I wanted to post my very unscientific battery testing results, in the hopes it helps someone. I bought a small solar panel / charging board / 2500 mAh battery from Adafruit, and I wanted to see how long it would last if I were popping online / offline every minute to post some data. I’m sure this panel / battery is way bigger than I need, and my duty cycle is way too frequent. The core could easily be slept way more often, and I could adjust the duty cycle based on the battery level indicator / charge indicators from the battery charger board.
Parts list:
- panel - https://www.adafruit.com/products/500
- charger - https://www.adafruit.com/products/390
- battery - https://www.adafruit.com/products/328
- case - http://www.amazon.com/dp/B00152RKK0/ref=pe_385040_30332190_TE_dp_2
- total cost, around: $77
My question was, assuming a fully charged battery and no sun, how long could the core last in this mode? What duty cycle would last indefinitely given some small amount of daytime sun?
Here’s my setup with the solar panel:
Here’s my setup after it was fully charged, with the counter, one tick every 2 seconds, the core sleeping 75% of the time, and connecting to wifi 25% of the time, leds on, etc. Here’s my test code - https://gist.github.com/dmiddlecamp/b2e39f6d48f33f39e16f
The final result was 62399 ticks, or about 124798 (lets assume 1% error, so +/- 1247.98 seconds), gives us a life of about 34 hours without any sun. The solar panel is rated to generate about 530mAh in full sun, I should only need a few hours of sunlight each day to break even, and any extra is bonus?
My next plan is to hook up a USB charging board to the battery as well, so I can charge my phone when I go outside to check on my core. 
Thanks!
David