It would be a 1-space item, made with a nanocarbon alloy, or a new item (who knows) and it would generate from nothing 0.2 energy bar per second, a low rate but infinite energy.
I think it's a good item and it would be very good for the endgame, you could put one of these and a portable oxygenator and make a very nice oxygen and energy supply.
Because of the rate it wouldn't be that OP, because the oxygenator uses more so you couldn't have a permanent oxygen supply.