You can get a 1000 watt small Honda or Yamaha generator, they are super quiet, burn little fuel for many hours but is not stand by, you would have to start them. You could set up a UPS that would run a small power head, the small ones are designed for computers which uses more power than a tiny PH. They would run it for a long time. The heater would draw lots of power, you would have to consider the heater size required for your tank, A big heater will run down the smaller UPS quick so you would have to go larger which they do make but cost more. If you can wait it out two or three hours I'm sure the temperature won't drop that fast to go without unless you have a small tank.