[namibj]

You don't really need a transfer switch, just a big relay to connect the grid and your internal wiring. Disconnect as soon as the grid drops out, and sync to the grid before re-engaging. It's not a full transfer switch you require, and should be in the price range of an resettable fuse of the same amperage rating. Just add a small solenoid to trigger the spring-loaded mechanism to disengage the switch, and use a slow-ish geared DC motor to re-load the spring and re-engage the connection. It should not be hard to enable the inverter to sync that way, and these fuses aren't expensive either. And if you cut the inverter along with only some of the circuits in your house from the rest, which are still connected to the grid, you can easily ensure that no high-power devices draw power and cause the inverter to either go into overload or have insufficient input power (solar/battery) to keep the output voltage on target.