300 watt heaters may be 300 watt, but the heater isnt using 300 watts consistantly,the heaters turn on and off to maintain temp unless for some reason your tank is cooling just as fast as your are heating. if it was like that, peoples electrical bills would be through the roof. a UPS would be your best bet, a heather is kind of non-essential in a temporary blackout, the fish will acclimate with the change in temp and should be ok till the power returns. after that 8 hour window, if you dont have power then your filter goes and well, dead in the water you are.