Chaff is cut to reflect and thus block the specific frequencies that are expected in a given threat environment.
The pieces of foil are very small. The idea is too short across, by comparison, very large spaces.
Hitting backup generators requires destroying them. Yes, they could in theory be located by thermal signature and a satellite Laser scanning for combustion signatures. This is how California and Washington state spot you are using your illegal wood stoves.
But the emergency generators will only run as long as the logistics in the country work. I assure you, they aren’t working now and delivering fuel is probably not happening.