Charting the efficiencies of boiling water

Water takes a lot of energy to heat up. If you’d like evidence of this, simply jump into a 50° F swimming pool on Memorial Day. Despite the difficulty of heating water, that simple act accounts for a lot of industrial processes. From cooking a steak to running a nuclear reactor, there isn’t much that doesn’t involve heating water.

[Tom Murphy], Physics prof at UCSD decided to test out exactly how efficiently he could boil water. Armed with a gas stove, electric kettle, microwave, and a neat laser pointer/photodiode setup on his gas meter to measure consumption, he calculated exactly how much energy he was using to make a cup of tea.

The final numbers from [Tom]‘s experiment revealed that a gas stove – using a pot with and without a lid on large and small burners – was about 20% efficient. A gas-powered hot water heater was much better at 55% efficiency, but the microwave and electric kettle had a miserable efficiencies of around 15 and 25%, respectively. There is a reason for the terrible inefficiency of using electricity to heat water; if only the power from the wall is considered, the electric kettle put 80% of energy consumed directly into the water. Because the electricity has to come from somewhere, usually a fossil-fueled power plant that operates at around 30% efficiency, the electric kettle method of turning dinosaurs into hot water is only about 25% efficient.

The take-home from this is there’s a lot of power being wasted every time you run a bath, make some coffee, or wash the dishes. We would all do better by decreasing how much energy we use, much like [Tom]‘s efforts in using 5 times less power than his neighbor. Awesome job, [Tom].


  1. Yorkshire says:

    As a tea drinker and a Yorkshireman, I approve of any efforts to make the preparation of my favourite beverage as cheap as possible.

    • The problem was his total lack of ballpark pricing, focusing solely on efficiency.

      Take the totally inefficient microwave experiment. 67 kJ is 0.01861 kilowatt hours. At 16.5 cents each, we’re talking three tenths of a penny.

      This reminds me too much of an ex-GF who always nagged me to remove my wall-wart when done charging my cellphone but never wanted to hang her laundry on the outside clothesline. (Hint: one year of squandered wall-wart usage is roughly equal to one load of laundry in the dryer)

  2. sinamox says:

    an analyzed pot never boils

  3. soupdragon says:

    With this kind of logic, electric cars are far less efficient than gasoline powered ones.

    • Tony says:

      When looking at the total system, yes.

      At the moment gasoline wins because it’s cheap, and electric cars as seen as poor performers because battery technology sucks.

      Gasoline contain a LOT of energy, and batteries will never match that. On the flip-side internal combustion engines are very inefficient compared to electric motors (most energy the burn is lost as heat) so it all may balance out in the end.

      I foresee a future when we all ride electric bicycles around…

      • Alan says:

        Oddly people measure the delivery cost of electricity, but not the delivery cost of gas/gasoline. It doesn’t arrive for free. Tanker trucks, pipelines and refineries all have their losses, some of them significant. Natural gas piping in particular is notoriously leaky. Exactly how much is lost at each step depends on the form of fossil fuel you are burning and how far it has traveled.

  4. rasz says:

    i smell a rat

    Was electric kettle 25% efficient? or was it 80% efficient times power plant efficiency?
    Does the professor think Gas is magical and comes from fairy ass? What was the point of all this?

    Let me guess, he is one of those global warming will kill us all professors?

    • Mental2k says:

      Didnt mean to report, damn phone.

      I’d assumed that as the majority of power stations are also fossil fuel powered that he was simply ignoring the processing costs of the fossil fuel. He could probably include it, but it would add effort and not make any significant difference to his findings.

    • lnmaurer says:

      The question about efficiency is answered in the article.

      The professor doesn’t think gas is magical. As Mental2k suggests, he figures the electricity comes from fossil fuels too, so he’s simplifying the analysis by defining efficiency as (energy deposited in to water)/(energy released by burning the fuel). If you’d have read the article, you’d have know this.

      • Megol says:

        In many parts of the world the electricity comes from windmills, hydroelectricity or nuclear power.
        I’d be surprised if my electric kettle isn’t at least 60% efficient even counting losses in delivery and generators.

    • ChemE says:

      Clearly electricity is what comes from a fairy’s pecker. Right now the cheapest energy plan available for me is *drum roll* tied to the price of natural gas. So the fuel for the turbine is the same as the fuel for the stove, and the energy costs in procuring that fuel and be considered equal and extraneous.

      The reality of the situations it that by burning the gas in a centralized location is can be scrubbed and pollutants may be captured (never mind the crap added to consumer NG to make it smell). In all likleyhood it would be most environmentally friendly to use the kettle, and probably cheapest too from a consumer perspective as electricity is sold in bulkier quantities and therefore lower prices.

      From a purely academic standpoint he makes a convincing argument that a gas stove has the highest overall efficiency and therefore uses the smallest amount of fuel.

      If you bothered reading the article the professor makes no indication to his political slant, and it appears the experiment stems from intellectual curiosity.

      Also, gas is not a proper noun…

  5. AB says:

    I think i spotted some kind of error in the logic. When using gas, the gas needs to be mined, transported, processed as well as it needed to be produced (organic reaction, bacteria, long time, pressure, whatever). Despite production, these steps do not influence the heat rating of the fuel itself but are to be taken into the equation.

