Actually it's quite smart and simple l: you take any outlet in your house!

The ac/dc transformer those things simply feeds it's energy into your house - after all a plug doesn't care in which direction power flows. .those systems have an upper cap though to prevent people from overloading the net.

It's really neat and simple! You basically lower your lower consumption by whatever these things produce.

I don't know or have seen anyone using these small balcony cells with a battery though - while technically not a problem I think there's simply not a use case: they don't overproduce that much at any given time anyway.