Really cool idea, loved it a lot, but some polish is needed. The turret's shot could be more precise and they lose power too quickly, leaving you almost no time to charge the generator. Especially in the third level, I was constantly going back and forth between turrets, trying to get some time, but I always failed. Finally, I just tried to only charge the generator which worked and I think this was not intentional.
But other than that, it's really cool! Good job!