you over estimate the battery backups and how much a flexwatt will drain them. i assure you it smuch faster then that. if you have one. just plug i your heaters, unplug it and check the clock. and wait for it to beep dieing.

Ive used mine before and only got a few hours for two snakes

BUT that being said if you have a dirty generator. AND one of the UPS supplys for your pc. You plug it into the generator and the whole idea of the battery ups will help regulate your power a lot.