[jstrom]

That's not necessary malicious on the part of the power company.

At a previous job, we built a system for a company that prepared batteries through multiple charge/discharge phases. During a discharge, the power was pushed back onto the grid.

The major challenge was matching the waveform of the grid power (occasional spikes confusing the zero-crossing, drift on the timer elements etc.). If you don't, it reduces the efficiency of the power company's transformer leading to replacement with a larger one, or damage due to the excess heat. You can imagine they're going to recover that expense from somewhere (and you and your dirty sine wave are pretty clearly at fault).

I would expect the home systems to be less precise than our industrial client's equipment.