You should not put lava / fuel inside the heat generator.. If you just put lava underneath / on top / on the sides or all of those.. it also generates electricity.. thus becoming an infinite source of power
I do find it to be quite a lot actually.. If you put lava on 5 sides of the generator, it generates 8 kW (so thats 8 kJ per second), which is only 1 kJ less than an advanced solar generator.. As a reference, the hydrogen generator generates 20 kW and the simple solar generator generates 1 kW.
Sorry for being off-topic, thought that this might be useful for the OP to know.
EDIT: Oops, only just see that Saman also said this