wanting from an economical and practical perspective.

Much of the problem comes down to the highly fluctuating amounts of energy being supplied, and the increasing mismatch between supply and demand the more intermittent sources are added to the grid. Having e.g. roof-based solar panels that feed into the grid contributes to this problem, causing increased local power surges whenever there’s a lot of sun in an area, even as feed-in tariffs are being cut and even abolished in more areas. This leads to both utility providers and home owners facing increased costs and decreased (financial) benefits.

All of this only concerns electrical power, of course. Homes, offices and the industry also require heating, hot water and e.g. steam for industrial processes. Here local options would seem to make significant sense where e.g. district heating is not an option. With the use of existing solutions such as heat pumps and hot water storage tanks, it would seem that here at least straightforward solutions exist.

Energy Sources

While it is possible to also charge a battery and heat water in a hot water storage tank from the local grid for later consumption (e.g. with off-peak rates), another energy source that is readily available is the Sun. Adding solar thermal collectors on the roof as part of a solar water heating solution can be cost-effective, depending on the solar irradiance levels (power per unit area). The effectiveness is here mostly determined by the payback period, which can range between in the order of 4 years to closer to 20 years.

An important consideration here is too whether anti-frost features are required. While a purely passive and thus rather cheap system would be fine in a warm climate, if temperatures drop below 0°C during winters, it’s essential that measures are taken. This can include adding anti-freeze to the water in the system, in which case a more expensive closed-loop system is required as well.

In addition to the heat from solar irradiation, the energy from the Sun can also be converted into electricity using photovoltaic (PV) solar panels. Currently, most roof-based PV solar installations do not have local storage, and self-consumption is not considered, with the prevailing business model being that of selling the produced power to the local utility.

Some locations may also have space for other energy sources, like a (small) wind turbine, but solar thermal collectors and PV solar panels are likely to be the primary sources of energy when other sources (like hydro) are not available.

Storage Solutions

Hot water storage setup using solar thermal collectors as well as resistive heating using (PV) electricity.
Hot water storage setup using solar thermal collectors as well as resistive heating using (PV) electricity.

As mentioned, heating water is a very common way of capturing and storing solar thermal energy. Many houses have a hot water storage tank in which a supply of water is kept at a specific temperature for immediate use. The main difference is in how the water in these tanks is heated. Often fossil fuels like mineral oil or natural gas are used, while in other areas electrical (resistive) heating is more common. When used in a context where solar thermal collectors and/or PV solar panels are available, the water can also be heated purely by these sources.

The advantage of this system is that it provides a potentially cost-effective source of hot water, which tends to be one of the more energy-intensive uses. It also provides an effective use of the electricity from PV solar panels when it is not used to charge batteries.

This leads to the other obvious storage solution in the form of large battery storage, such as Tesla’s Powerwall and similar offerings by competitors. Recently, Broughton et al. (2021) detailed the economics of battery storage for residential solar customers in Southern California.

In California the feed-in tariff has been dropping for years now, with the current NEM 3.0 program reducing the financial incentive to produce PV power for the grid. Not surprisingly, the introduction of Time-of-Use (TOU) with the NEM 2.0 program, saw the number of battery storage installations by home owners already increase significantly.

The conclusion by Broughton et al. was however that with the payback period when having a single Tesla Powerwall 2 system installed, along with a PV solar installation, was too long for this to make any financial sense. Where having battery storage like this installed does make sense, is when the grid power is unreliable, as is increasingly the case in California.

This then makes the argument to instead charge a BEV with the power from a (roof-mounted) PV solar installation, and have a charger that is capable of inverting the energy flow so that the BEV can act as an emergency battery to power the house. Even so, the economics of whole-house battery storage do not seem to be quite there yet when one lives in an area where utility-provided power is an option.

Wrapping Up

The general theme here when looking through the recent studies on home energy storage and related fields seems to be that storing energy in large quantities is only likely to be economical when it concerns heating up water in a hot water storage tank. The payback period here can be rather minimal when the conditions are right, and even adding a PV solar panel for resistive heating of the water may make financial sense depending on local factors.

This is also the case with battery storage solutions. Considering that the main cost of a BEV is due to the battery pack, it isn’t too surprising that something like a Tesla Powerwall costs about the same as a BEV. Here one could run the numbers on whether perhaps running a large array of (sealed) lead-acid batteries might nudge the numbers closer to financial sense.

Either way, the topic of home energy storage is not one which is likely to go away any time soon. Even if one’s intention isn’t to go off the grid, there are many other incentives that would lead one to look at the options. Whether it’s cost-savings, or having a backup option when there are rolling blackouts as we have seen around the world the past years, there are many reasons to take a look at the available storage options. But we simply didn’t come up with much.

What are your experiences and thoughts in this regard? Since the Hackaday community tends to have a lot of tinkerers, no doubt some of you have implemented any of these generating and storage systems, or have at least looked at the cost picture. If you have a whole-house battery system and/or solar-powered hot water system, how is it working for you?

156 thoughts on “Ask Hackaday: How Can You Store Energy At Home?

      1. Most vehicles have a requirement of a 10 year lifespan. So after 10 years the battery should have 80% of its SoC. After that it tends starts to degrade more rapidly.

        This is based on working for an OEM on theie BEV in 2019 so my info might be out of date.

          1. We have buses of 4 years old (that do 3-4 full charges/day, a little more than a normal car, they have done almost 450000km/vehicle). They were required by contract to have 87% of their original capacity by this time. Some have (and even higher) while some are significantly lower, the worst few being at 67%. This is a warranty case.

        1. One of the Teslas here at work is on a mobile charging station (van with a diesel generator) once every other month or so. It’s three years old and has an effective range of about 100 miles according to the owner. Tesla can’t get him another car or a battery but is willing to help him with their van until it gets worked out. So yeah , clearly a warranty problem but I wouldn’t bet on those 7 years.

