The present invention provides an electric quantity computing device, an electric quantity computing device, an electric quantity computing system and an electric quantity computing method. When solar distributed power supplies and an accumulator are equipped in a requirement user, whether the electric power of the distributed power supplies or the electric power of the accumulator is used as back-flow power can not be determined. The electric quantity computing device of the invention is provided with the following components: a unit which measures output electric quantity of the distributed power supplies, and a unit which measures charging/discharging electric quantity of the accumulator. Consumed electric quantity of the requirement user and the back-flow power are summarized. One smaller electric quantity in the electric quantity which is obtained through deducting the discharging electric quantity of the accumulator from the back-flow electric quantity and the electric quantity of the distributed power supplies is estimated as the electric quantity that is generated because of power generation of the distributed power supplies.