Fusion Cells are unlocked after Thermonuclear Cells, for a cost of 500 billion RP.

For $800 billion, you get 2.5 billion heat/tick, for 800 ticks, so one fusion cell will give you 2 trillion heat.

After unlocking, you can get Thorium Cells, which cost 50 trillion RP, Groundwater Pumps, which cost 1.2 trillion RP, and Generator 3, which costs 25 trillion RP, and Banks, which cost 300 billion RP.

You can automate the process for renewal for 8 billion RP.