    1. I live in a place with no feed-in-tariff incentives. We’re currently paying 26p per kWh and you can generally get about 7p per kWh for exported energy from newly-installed solar. So actually your best strategy is to use the solar to reduce your energy imports, giving you 26p per kWh gains on your electricity bill. You can either install a powerwall, giving you about 5-6 year payoff currently, or you can just move as much energy use as possible to when it’s sunny (run your dishwasher, washer, dryer etc in the middle of the day).

      1. I should add that the cost of installing solar panels (at least where I live) is scandalous. I can, fairly straightforwardly, source panels at well under 50p per W. The cost of an installed 5kW system is well over £1.30 per W. The inverter is likely to set you back about £600. So about £3k for the equipment, leaving £3.5k for someone to mount the panels and plug it all in. WTF?

        1. I agree. So I bought secondhand panels at 24p/W (measured) and a secondhand inverter (£100, plus a spare another £100). Cables and connectors is another £30.

          Got me 2kW for £630.

          1. As far as I can tell, this is missing the important part. Even if you’re prepared to mount the panels and do most of the wiring yourself, the expensive bit seems to be the grid connection. You need a meter than can measure import and export. You need the relevant certification (though regulation around this is improving). How did you manage that cheaply?

          2. No certification needed if you don’t want to get paid back for electricity generated.

            Since the payback amounts are only one tenth of what you pay for use, it’s barely worth it.

        2. That seems pretty reasonable to me. Think about the costs the installer incurs: hiring workers to clamber on a roof, insuring them against injury, insurance for damage to the structure, trucks, equipment, consumables, taxes, and enough profit to make it worth the hassle. And probably other expenses I haven’t considered.

          1. No. Just no. That is an OUTRAGEOUS amount to charge.

            I’ve just had two gas engineers and an apprentice spend nearly two days on site to install a boiler (and make some associated changes to our plumbing). They’ve charged around £1k for labour. How can labour for fitting a solar system possibly be 3.5 times that? All the overheads you mention apply to them as well. Similarly, a bathroom fitter who quoted for me a week ago charged £400 per day for a plumber and a builder. At that rate, the solar installer is quoting NINE DAYS to mount panels and do wiring.

          2. @Tom, and TBH £1000 is utterly unreasonable for fitting a boiler.

            The problem with the UK now is that pretty much everything requires a certificate for a jobsworth to milk you for.
            There is no official room left for the individual who often knows more and will do a better job than the person with the cert looking to do 4 jobs a day not 1 and cuts massive corners. No one checks the work of the man with the cert, but I can count on my hand with number of tradesman Ive seen doing a good job.
            My simple solution is not to play that game and I do all work in my house myself.
            But what if you come to sell you ask? If you understand how indemnity insurance works, dont worry.

        3. There are a lot of companies under fire for ripping off customers and making false claims that their panels will give them income.

          As for storage solutions, depending on your requirements good old lead acid batteries are great if you maintain them properly. If ya really want to knock your power bill down, dont use a dryer, hang dry to let your a/c do the work and cook outside during the summer months. Then you arent paying to simultanioisly cool and heat your house at the same time. I have a small toaster oven i chuck on the porch for summer time baking needs. use the grill for everything else. Induction burners dont let off a boat load of heat so i use those instead of gas or old fashioned electric. Air conditioning however, is an absoloute requirement for my humid climate and cost me some 350 bucks last month… the price gouging is real during the summer. It can be expensive to run any type of a/c on solar, but if i had the cash i would definitely drop it on a dedicated mini split run mostly or entirely off of solar. Pretty sure there are kits you can buy, unsure of how good they are though.

          1. We retro fitted our100 year old house with minisplits.
            3 wall units on the South side, 1 large outdoor unit.
            We also have gas forced hot water and electric baseboards.
            Payback ? meh.
            Comfort, A/C and zonal heating, a big plus.

            Baseline the gas heat at 65 in the winter, suppliment with the splits.

            No grid ?
            We won’ t be alone in the dark.

            The propane genset we had installed at camp only gives 8 hours of power per fill, most of it wasted, running on small or no load.

    2. I’ve often thought about that and wished my area had separate peak/non-peak prices so I could game it that way. Then I remember that our flat rate is probably lower than most places non-peak rates.

        1. Gasoline properly stored in an airtight metal container has a shelf life of up to 25 years according to a long-term study of the German ADAC (though this was tested with old canisters of leaded fuel so YMMV with modern unleaded fuels)

      1. And go and fire it up regularly, check the oil, check the plugs (if not diesel), and generally maintain it even if (and especially if) you rarely use it. An unmaintained and untested generator backup is no backup at all.

        1. Yep…. But we are not at that point yet. Currently we have nice steady state power with coal and hydro. But with more solar and wind going in (and more people moving into the state), we are expecting to eventually see rolling brown/black outs to happen on those cold nights, and really warm days…. Every year, we see our peak power need hit records…. Take today, we have high temps, but wind generation is in the single megawatt digits which it can be in the hundreds if wind is actually blowing…. Gotta love that ‘renewable’ energy that people seem to want but expect ‘always on’ electricity to the house.

        2. You’re making it sound harder than it needs to be…

          Checking the oil isn’t hard and generally doesn’t get low if the unit isn’t used a lot. Plugs also don’t generally go bad from sitting. 90% of the issues people have with small engines is related to the ethanol in the fuel and letting it sit in the carb and gum up the jets.

          I’ve literally done zero maintenance on my generator in like 6 years by only storing ethanol free fuel in the tank. If I end up putting ethanol in it after prolonged running I make sure I run the fuel dry out of the carb. I do this with all my non heavily used equipment and like I said, little to no maintenance on any.

          1. I’d actually agree with you Flywheels are probably the cleanest energy store if done right, but the engineering challenges and frictional losses make it more suitable when you want a giant capacitor able to dump lots in a short period rather than a battery..

    1. Natural gas may actually be the way to go. Run it off the line to your house for normal emergencies, and for when things get really hairy, have a big tank you can switch to.

      1. Unfortunately for the natural gas generator option, it was actually more expensive to have that installed as opposed to the solar and batteries. So we have great power from the sun.

