Automatically switch between household power and inverter when power is lost?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Automatic transfer circuit.

Figure 1 shows one way of doing the job. 


RLY1 needs to be a contactor (heavy-duty relay).
Coil voltage = 120 V or 240 V AC, depending on your supply.
Contact rating 10 A inductive or motor start rated at the AC voltage you are switching.
Contact arrangement: 2-pole, changeover.


When mains power is lost (as shown in the schematic) power will be supplied from the inverter. When power is restored the relay will energise and power the pump from the mains.

Normal safety precautions apply. Install in box that requires tools or key to open. Gland all cables to prevent chaffing. Add a fuse to cut supply to the coil in the event of a solenoid fault. It may prevent a fire.