This is what I do for 10 to 20 hours and below. A portable generator will be used beyond that. Apex is programmed to recognize power outage and cut everything but return pump, 1 heater, and some flow. The AMS inverter has a transfer switch built in.
https://www.reef2reef.com/threads/battery-back-up.644967/post-6478164
https://www.reef2reef.com/threads/battery-back-up.644967/post-6478164