      1. Not much, as even if you assume 100% efficiency the small volume and height gain just isn’t holding a great deal of energy, do the math on lifting whatever volume/weight of water you can fit however high you lift it and just assume no losses, then compare to a few Lead acid’s.

        Its still a great cheap method to build if you don’t have impractically high energy demands or if you happen to have a large reservoir a long way up in the mountains somewhere then you can get good lift height and total volume so actually have vast energy stored. But at the more domestic scale its not the most practical in and of itself.

        I can see some great setups out there where it is a real winner over other options – but that is when its not just a gravitic potential energy store but part of a more integrated system. So perhaps its also your fish farm, drinking water reserve, used to power your homes ‘AC’ evaporative cooler, or maybe its also a thermal battery that can likely pull double duty and keep your solar PV panels cooler so running more efficiently.

        But all of these still require very very large tank(s) a rather long way up even for small energy demands – so Bojho Bill likely has more than enough for themselves during the odd blackout as old commercial water towers are generally quite tall and high volume, but still too small for any great sustained demand.

          1. I know, but that isn’t the biggest issue with pumped water – its the volume and heights required to actually hold significant energy – so assume 100% efficent to make the mathematics trivial as its just work over the distance and still at least for personal level use be shocked at the volume required.

        1. A 100t weight lifted 100m stores around 10^8 J. That’s 27kWh in the bizarre units that household energy is usually measured in.

          It’s not peanuts, but you also don’t have a 100t weight or the ability to lift it 100m in the air.

          1. ” but you also don’t have a 100t weight or the ability to lift it 100m in the air.”

            You obviously have not met my mother-in-law!

  1. As an RV’ers, we are surrounded by discussions of lithium batteries. Ourselves, we have 4x100ah lithium batteries. Now, the ones we have cost about $1000 each. I won’t mention the brand, but they are the “Kleenex” of batteries, in that they have the biggest of the marketing budgets, etc. There are many, many quality lithium battery makers that can cost 3/4 or even 50% of those. The question will be “longevity vs. cost”, as with many things. Do you intend to keep this house long enough to recoup your costs. Solar is more efficient with each technological iteration (up to a certain point, of course) and the batteries keep coming down in cost. Coupled with a good size inverter, you can set yourself up pretty good. I highly recommend Will Prowse’s YouTube channel for all things solar, lithium, charging and just off-grid power in general.

    1. I purchased an 18kw battery from a 2016 Chevy volt in 2016 for $2900 delivered to AZ.
      The prices are still around $150-200 kwh delivered . Then you have to make it work .
      I would never do grid tie as you are only helping your friendly utility companies meet their green energy requirements . It is way cheaper than the media implys to go off grid. I purchased used 250 watt panels for $50 , which I have 18 . An outback controller and inverter and I rarely have to check it.

        1. @David Harvan. I hadn’t thought of that. Not a bad idea for a DIY system

          @Dude. Second this. And you also have to consider for most people they’d have to pay an electrician to install a custom system which would also be expensive.

          It’s the same with DIY packs from old 18650 cells. Really cool idea if you have the skills, time, space but if you don’t it’s not efficient.

  2. Any source or further explanation on how mineral oil is similarly used like natural gas (I assumed burned) to heat water? Its the first paragraph under the “storage solutions” subtitle.

    1. In Sweden there was addon burner for a normal wood fired boiler so you could burn diesel for heat.
      There was also specialised oil burners but I have not seen one in a house.
      Wood-fired boiler, electric heating and heat pumps are the most common ways of heating in Sweden as far as I know.

      1. Im from Finland, in a older normal one family house the heath from the energy source is distributed with hot water to radiators in each room.
        In the fifties and beginning of the sixties the energy was wood or coal, in mid sixties an oil burner was added to the boiler and replaced coal.
        For a while in the seventies direct electric heating was often installed.

        Today conversion to wooden pellets burners is done or heath pump technology from air to water or more efficient water to water with a primary circuit of a bore hole down in bedrock or buried in a field at frost-free depth or lake or sea.

        Heath pumps do use electricity to convert low temperature to higher temperature saving electricity compared to direct electric hething

  3. A surprisingly cheap way to store heat is to use an insulated tank with 500 gallons (4000 L) of molten paraffin wax. The latent heat of crystallization is very much large enough to be useful and you don’t have the terror of 4000 kg of steam in the basement.

    Talking to my friends in the generation and distribution side of things, I was surprised to learn that the sheer mechanical inertia of a 3600 rpm steam turbine is enough to handle transients on the order of 1/30th to 1/20th of a second – two or three cycles, in other words. In the big fossil plants there is “quite a few seconds” worth of steam pressure in the boilers, pipes, and accumulators.

    It turns out the fluctuations in production from wind and solar haven’t actually been as bad as first feared. Video cameras pointed upwards and what is probably some OpenCV action predicts cloud shading seconds to minutes in advance. Aeroderivative peakers can go from idle to full bang 10 MW in a matter of a few seconds. Evidently it’s gotten practical to use ultrasonic and laser wind profilers to steer wind turbines into the gusts before they even get there, but it’s even more important to have a few seconds of notice before the gusts die out. Neither of these is “storage”, but it goes a huge ways toward solving the need for storage.

    I wrote off “power” as the boring side of EE decades ago, but the last few years I’ve started to reconsider my position. :-) Also I saw a many-MW SCR (mosfet? triac? dunno.) being delivered a few years ago and I was thinking “you don’t just order that puppy from Digi-Key”. It came in on a flatbed trailer and they used a crane to unload it. There’s something undeniably cool about that.

    1. I think I’d be more terrified about four cubic meters of hot oil than hot water. Especially for what happens when you mix them together, as is apt to happen if there’s a leak.

      1. Why would you be afraid of hot oil and hot water mixing*? It’s hot oil and oxygen mixing that you need to be afraid of :)

        *Given the context, since paraffin melts at about 70C, and since I don’t think anyone would make a system hot enough to flash-boil water in their house (primarily since I don’t think a person who would try it would survive long enough to complete it), I don’t think you’d get a steam bomb of hot oil.

