That depends on a few factors – how big the Yeti is, how big the Boulder is, how cloudy it is, how hot it is, just to name a few. Goal Zero have created a handy calculator that takes all of these factors into consideration. Also, great news, all the Yetis allow for something called “pass-through power”, this means you can be trickle charging your Yeti and using it to power your gear, all at the same time.