The quest was for a reliable, cost effective backup system that would come on automatically in the event of a power failure and run my MAG18 return pump for 8 hours...
The result cost under $175 total, and consists of a 95ah deep cycle battery, power inverter, automatic trickle charger/maintainer, and a DIY transfer switch.
The main unit is in the basement, with only an outlet box in the cabinet.
This outlet has a cord that plugs into the main GFI, and routes power through 2 legs of 4 conductor wire down to the UPS. Either house current is sent back up, or it is switched over to inverter power.
Here's a top view of the backup
The heart of the system is a transfer switch that consists of a relay normally energized with house power, and when that is absent, it turns on the 300w AC power inverter, and sends that power back up to the outlet.
There are a few other bells in there, like a test switch, audible alarm, and indicator lights.
It took quite a while to design, build, and debug, but so far it has tested out perfectly, and at a fraction of what a similar commercial UPS would have cost