  4. No amount of storage will always be adequate as it isn’t just the predictable 24 hr cycle that you must consider, there is the seasonal and even less predictable variation in solar and wind power that you must account for or you are going to run into problems. So what is the worst case scenario in terms of disruptions to power generation opportunities? Another volcanic winter, which will last for several years and could drop your solar power production down to mid winter levels for that duration while your heating costs stay at winter levels. So what is the probability of that happening again? About 100% because it really is just a matter of time, the only question is will it happen tomorrow or in several centuries from now, and we just don’t know. Look at that Tonga blast recently nobody predicted that, but it is still affecting the upper atmosphere, and it was just a baby compared to others that we know have happened since humans started wearing pants and bossing each other around. Some form of nuclear power, preferably fusion power is humanity’s only hope. That is my conclusion after spending a quarter of a century of interest in all forms of renewable energy and energy efficient architectural design. I really have spent that long making the most of knowledge resources such as libgen and I honestly think that “renewable” energy sources need to be renamed to “opportunistic”, make watts while the sun shines, but be prepared for the storm.

    1. >“renewable” energy sources need to be renamed to “opportunistic”, make watts while the sun shines, but be prepared for the storm.

      I like that way of looking at somewhat, its certainly a good description of how you have to treat your ‘off-grid’ system. Not sure I can agree the “opportunistic” supplies are as bad as you make out though. They are pretty damn cheap to build, in the case of solar the panels themselves should last functionally for a lifetime, probably more if you avoid damage and even on a bad day they make something. So you can reasonably affordably if you have the land area oversupply even to the extent that volcanic winter isn’t a major problem, just means you have to be careful rather than profligate with your energy use.

      I do agree nuclear really shouldn’t be as vilified as it is, and its a really solid choice especially if we ever manage to create practical fusion reactors. Also agree that you can’t hope to store your way out of any trouble – though the same is true for nuclear and fossil fuel too, can’t stockpile enough to outlast any possible trouble, however the energy density of Fissionable fuels is very very high, so a more manageable stockpile volume goes quite a bit further than that volume of battery.

    2. Thank you! I’ve been pushing for the concept and development of “seasonal storage” for some time now. Admittedly, I’m thinking more about disconnecting from the grid than surviving a volcanic winter. I have not yet found an easy solution.

      Consider a typical home with enough solar power to zero out your annualized electrical bill, usually somewhere around 15-20kWh. You’ll still pay during the winter months when your solar doesn’t cover your consumption, but that’s offset by the refunds during the summer. And you’re still dependent on a connection to the grid; it becomes your seasonal battery.

      What would it take to locally store enough during the summer to make it through the winter? If you sum up the daily deficits through the entire fall-to-spring cycle, you’re likely to end up in the 5-15MW range. That is a huge amount.

      If there were a way to generate liquid propane out of air, water, and electricity, you would need to generate roughly 1000-2000 gallons over the summer months to supplement your winter solar production. If you prefer nitrogen instead of carbon, the quantities and storage methods of anhydrous ammonia are similar to LP. That’s a biggish tank farm for a residence, but not outrageous.

      To create that using surplus solar production during the summer, after considering conversion losses, you’d need to significantly increase your solar size. Fortunately, that would have the benefit of decreasing your daily deficits during the winter season. I’m looking forward to the day we can do that at residential scale, but we’re just not there yet.

  5. Buckminster Fuller put quite a bit of thought into home energy storage. His solution was compressed air. He designed a flat wind turbine that took advantage of the laminar flow of wind over his geodesic home structures. The compressed air would be stored in underground tanks and routed via outlets to the home for use in pneumatic appliances.

  6. I use plain water to preheat DHW into my gas heater. I only get 110 degrees or so because I was too cheap to put it on the roof where the snow falls, but on a south wall it works more efficiently in the winter when it needs to. I have two 55 gallon poly drums and dip tubes into each to pick up the heat. The collector is EPDM and freeze-proof. “Solar-Roll” it was called, used for swimming pools but chlorine rotted them out; plain water works fine. A differential thermostat (LM339) runs the circulating pump. I was too cheap to use commercial thermal sensors so I used 5 1N4148’s in series (10mV/deg. C) for each. No expensive 555s were used.

    The system has been running since the Reagan era Solar tax credits, and I replaced one 2N7000 after a near lightning strike. I just replaced the water heater, which has been loafing along about 37 years. It was from Mongomery Wards.

    Unfortunately I have no flow meters or data recording so I don’t know its performance. But our energy bill is around $120 a month averaged year round. That’s gas plus electric.

  7. “Having e.g. roof-based solar panels that feed into the grid contributes to this problem, causing increased local power surges whenever there’s a lot of sun in an area”
    This sounds like a design issue in the installation, which should certainly be fixable. Do you have any data on this issue?

        1. The problem is political: every producer is an individual seller who wants to sell everything they can to get either subsidies or net metering, regardless of what happens to the power prices for other producers.

          The regulator you need is not an electronic component, but a government entity that would be tasked to limit how much each can produce and sell, but that would require you to implement a command economy.

          1. Or, you could take away the net metering and the subsidies, at which point they’d just destroy their own profits, but hey – a solution that doesn’t require the state to do anything is not pleasing to the voters who want the state to “do something”.

          2. Or create a program to incentivize operating loads in a way that tries to match production. Have it be decoupled from the need to own on site wind or solar so that anyone connected to the grid (including those living in apartments) can participate.

          3. >a program to incentivize operating loads in a way that tries to match production

            Like giving out free power for people at low demand periods, so the utility company can earn tax credits for every kWh of wasted power while the consumers pay taxes to fund someone else’s pool party?

        2. Yes, poor choice of words. Surges makes one think of lightning or other surges that burn out equipment. He is speaking about local oversupply. That would only be a problem if the solar power production in a neighborhood exceeds the demand of all the customers plus the ability of the transformers to backfeed to the grid.

