This is great. I love how every upgrade makes me re-think my configuration. You have raised the bar for idle games.
a suggestion: Have each generator show how much heat it's currently converting, instead of just it's max.
Towers get the same exp even if you don't upgrade them. So if you find yourself with extra money, put off upgrading them until you've got every tower placed. You can easily get at least 10 or more of your towers to max exp by wave 45 this way.
This game suffers being too easy, and the level designs could use improvement. Too many of the levels suffer from having too many of the pieces already put together. It's like a jigsaw with half the pieces put together. Which is why I think this game should be an engineering game instead of puzzle game. Other than that it's a pretty good game, 3.5/10, rounded up.
So I invade an alien planet and the only dangerous creatures can't move. It's all right though, I killed them and their children and saved the day. Its a craptastic version of Avatar but so subtle it doesn't even know it.