It's not cheap but a company called Tripplite makes power inverters/chargers that will work as back ups. I use one for my salt water tank. You plug it into the wall, connect a couple car batteries to it and plug in your heater to the unit. It will keep the batteries charged, automatically switch to battery power and covert it to A/C when the power is out. When the power comes back on it will switch back to A/C from the wall and start charging the batteries again. Denpending on the amount of batteries you have, their rating and what your equipment draw is you can get 4-6hrs of back up heat. Could make the difference..... General-Purpose Inverter/Chargers