          Two points:
          – That would be a LOT of solar power
          – There would be some problems with backfeeding the power through the distribution transformers as the power line protection system is not designed for power flowing the other way. The relays would have to be recalibrated.

          1. >– There would be some problems with backfeeding the power through the distribution transformers as the power line protection system is not designed for power flowing the other way. The relays would have to be recalibrated.

            This seems unlikely. Most of the distribution protection I’ve seen is nondirectional overcurrent relaying, fusing, and reclosers using nondirectional overcurrent. Sending power upstream would just decrease the current until you reached some high level of generation (unlikely with a home setup). Most likely, that power is going to the other homes on the feeder anyways. And yes, what you describe happens (upstream flow), the relays don’t trip, and life moves on.

          2. @Andy: the overcurrent relay protects the wiring behind it. If you feed additional current, any branches can be overloaded and burn everything down without the circuit breaker noticing. And with the voltage, on a long distribution line the feeding transformer may be adjusted to be up in the tolerance band, so the last house on the line doesn’t brown out on switching on the coffee maker. But if you now try to feed back, this house will quickly get well above maximum voltage, so you have to adjust the transformer voltage, which in turn may require thicker cables or a decrease in allowable max load to keep an acceptable voltage. This may or may not be the case, but the electricity company has to check it in every case before allowing backfeeding.

          3. There is no “behind” when you’re talking about bidirectional current flow. The fuses and reclosers will trip long before anything melts. They’re normally limited by the transformer capability. You’d probably trip the fuse on your pole or the next upstream fuse. That power has to go somewhere, and the fuse can see that current. Downstream of your hypothetical house at the end of the feeder would only have enough current to satisfy demand, which is already acounted for.

            As for the LTC being overwhelmed, this is unlikely. Let me explain.

            Let’s say you put a 4kW solar system on your house. You start generating at full capacity without voltage control and no demand at your house. Well, there are other houses on the feeder with demand. The impedance is lower than the system, so your solar flows to their homes. By the time you get to the LTC, voltage drop on your hypothetical long feeder would keep the voltage in an okay range. However, if using an IEEE 1547 inverter, voltage protection is standard, so your inverter would just experience momentary cessation whenever the voltage got too high. A more modern inverter would even control right to the voltage level and keep it in check. It uses phase angle difference to transmit power.

            Now, if these assumptions aren’t met, it would be different. If everyone had solar generating on the feeder, it might lead to issues, but probably not. What you’d see is that people’s inverters would cycle into and out of momentary cessation to keep the voltages from being too high. Each LTC would, assuming auto LTC, bump down a couple taps on each one to compensate.

            I sincerely doubt a study would be run for each individual install. What would eventually result in a study would be power quality monitoring showing voltages out of range or repetitive fuse blowing. I mean, sure you could run that study for each install, but it would be a waste of resources.

          4. > There is no “behind” when you’re talking about bidirectional current flow.

            Exactly that is the problem.
            Let’s have a typical setup from around here[tm], one transformer with 250kW feeds 10 houses which are fused (and behind the fuse wired) to pull 45kW each. The feeding line is calculated for 250kW, and is protected by the transformer fuse. Now put 10kW solar on each house and experience a short circuit with a resistance in the single digit Ohms at the end of the line. The transformer fuse puts 250kW, and each house adds another 10kW, so at the end the line is well overloaded, and nothing is able to switch off the fault or even detect it, since all currents at the fuses are in spec. The insurance company will not pay a single cent for the resulting damages, and rightly so. That is why for every single backfeeder is checked wether it can result in an undetectable overload condition on any part of the network.
            In the mentioned case one could lower the fuse rating (and risk power outage on high demand situations), or dig in a stronger feeder cable, or deny the solar cells. Or one could find that the cable is already rated for 350kW, but one has to check that, and document that it was checked.

    1. Ha! You Americans who don’t care about storage volume in your multi acre houses!

      So jealous.

      Here in the U.K. we have small houses in increasingly small plots. I’ve replaced paper books with ebooks to reduce the physical storage of a bookshelf.

  8. I’ve been hoping for a breakthrough in super cheap mass electricity storage. I have a bunch of space in the back yard that I’m happy to fill full of plastic 55 gallon drums if I could figure out how to push electricity into them and not be horribly toxic or explosive.

    Failing that I’m kind of interested in this sand heat storage idea. It would be nice to build a big really well insulated sand tower in the back yard full of coils – then pump something (again, non toxic) from (water coil) solar roof panels into it when it’s warm and back out through coils in the floor (installed but unused) when it’s cold.

    The advantage of sand over big water tanks is that sand can go above boiling? But then what do I pump through it.

    1. What you describe is just like an electric storage heater. TO heat it you “pump” electricity through. TO cool it you have a loop that you pump water or mineral oil through. Or air.

  9. Shifting your usage is another option, put your washing machine on when it’s sunny, as with other optional items.
    Even a freezer doesn’t “need” to be plugged in all night, so maybe put that on a timeswitch.

    I have a number of appliances that use lots of power for short times. A 1kWh/3kW peak discharge battery system would be good enough to smooth out some peaks and get payback in the evening.

    Definitely water heating with the excess after that, but needs an intelligent controller to inject power that matches your PV excess output.

    Are there any open source projects out there for monitoring/controlling:

    – PV output
    – net grid in/out power (including direction)
    – power dump into appliances (when there is an excess?)

    I was thinking earlier, that some intelligent power sockets would help, so you have some sockets in the house that are on “optional” power, and come on when the PV is doing well. You could then put your washing machine etc on them, the washing machine would stop and start (most do automatically) as power was available.

    1. That’s called deferred loading, and it is one proposed solution.

      With one obvious problem: deferred loading tends to stack up, so when power does come available the latent demand overtakes it. People who have been putting off washing their clothes, whose fridges are just about melting, their houses are cold and their batteries empty, they can’t wait indefinitely – so you’re building a system that is prone to developing a demand tsunami on a system level.

