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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Yes, I'd think that would work fine - the processor can detect the voltage on itself so if the voltage is below 3.3v it knows that the capacitor has discharged past that point. It'll run until about 2v I believe so as long as you have over 3v on the capacitor and WiFi is off you could be pretty confident there'd be enough power to write to flash.
There have been a few posts about powering from solar. Personally I've found the supercapacitors I tried a bit of a mixed bag and I don't really feel confident recommending anything.
However what I'd suggest is you just buy a cheap solar USB battery bank. While you can't run directly off USB since the USB power supply in the bank will use too much power, you can just crack it open and connect the Espruino directly to the battery inside - then you have a neat solar + battery solution in a decent case.
If you really wanted to go for something neat you could pull the big battery out of the solar USB charger, and replace it with a smaller battery and fit the Espruino WiFi in the remaining space.