The most economical backup is a $20 penn plax air pump that comes on when power fails. It uses two AA BATTERIES and will last several days running.
I use a 75A power converter used in rv’s and charge 12vdc batteries. Because I have extended systems, I run continuous 12VDC air pumps with no switching device. For systems with sumps, I use an inverter to run AC PUMPS, again no switching devices. All diy.
Know this about Uninterrupted Power Supplies, I use them for life support systems only. If you want to operate everything, then get a whole house generator.