      See e.g. the problem where building more roads leads to more congestion because people were holding back on driving cars because of the poor situation. The usual social planner’s options there are to either build enough roads, for which there isn’t ever enough funds allocated, or to keep the road infrastructure limited to the point that people just won’t drive and cite the original problem for why you won’t solve it. All the load-adjustment schemes eventually fall down to this.

      1. I am not happy with timers for load shifting, since the power grid has already a load balance indicator built in: the frequency. Just measure it and adjust the refrigerators temp a little up when the frequency is low. This helps stabilizing the grid on the minutes time scale. If you stay in the tolerance band for the temp througout the frequency tolerance band, no load tsunami will happen.
        Take a CD4060 (or CD4040) which counts an oscillator during one half wave, then compare it to the averaged last 1000 or so counts, and you know wether at the moment is too much or too few energy in the grid (start with an estimated count, add one if the actual count is higher, substract one otherwise. This scheme is very simple, but may take some time to give meaningful readings). Hook up a display, and you can get a feeling for it and use your kettle to help grid stability.
        This does however not help on a sub-second and a days/weeks time scale.

        See for the european grid.

        1. You can’t use frequency as a measure of total demand. The frequency is always automatically adjusted to 50 Hz very quickly when demand changes. It is accomplished by generators that can increase/decrease output very quickly. So every time hour of the day it will stay *very* close to nominal and move up and down tiny amounts.

          1. You can’t use frequency as a measure of total demand because it’s arbitrary. It’s just required to stay within a certain band over some days or weeks. The frequency can be stable at 49.98 Hz instead of 50 Hz with supply and demand balanced because the grid operators don’t want to pay the cost to run peaking turbines to return it back to 50 Hz. They’ll just wait for the night when the demand goes down to nudge it up.

          2. You can’t use frequency as a measure of total demand, because df/dt is a measure for the power imbalance in the grid. The automatical adjusting is called load-frequency control, and I just suggested a simple method of participating in that.

  10. “… seems to be that storing energy in large quantities is only likely to be economical when it concerns heating up water in a hot water storage tank”
    You mean… nuclear fission? :) That certainly is the only sustainable economical grid scale solution.
    Uranium is the ultimate energy storage solution <3

  11. Storing hot water is good.

    What about ice? ice can store a lot of energy to help with household refrigeration and air conditioning in hot countries (now including Europe BTW). Latent heat of ice is 33600 J/K so a cubic meter of ice could store 33 Mega joules or about 9 kilowatt hours. Ice can be stored for days or longer with reasonably cheap insulation. I think ice used to be stored for months pre refrigeration days in ice houses.

    1. I saw this recently ( – The manufacturer admits that latent heat storage as a backup system is costly, and wasn’t very popular.

      Interestingly though, seasonal heat storage is a thing, but with massive amounts of water stored in a gravel-filled foil pouch that is buried. The water temperature can reach 80°C or higher, but heating is much simpler than cooling in this case, as you only need to exchange heat from a solar collector.

    2. I’m wondering about the return on an ice storage system here in Southern Minnesota.
      Along these lines.
      A modern version of the ice house.
      A large insulated tank below the surface grade with a large, insulated, movable lid.
      Just above the bottom of the tank is a spiral of Pex tubing filled with an antifreeze.
      During the really cold days/nights of winter (around here) when the temperature drops below 0 F to slowly pump in water at a rate that it will freeze quickly. When the tank is full of ice or the outside temperature rises too much, stop the water and place the lid on.
      This freezing process may need several cold spells to complete.
      When July 1st arrives; begin circulating the antifreeze to a radiator in the house.
      Collect the condensation and move it outside (e.g. water the garden).
      As the antifreeze slowly warms up in the house it is returned to the tank and slowly begins melting the ice block. Circulate the antifreeze only on days/nights it is needed.
      Depending on the size of the ice block/tank there could be enough thermal transfer to negate the need to run an air conditioner during the summer/fall. Electricity would be needed to run the antifreeze pump, the well pump to fill the tank, and a fan to push warm air through the radiator.
      That electricity could be supplemented with wind or solar.
      A heat pump with a heat exchanger could also be incorporated.

      1. Do the computation, and you’ll realize it’s not possible. Just to cool a medium room, you need 5kW of power (or with a heat pump, it’s down to 2kW). On a single day, it’s 48kWh (for a heat pump, but for this scheme, it’s twice that, around 100kWh), so for a single day of cooling, you’d need 10 metric ton of ice. That’s 900m3 of ice for the 15 June to 15 September period, and in reality you can double this, since the insulation isn’t perfect, most of the ice will have melt before the 15th of June. This means for a single room of, maybe 20m2 you need a surface of 90m2 over 10m deep (to 20m deep depending on actual efficiency of the system) to store enough ice to cool it.

        That’s impractical.

        1. What you need to keep a room comfortable is going to be far too varied to state it needs x.

          If the place is really well insulated then almost all the cooling/heating requirements are driven by what is in the room, as its just not exchanging energy with the outside world very well. So assume one person in an amphitheater or an airing cupboard and your cooling needs are going to be basically the same – just enough to offset the heat generated by that person. Which means something in the region of 100w for an idle human if wiki is to be believed.

          Obviously the real world doesn’t have that magic level of insulation, but still the point stands to maintain a temperature inside something insulated reasonably well has less to do with the volume and more to do with what heat sources you put in it. Only time the volume really comes into play directly is if you have let the whole volume reach a higher temperature and so have to cool all that air mass.

          Obviously dealing with humans makes it more complex than that as we want windows that tend to be less insulated than the walls and you need to exchange air with outside (as nobody has their own personal life support bubble do they??). So there is always going to be some energy loss through the heat-exhangers on this air path in an efficient building, and leaky seals around windows and doors in most buildings. But its so very varied building to building and user to user!

        2. Holy moly!
          48kwh per day for one air conditioner! I use 18kwh per month with a 6kw AC turned on when we need it (currently for heating because I am in Australia) plus I have electric hotwater, electric jug and induction cooking and a fridge in an essentially uninsulated house. Do have a wood fire for most night time heating, though. I use a bit more in Summer for cooling.

