Thanks for the plug
@spyder813 
I'm not sure what powerhead you mean (I don't think Kessil makes powerheads), but 7W is not too far off. Most DC powerheads probably use around that. I have a Jebao PP-4 that uses between 5W and 10W on its "random" mode, which goes from 30% to 100% randomly.
Personally I would not recommend running your heater in an outage. I say this for several reasons. First and foremost, heat is not a primary concern in an outage. The biggest problem is keeping water moving and keeping it oxygenated. Plus, as you said, you have a generator. Once you're aware your power is out, you can switch from battery power to the generator and start heating the tank again. So realistically, your tank is only going to be without heat for 8 hours or so. Water takes a long time to change temperature. Not a huge deal, in my opinion. Second, and probably more importantly, running a heater is an extraordinary load in terms of how much power it draws.
To give you an idea of numbers here, let's assume your heater runs 50% of the time in any given hour. So, your power demand is 257W (heater + powerhead) per hour if you run the heater and that pump you mentioned.
This battery supplies 100Ah and costs $165. Running your heater and your powerhead, this battery would only last between 2 and 3 hours. And that's for a $165 battery. And, if your heater runs more than 50% in any given hour, that 2 - 3 hour estimate could turn into one hour pretty quickly. If, however, you just ran the 7W pump instead of the heater, the same battery would last you 125 hours. That's over 5 days. If you just ran the pump, you could get away with
this $33 battery and get about 22 hours of runtime, more than enough time to get home and get your generator up and running.
I don't mean to totally dissuade you from running your heater on a battery backup. Just know it's going to increase your battery costs likely by an order of magnitude as opposed to just keeping some pumps running.
