2% is probably near the theoretical maximum, too. Actual output considering weather, efficiency losses, etc is probably less than half that. Solidly in "not-worth-it" territory for most use-cases. Heck, my car won't even properly charge on a 120V outlet when it's too cold, because it needs to heat up the battery to a safe temperature first and at -20°C that takes more than the ~1kW available from the outlet.