          Talking of impractical – maybe you should look at ways to reduce the time you run your AC instead of worrying about storage.

      2. I heard of this being done by some Mennonites who don’t use electricity. They converted a livestock barn to a cold storage for their vegetable business. In the winter they filled the old silo – or maybe it was an old manure pit with snow. And they were somehow able to use that to refrigerate their vegetable storage all summer.

        1. We have a number of Mennonite “enclaves” around here, I have a great respect for them.

          BTW, they will use the electricity grid around here, as well as ICEs to get work done.

    3. I was going to say that thermal storage in the other direction works, too. In fact, it would be better in some respects as the temperature would be near constant as the storage is “charged” or “discharged”. And unlike batteries, there’s no inherent limit on number of cycles and it can be left fully charged, fully discharged, or anywhere in between indefinitely with no impact on lifespan.

    4. This is already used on larger scales, such as campuses or larger buildings, but typically only for daily cycles, with hydronic cooling.
      You can either size your compressors so that they run constantly, but the excess cooling is used to cover the high TOU periods, or oversize so they mainly run off peak and the ice covers all of the cooling needs for most of the day. These systems can also offset or completely fill DHW needs of necessary.

      Heat pumps always have a hot side and a cold side, and society typically requires heating and cooling simultaneously in different places (fridges, hot water, etc) so we can get much more energy efficient just by realizing our heating and cooling needs as a whole and letting them support each other. District heating and cooling on the large scale, but I can imagine loops on apartment or even single home scales as well. If you pair it with a ground source loop you can even do seasonal time shifting.

      1. Again, if you take numbers into account, it doesn’t work. Sure, your fridge will generate heat in winter but it already participate to heating your house. The heat of the AC is a waste in summer, you don’t actually need it, even for heating the water (since it’s a low quality heat, too cold to be useful). The issue is not the split between heating and cooling in a house, it’s mainly because we have left caves millions years ago and expect our house to behave like a cave all year long.

        1. If you already use electricity for (resistive) heating, you may put water to freeze in the fridge, and throw out the ice afterwards. So you gain the heat generated by the freezing process. Mind that the fridge may not be made for putting through huge amounts of energy, and that the ice should melt outside to not reclaim the generated heat from the room.

  12. We use the house as thermal storage – use the heat pump to overheat/ cool (68F winter/ 78F summer) when the sun’s out, then let the house coast until bed time. Zero cost, zero extra hardware.

    1. Haha i thought i was the only one!
      Also if we are gone during the day, the AC gets turned off. Most people don’t understand exponential heat decay, so think running the AC when you’re not home saves electricity cost.

    1. Swimming pools aren’t that typically great as energy storage devices. A pool cover can help reduce losses to the atmosphere, and in-ground pools are obviously better than above-ground pools, but the reality is that they are quite inefficient.

      Moreover, I’m a little confused as to how you are thinking about this. Are you trying to heat the pool and somehow get that solar energy stored in the pool, with the side effect being a cooled house? That’s kinda backwards – the heat pump is more efficient with cooler water, so why heat the pool?

      The answer to that is that you have to – heating the pool is a side effect of using the heat pump. From an efficiency standpoint a warm pool is less efficient than a cool pool, but still vastly more efficient than an air-source heat pump. Another benefit is that people often want to heat their pools for their intended purpose, so it’s a win-win. Think of it as cooling the house with solar, gaining massive efficiency gains by using the pool and getting free pool heating too.

      Google heat recovery pool heater. Honestly, given the typical costs of heating pools, they should be required.

  13. This is missing seasonal thermal storage.
    Imagine a large water reservoir (40m^3 or more), well insulated and located in the building. You charge it up during the summer with solar thermal collectors. The heat should be enough for all of the winter.
    Any losses of the storage will heat the house – so the heat is not really “lost”
    One might have an additional emergency heater in case of a really bad season… And with this amount of storage, you’ll need to build the house around the storage.

    I love this concept because it is conceptually simple – only water and water pumps. No refrigerants, no heat pumps, only little electricity for the pumps, some smarts for where and when to pump the water.

    In German “sonnenhaus” and “saisonspeicher” is available and I’ve even visited one. Economically unfeasible with current energy pricing

  14. Off-grid with 1320ah of flooded lead-acid (FLA) cells, installed 2009 (replacing 1100ah of FLA), still good for overnight needs – refrigeration mostly.

    AUD$12K to replace at today’s prices.

    So, 2022 – 2009 = 23 years. $12K/23 years = $521/year

    Plus the cost of panels and backup generator+fuel. But I call it a bargain to be {nearly} grid-independent.

    1. Sanity check says it can’t be 23 years old if it was installed in this century.
      2022 – 2019 = 13 years, so almost 1k/year. That’s more than my electricity bill (probably not at the current prices)

  15. Before even considering what energy storage device is more efficient, we should address the elephant in the room. Pure sine AC inverters are expensive. There should be a mandatory switch to DC energy. We just need to agree on a proper voltage. I declare 96V (12v x 8) that proper voltage.

    Until we switch from AC to DC, I see pointless to even have a proper (efficient) energy backup system, since most of it will get lost in DC to AC conversion.

      1. Speaking about elephants, and the War of the currents

        AC vs DC will always be a taboo topic. AC made sense because it was more efficient, for transmiting electricy over long distances, but not anymore. High voltage DC is more efficient in transmiting energy over a long distance.

        IMHO, it doesn’t make ANY SENSE to still use AC. Switching to DC would have a direct benefit, such us;
        1) not requiring wires as thick as the ones with AC
        2) not requiring to “synchronize”, in case you need pump some energy to the grid
        3) less components and less costs, for inverters, and energy backup systems
        4) more efficient in terms of up/down converting voltages

        sure, it was “cheaper” to up/down convert voltages using simple transformers and AC, but not anymore. The costs associated with electrical wiring is higher, and the need for simpler way to upload energy to the grid is increasing. There is a very high costs associated with switching from AC to DC, but in terms of benefits, it is worth it, considering that we will use less materials, and make it easier to contribute renewable energy into the grid.

