Capacity & Energy
When it comes to measuring electricity, it is important to distinguish between capacity and energy.
Capacity is the capability to generate or transmit electrical power, or the ability to control demand electrical power. It is measured in megawatts (MW).
Energy is the quantity of power that a generator produces over a specific period of time. It is measured in megawatt-hours (MWh).
NYCA Energy Production – Electric Energy Production in New York State by Fuel Source
To determine how much energy a power plant can produce over a given period of time, the capacity of the plant can be multiplied by that time interval, in hours. For example, a power plant with a 1 MW capacity operating at full capacity for one hour will produce 1 MWh of electricity (1 MW x 1 h = 1 MWh).
Capacity Factor measures actual generation as a percentage of potential maximum generation. A generator with a 1 MW capacity operating at full capacity for full year, or 8,760 hours, would produce 8,760 MWh of electricity and have an annual capacity factor of 100%.
Annual Capacity Factors for Zero-Emission Resources
Of course, power plants do not operate at their full capacity all the time. A unit’s output may vary according to weather, operating conditions, fuel costs, market prices, scheduling instructions from the grid operator, among other things. The ability of generators to operate at full capacity also varies by the type of facility, the fuel used to produce power, and the unit’s technology, etc.
For more detailed discussion of capacity, energy and capacity factors, please see our 2018 Power Trends Report.