a little trick: how to combine 3 AC backup devices to extend the available power of AC backup systems.
You can only get up to 1500 Wh battery power AC backup systems for a reasonable price.
Here a way to combine 3 to triple the available power: . The relay is ON when system delivers power. When A is empty it switches to B. The switching AC interruption is covered by system C. When System B is empty ,system C will deliver the power. Hence you have 3* 1500 Wh available.
The relay is a simple 120V (or 220V) relay with 2 independent switches. They switch both Line and Neutral from A and B to C. If A delivers power it is switched to A, once A done, the coil loses its power and it goes to off mode and routes B to C.
Seems to work fine.
If you try this make sure that everything is well isolated.
Karl