Also chargers and transformers are not perfectly efficient. Chances are you aren't transmitting power from this gizmo to your house at USB voltage (horribly ine

Also chargers and transformers are not perfectly efficient. Chances are you aren't transmitting power from this gizmo to your house at USB voltage (horribly inefficient) so it's probably being sent at a higher voltage (110V seems the obvious option) which means converting it back down using an power brick, so the Wh capacity of the battery understates the power requirement for running the iPad by the inefficiency of the power brick.