Fusion Cell

Fusion Cell is a straight-up upgrade to Thermonuclear Cell. Higher heat output, higher price, no other differences.

It produces 2500000000*800=2000000000000 (2 Т) heat in it`s lifetime (without upgrades). Subtracting the cost, it`s pure profit is 1200 B.

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

Profit per tick
The profit increase with each level up increases logarithmically, so it's not worth the cost to go past about 2 or (maybe) 3.