Because most electric utilities are required to accept electricity from you when your solar panels generate more electricity than you need at the time, a special electric meter is used that can measure both the amount of electricity going to the grid (local electric company you are connected to) and the amount of electricity coming to you when you are not generating all the electricity you need. Both of these meter readings are read, usually monthly, and the amount of electricity you sent to the grid is subtracted from the amount of electricity that came to you from the grid. If more came to you, you pay the electric company. If more went to the grid, that amount is credited towards your next month’s bill and you pay nothing.