What sort of backup do you want? If you are looking for heat and pumps for a multi day power outage your battery would be thousands of $$s. What a lot of people do is just buy a battery for an air pump to at least keep a sponge filter going (bio and oxygen).
If you are in a place that annually loses power for a few days (like my aunt in the woods of New Hampshire) I'd get a gas generator.
The other big difference is if it's also running for seamless transition or if you need to switch the plug yourself. An UPS (uninterruptible power supply) will set you back $1000 before you even start adding batteries to it.
Power back up is all about budget and goals man. What do I have? Nothing.
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