Measuring power usage over time? What kit is that?

Quick example that I just tried here... Put a 0.33uF supercapacitor between Gnd and 5v on the Espruino, and measure the voltage across it.

When charged up and after I unplug USB power, I get 4.35v, so I put 0.33F, 4.35v and discharge to 4v in here and get 32.083 µAh. I then figure out how long it took to get to 4v, which looks like around 7 minutes in my case.

32.083 uAh / (7/60) = 275 uA

So on a 700mAh battery that's 700 / 0.275 = 2545 hours = 100 days. ish. The battery can self-discharge, and the voltage may drop to a level that's too low before its full capacity has been used.