A RTX 5080, not the lowest power card.

Not the lowest power, but surely it uses less power than a microwave. 360W TDP, 850W required system power, while my microwave is 1000W.

The difference is most prompts take far longer than a single second to return.

Yeah, but couple hundred watts certainly shouldn't dim your lights.

And if it does, you should get your wiring checked! If voltage is sagging enough to dim your lights with such a small load, that indicates a lot of resistance somewhere in the wiring, which could lead to fires.