Isn’t that essentially the same as a UPS, provided of course that you keep it charged?
The point I was trying to make is that a sudden withdrawal of power isn’t good for the SD card, which is what I anticipated you’d do if you somehow powered your Pi only when the sun was up. And I wasn’t considering the holes in the data you’d have overnight. If you go on to the arrangement you envisage here, you’ll need the emonPi to be permanently powered anyway, or you wouldn’t (of course) have any consumption data overnight.