Plant load factor

The net capacity factor is the unitless ratio of an actual electrical energy output over a given period of time to the maximum possible electrical energy output over the same amount of time. The capacity factor is defined for any electricity producing installation, i.e. a fuel consuming power plant or one using renewable energy, such as wind or the sun. The average capacity factor can also be defined for any class of such installations, and can be used to compare different types of electricity production.

The maximum possible energy output of a given installation assumes its continuous operation at full nameplate capacity over the relevant period of time. The actual energy output over the same period of time and with it the capacity factor varies greatly depending on a range of factors. The capacity factor can never exceed the availability factor, or the fraction of downtime during the period. Downtime can be due to, for example, reliability issues and maintenance, both scheduled and unscheduled. Other factors include the design of the installation, its location, the type of electricity production and with it either the fuel being used or, for renewable energy, the local weather conditions. Additionally, the capacity factor can be subject to regulatory constraints and market forces, potentially affecting both its fuel purchase and its electricity sale.

The capacity factor is often computed over a timescale of a year, averaging out most temporal fluctuations. However, the capacity factor can be also computed on a monthly basis to gain insight into seasonal fluctuations. Alternatively, it be computed over the lifetime of the power source, both while operational and after decommissioning.

Nuclear power plants are at the high end of the range of capacity factors, ideally reduced only by the availability factor, i.e. maintenance and refueling. The largest nuclear plant in the US, Palo Verde Nuclear Generating Station has between its three reactors a nameplate capacity of 3,942 MW. In 2010 its annual generation was 31,200,000 MWh, leading to a capacity factor of:

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