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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.
Espruino_user_dk
Ok, cool. That's pretty easy then - I guess your best bet is to put a resistor right across it to give it something to put power into. Do you have a volt meter? If so you could experiment and pick some resistor value such that the voltage only got to the near maximum when you had the most light you ever thought you'd get.
You connect panel negative to gnd, panel positive to C0, and put the resistor across gnd and C0.
Then, working out the power is easy. You measure the voltage and know the resistance so current is I=V/R, and you multiply that by V again to get power, so it's power = V*V/R.