[VoltLog] got a hold of a prerelease unit of Joulescope — a DC energy analyzer that promises to make it easy to optimize power and energy usage of your electronic designs. You can find his review in the video below. The device is a very fast ammeter and voltmeter. Given that, it is easy to compute energy and, over time, power.
The device is set to retail for about $400 according to a letter in the video, although the website mentions closer to $800. Both of those seem to be a bit much for a piece of specialty gear that is really just a fast analog to digital converter and some software. To be fair, the device can read ranges between 18 microamps to 10 amps with resolutions as low as 1.5 nanoamps on the lower side of the range. Is it worth it? That will depend on your application and your price sensitivity.