How to measure power / voltage from solar panel using Espruino

It might be that while the solar panel produces enough voltage, it doesn't produce enough current (and current*voltage=power). By default, Espruino doesn't go into ultra-low power mode either, so you might want to write some code like:

setInterval(function() {
  digitalPulse(LED1, 1, 20);
}, 5000);
setDeepSleep(1);

and then save it. That should blink the red LED every 5 seconds, while enabling deep sleep the rest of the time.

You'll also probably need to add a capacitor (a supercapacitor would be awesome, but something greater than or equal to 200uF should be ok). This is so that when Espruino 'wakes up' and draws around 30mA of power for a brief instant, it can get that short burst out of the capacitor rather than the solar cell.

Finally, when Espruino starts, it takes around 1 second to boot (it's waiting for a USB connection - I'm planning to change that soon though). During that time it's still drawing 30mA, which could also discharge the capacitor - so it's actually helpful to have the solar cell + capacitor plugged in, and to plug Espruino into USB for a few seconds initially to get it started, and finally to unplug USB (and then it should keep working).