plug them in to a wall outlet to reduce your net usage.
Yeah I think that's why they're illegal in the U.S. The electrical systems for a lot of homes are old as hell and this can start a fire. It happens all the time with people using generators and male-to-male power cords and plugging into the wall to feed back the power to the electrical circuit
Maybe (I am not an electrician) but it's also sort of a fire/shock hazard to have a device putting out electricity that has a male plug.
The plugs in Germany/other countries might have a better design when it comes to that, the ground connection is on the sides rather than directly across from the positive connection so it's a little less likely to short stuff out if it just falls out of the receptacle.