A heater on a battery back up will likely last 2.5 seconds. The purpose of a battery back up is to aerate the tank to keep it oxygenated. It gives you a chance to get home in a timely manner and, if need be, hook up the tank to a generator to power it. I agree with above, you have to look at the history of any power outage in your area and access the situation from that. Me personally, the longest power outage I have had in 10 years at my current house is 5 hours. As a result I have my flow pumps on a couple of battery back ups. I have Maxspect Gyres and Ecotech pumps, so I use the same namesake battery back ups for both. It gives me enough time to get home, in a timely manner to do whatever needs to be done for the tank.
Currently, I run a Neptune Cor 20 pump, and it will take 7 of those pumps to equal the output of one of my 300 watt
heater.
I use the Ecotech battery back up on my Mp40, and a IceCap battery backup on my Gyre. I have never had an issue with either.
I am currently upgrading to a bigger tank. Sorry to keep adding to this post, LOL, but I plan on going with 2 MP40's and 2 Gyre XF350's, so I will likely double the battery backups. You need to look at the likelihood of power outages, the common length of outage, and your work schedule and how quick you can get to your tank. Myself, I can sometimes break away from work quickly while other times it's impossible, it just depends on the job and time of year.