    Whenever the efficiency of devices using fossile fuels or gases are involved that fact is usually ignored, because the primary energy used is calculated using the heat rating alone (i hope i used the right translation, i mean the excess energy of the exothermic reaction) of the fuel, but it ignores the way that matter used to be formed and that it needed logistics to get there.

    When comparing these values to electricity out of the outlet it can not be much better. Of course the same principle applies for fossile-fueled power plants and their infrastructure, but not for other forms of generating electricity, e.g. local photovoltaic and the like. It remains a question of energy balance.

    Think about it…

  6. AB says:

    Of course finding the most efficient way to heat water is kind of important work, but mostly the decision in industrial processes is based on which form of energy you have as excess based on the main process, location or cost rather than being more efficient on its own.

    In other words: A less efficient process might be the better choice if the cost for the primary energy is lower.

    To get equipment more efficient one can still try to minimize losses or convert them and get them back in the main process.

    • Jonam says:

      I like the Japanese approach to boiling water. In all of the hotels I stayed in, there was an insulated electric kettle with a day or two’s capacity for tea.

      You’d fill it and boil once and then leave the kettle on all day and night. A thermostat in the kettle would ensure the water stayed at the right temperature by occasionally turning on the heater. I rarely noticed that the kettle would turn on and I had access to hot water all day.

      Much more efficient than the shiny stainless steel electric kettle I have at home which loses the heat in the water after a few minutes and I have to reboil all day to have tea.

  7. WA says:

    Ultimately, this professor demonstrates why we should all be skeptical of any authority making claims…

    By including more of the activities in one side of the equation (electrical generation methods), but not the other (gas) he effectively skewed kis answer to make the point he desired…

    A very typical mechanism of the demagogues.

    • lnmaurer says:

      What do you think his desired point is? His conclusion was that boiling water in an electric kettle and boiling water with a gas stove were comparably (in)efficient (at ~25%), although the kettle won _if used properly_.

      If — as you suggest — he took in to account more of the inefficiencies on the gas side, gas would be shown to be even less efficient, and the electric kettle’s margin of victory would increase. In short, it wouldn’t really change his ranking.

      • WA says:

        Frankly I don’t know (nor care) what his agenda is, but I am certain that he has one and that it colors his approach.

        From the information provided, he compared the efficiency of the electric mode AND the efficient of generating the electricity (in small part), but he only compared it against the efficiency of the gas heating without taking into account the energy required to produce the gas…

        Invariably the only way to truly compare efficiency (without including costs) is to estimate the total energy consumption used for the event… and such calculations are lengthy and time consuming… To the point that as far as I know they are never done.

        What is done involves simplifying assumptions that always reveal the bias on the part of the person (or persons) performing the ‘analysis’…

      • Tony says:

        The effort required to produce the gas (assuming it’s natural gas) and pump it to your house is fairly minimal compared to producing electricity.

        There are natural-gas powered power stations, so the question you want to ask the professor is: is it better to have the station convert the gas to electricity, or pump that same gas directly to my stove?

        The professor is saying it probably doesn’t make much difference, electricity wastes most of the energy at the power station, while gas wastes most of it on your stove.

      • @ Tony:

        Natural gas power plants are insanely expensive to run. Coal is the preferred method of making electricity (at least in the US, and in Germany as well), with oil slightly better.

        I live in the only town in my state with a municipal power plant. It’s a natural gas plant, and it’s basically never on. It’s meant to pick up in very high demand situations, like 2 in the afternoon when it’s 105 degrees F out.

        If Sim City 2000 taught me anything, it’s that you have to wait until 2050 when fusion plants are available.

  8. William Tjaden says:

    I just gotta ask… Who in their right mind *boils* steak?!?

  9. dave says:

    Yes, but which option is cheapest?

  10. Donald Becker says:

    Even if his gas comes from fairy ass, that fairy had to get to work somehow.

    Extracting and transporting gas takes energy too. It’s not like they hook a pipe up to ground and the well pressure drives the gas across the country and into your house. There are massive pumping complexes that support pipelines, and smaller pumps throughout the system.

    Doing an end-to-end energy use estimate is almost impossible. Do you count the energy used for the pipe? The diesel used by the construction machinery that installed the pipes? It all takes energy and dollars, and dollars are the only thing you can be certain is never skipped when counting.

  11. TomV says:

    One option is missing, heating water directly from solar power. Turns out it is pretty efficient with the right setup (>80%) and requires marginal amounts of sun. (I must admit i did not take the efficiency of the sun into account)

  12. Kris Lee says:

    When you realize that the gas combustion products are cancerogenic and they stratify on the every surface in the kitchen and they mix with the dust in the air while doing this then you realize that there is no point in caring about the efficency between gas and electricity in your kitchen.

  13. HackJack says:

    Even after reading 10 minutes, I still don’t know which option is cheaper. I don’t care which one is more efficient. I want to know which method cost me less. Microwave, Electric Kettle or Gas Stove. In a free market, efficiency should ultimately be reflected as cost.

    • ab says:

      If only efficiency should be reflected as cost you´d need to ignore things like pollution and safety. It is very efficient to have some nuclear reaction heat water… but it´s not very popular in a home.

      You are probably right that efficency in capitalism is partly reflected as cost, but one should remind that exploitational processes don´t last forever as well as cost and value are not always proportional on a market.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s