Ya i have a portable gas generator capable of powering my entire house LOL (way to big lol) but i like the idea of a natural gas one, never thought of that. That said, I insulate 4 sides of my aquaria during the fall/winter (its removable, also only ever had built into walls tanks) so they dont lose heat fast unless its really fracking cold out.
ANYWAYS, I did a quick google of battery backup air pumps, and I found this page just for some examples
http://www.aquacave.com/searchresult.aspx?CategoryID=70