Jason's explanation is very clear. Another way of looking at this is to look at your energy meter on your house. Old meters have a spinning disk that you can see through the glass. If power is being pulled from your house to charge your Hummer EV (and for the sake of the argument no other circuit in the house is on), then the RPM (how fast it is spinning) of the disk represents the the kW of power flowing into your Hummer at that moment. And how many times the disk rotates represents the kWh of energy that has gone into your Hummer.