The extra solar panels installed on the grid to feed those carts don't have zero environment cost either, though. And neither do the batteries required to hold

The extra solar panels installed on the grid to feed those carts don't have zero environment cost either, though. And neither do the batteries required to hold that energy until the carts are plugged-in, which would likely be at dusk/night, when they're not in use. (Grid solar -> grid battery -> cart battery) involves more losses than just (cart solar -> cart battery).