Not a specialist, just from what I heard: There are two things that make it work. First they are not really "independent" like the title says. They sync with the grid frequency. If the grid is down they shut off for safety. The other reason it works is that the grid power inside the home is just what you get as incoming power 〜230V. For example, I think in the US you get 240V or so delivered to your house, but 120V from the plug.