[MN Renovator]

I think as far as compressed gasses go that propane is supposed to be the cleanest. It's MUCH more expensive than NG though, and of course also needs delivery or a tank small enough to be portable to be able to fill it from a station. If your goal is cheap electricity and heating done at the same time, you might be able to reach that goal if you do it right. The environmental aspect, I can't comment on as I don't have information to back anything up. I'd imagine if you were heating your house and your water with natural gas anyway that this might still be cleaner due to the dual source nature but it might be hard to figure. If all you've got running is your refrigerator intermittently at night, you'd better be pumping the excess energy capacity into batteries or you've got an extremely inefficient system. It all needs to be planned and executed carefully.