Abysmal. A current design you can see is the tritium nuclear battery. I misspoke, it is beta particles that generate the photons. The reaction produces nanowatt

Abysmal. A current design you can see is the tritium nuclear battery. I misspoke, it is beta particles that generate the photons. The reaction produces nanowatts. If you remake this system with quantum dots and layered materials it is actually useful and can power small electronics, like a bug. But it needs light shielding.