        Tesla was right about using AC back in the era, because he was smart in using what was available in that time. But the real greatness of him wasn’t creating AC. His greatness was achieving a realistic goal with what was available back in the era. That same Tesla would use DC, considering how efficient he was at using the available resources we have today.

        That doesn’t mean AC has no place in todays world. We still use it for wireless charging, and other wireless stuff.

        1. You may think about repairability when there is a power semiconductor shortage, and you may think about arcing. Hint: the arcs disappear when the current goes to zero, which is twice a periode on AC. With DC you will need converters anyways to get to the needed voltage, so you can as well feed them with AC. Plus load frequency control does not work with DC, so you are left with one parameter to control voltage and power flow, which are (mostly) independently controllable with AC.

        2. There is one huge gain to AC you haven’t accounted for – you can have powerful motor (among a few other things) that just work reliably and effectively with no extra circuitry. We don’t want or need silicon chips in everything, its a waste of effort and another point of failure.

          DC has its place, and AC has its place. At this point AC transmission lines just make sense, as the world is already covered in them, and they are pretty damn efficient, so even if DC is now a little better efficiency (which is something I’ve seen no evidence of) its alot of cost for very marginal gains. Yes feeding lots of little energy sources into an AC grid has some challenges, but you’d have challenges in a DC grid too, just different ones, and you’d have spent a fortune changing everything to learn just how challenging those are to deal with in practice.

          Mains electric being AC also makes sense for a prolonged period from now still, with the age of the internet many things now use that for any clock requirements but plenty of stuff out there that doesn’t need to be smart enough to use NTP it just needs a rough counter that keeps its accurate enough – which AC provides, and sometimes importantly provides synchronously to a vast area.

          Now if you want to argue new buildings should have a low voltage DC and HV AC mains circuits so your local DC energy stores can be put directly to use on some of the many devices that are DC that I could perhaps see as a valid argument.

    1. I think you need to check the efficiencies.

      The system powering my house right now shows 4.4kW from the solar panels, 300 watts feeding the computer and other appliances in the house, and 4.0kW going to the grid. That’s a loss of around 100 watts. Out of 4.4kW, that’s a loss of less than 3 percent. In other words, the inverter is better than 96 percent efficient.

      That’s very far away from your “most of it will get lost in DC to AC conversion.”

    2. I think you need to check the efficiencies.

      The system powering my house right now shows 4.4kW from the solar panels, 300 watts feeding the computer and other appliances in the house, and 4.0kW going to the grid. That’s a loss of around 100 watts. Out of 4.4kW, that’s a loss of less than 3 percent. In other words, the inverter is better than 96 percent efficient.

      That’s very far away from your “most of it will get lost in DC to AC conversion.”

  16. Efficiency is everything.

    Modern batteries are better than 90% efficient. It’s difficult to beat that, and with Time Of Use metering, it’s relatively easy to make that 90% work for you. Without TOU metering, it’s always a 10% loss. Perhaps power backup functionality makes it worth installing – you’d only eat that 10% when you actually use the power.

    Thermal storage can be very efficient, notionally close to 100%. That doesn’t always work in your favor though. Hot water storage is seen as inefficient today as tank-less systems are taking over. It’s certainly true that heating water on demand is less wasteful than having hot water sitting losing heat to its surroundings when looked at from only the water standpoint. If you’re in a winter heating season and the same boiler is heating the radiators, then it’s a wash though as heat lost by the water tank merely reduces the heat needed via the radiators. It’s the summer cooling season where water tanks are inefficient as every joule lost by the tank is fighting the AC. You end up paying for it twice.

    The best thermal storage is a massive thermal sink in the middle of your home, well insulated from the ground and roof but open to the interior air. Combined with good house envelope insulation it will help keep your interior temperatures constant at close to 100% efficiency.

  17. Heat pumps with spent uranium rods in the winter. Take the heat and use it for your home. In the milder months they go into storage OR are used to heat water. Can rods from reactors that generate heat but little to no radioactive effects be used to make heat pumps or water heating more energy efficient?

    1. “Spent” uranium rods can be very “dirty” (plutonium isotopes and such mixed in).
      And yes, the radiation (not the heat) they give off can ionize water particles.

      1. Is there no way to mitigate the radiation? Lead? Concrete? Keep in mind for the use I have in mind I just want heat, and mass is not critically important as the rods would be external to the heat pump (at least that’s how I see the setup, testing would tell if it was useful).

        Mind you, a heat pump that utilizes a water heater as a heat sink would be interesting in the summer months. Taking the unwanted heat out of the home and putting into hot water for home use or even keeping the pool water at an agreeable temperature.

  18. Sorry – I was distracted – the original 1100ah batteries were installed in 2000. So yes, about 1K per year. Even at that price, it’s still cheaper than mains electricity around here. I know people who pay that each quarter! Of course, they have ducted aircon, a swimming pool, and lots of domestic electric gadgets like breadmakers, etc.

  19. Solar hot water is fantastic. I’ve been using a stored/pumped system for the last 17 years and it’s pretty much free hot water almost all year.

    I also incorporated solar water panels into my heating system for the winter – it feeds into underfloor heating (and cooling for summer) and is part of a chain that comprises of heat pumps plus oil boiler (as a last resort).
    Details are on

    The heat pump(s) run from a solar system with 8kW of panels and 21kW lithium batteries.

    Around mid-day (in summer) the batteries are full so excess power can be used for washing/drying etc.

    I’ve also developed a system to control use of the excess power (plus a load more things) using smart sockets. It’s very much plug and play, no Pi’s, Node-red etc..

  20. “… it isn’t too surprising that something like a Tesla Powerwall costs about the same as a BEV.”
    Nonsense. Where the heck did you see a BEV that is as cheap as a PW?

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