From what I can tell (as someone who's learned a bit more than average about it but definitely doesn't pretend to have a lot of knowledge) it's true that generating the same amount of power from a central generator is certainly better than having a bunch of crappy little gasoline engines generating it (gasoline lawn mowers are another example of a terrible polluter that people just take for granted), but central power generators can nonetheless be awfully polluting, and most indeed are. E.g. did you know that more than half our electricity in the US still comes from burning coal? (See http://www.sierraclub.org/cleanair/factsheets/power.asp for details.) Part of the problem is that an awful lot of power plants are very old, not meeting any kind of decent environmental guidelines - certainly we have the ability to generate power more cleanly, but apparently as a nation we lack the political will.