Glossary: Energy Measurements
When learning about energy efficiency, it may be useful to
have a basic understanding of how energy is measured. Many of these definitions
were obtained from the Solar
Glossary of the Department of Energy’s Office of Energy Efficiency
and Renewable Energy.
Ampere (amp): A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere. Amperes are used by utilities and electrical engineers to measure electrical flow.
Joule (J): A unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second (synonymous with watt-second).
Kilowatt (kW): A standard unit of electrical power equal to 1000 watts. The term “kilowatt” (in addition to the measurements of “watt” and “megawatt”) is commonly used to describe the capacity of an electric generator, particularly in reference to small solar photovoltaic and other generating systems.
Kilowatt-hour (kWh): 1,000 watts or 1 kilowatt acting over a period of 1 hour. One kilowatt-hour is equal to 1,000 watt-hours and is equal to 3600 kJ. The primary difference between a kilowatt and a kilowatt-hour is that “kilowatt” measures the capacity of an electric generator and “kilowatt-hour” measures the actual amount of electricity it produces over a certain period of time.
Megawatt (MW): A standard unit of electrical power equal to 1,000 kilowatts, or 1 million watts. Like watts and kilowatts, the term “megawatt” is used as a standard measure of electric power plant generating capacity. It is most commonly used for large systems like wind turbines, biomass plants, and coal, natural gas, and nuclear plants.
Megawatt-hour (MWh): 1 megawatt acting over a period of 1 hour. One megawatt-hour is equal to 1,000 kilowatt-hours or 1 million watt-hours. The primary difference between a megawatt and a megawatt-hour is that “megawatt” measures the capacity of an electric generator and “megawatt-hour” measures the actual amount of electricity it produces over a certain period of time.
Ohm: A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere. Ohms are used by utilities and electrical engineers to measure the resistance of wires conducting electricity.
Volt: A unit of electrical force equal to the amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm. High-voltage electricity moves faster than low-voltage electricity, as seen in the difference between high-voltage transmission lines used to move electricity quickly throughout a region and lower-voltage distribution lines used to move electricity directly to customers.
Voltage: The amount of electromotive force, measured in volts, that exists between two points. Voltage is used to describe the amount of power produced by a generator.
Watt (W): The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage). The term "watt" (in addition to the larger measurements of kilowatt and megawatt) is commonly used to describe the capacity of an electric generator. For example, a 1,000-watt photovoltaic system has the capacity to produce 1,000 watts of power at any given time, though it may not consistently produce this much.
Watt-second (Ws): One joule equals one watt-second.
Watt-hour (Wh): The energy produced by 1 watt of power acting over a period of 1 hour. The Wh is the basis for the more commonly used measurements kilowatt-hour and megawatt-hour.