You can measure the flow of power by calculating the vector product of the current and voltage in the power line. EEVblog has a very good explanation video how

You can measure the flow of power by calculating the vector product of the current and voltage in the power line. EEVblog has a very good explanation video how this is done in an (analog) domestic watt-hour meter: https://www.youtube.com/watch?v=P_3DXcB9-xE https://www.youtube.com/watch?v=P_3DXcB9-xE