A single phase energy meter is a device used to measure the amount of electrical energy consumed by a single-phase electrical system. A single-phase electrical system is one in which the voltage oscillates in only one direction, it is the most common type of electrical system used in residential and small commercial settings. The meter records the total amount of electric energy consumed in kilowatt-hours (kWh) and is typically located on the outside of a building or inside the main electrical panel. The reading is used to calculate the cost of the energy consumed based on the rate charged by the utility company. Single-phase energy meters are relatively simple in design, and typically consist of a rotating disk or an electronic register that records the energy consumed. They are typically read by the utility company on a monthly or bi-monthly basis.