[thaumasiotes]

That's not just a problem with being hooked up to the grid so you can altruistically use less of your own energy and donate the rest to other people.

It's also a problem for billing Dave Rosenthal appropriately. Assuming he turns on lights at night, he's not using his own energy. No amount of solar generation will let you use power when the sun isn't shining. So he's not reducing the demand for (say) coal at night.

But because he's being billed wrong, rolling over daytime power he doesn't use as if it were the nighttime power he does use, people still think he is.

[Taniwha]

grid batteries do exactly this