What is capacity factor?

Capacity factor tells you how much energy a plant really produces compared with what it could produce if it ran at full power all the time. It is a basic number for comparing wind, solar, gas, nuclear and other sources. Planners use it to convert a plant's nameplate size into the energy that can be counted on, and so to compare costs fairly.

How it works

The formula is capacity factor = actual energy produced / (nameplate capacity x hours in the period). A low value does not mean a plant is poor. A plant that runs only at peak hours may have a low factor by design, while one that is meant to run continuously aims for a high one.

Three things pull the factor down: the fuel is not always available, as with wind and sunlight; the plant is shut for maintenance; or the plant is held back because the grid does not need its output. Because the cost of building the plant is fixed, a lower factor spreads that cost over fewer units of energy and raises the cost of each one.

Example

Say a wind farm has a nameplate capacity of 100 MW. Over a year there are 8,760 hours, so the most it could produce is 100 x 8,760 = 876,000 MWh. If it actually delivers 306,600 MWh, the capacity factor is 306,600 / 876,000 = 35%. A gas plant of the same size that delivers 438,000 MWh has a capacity factor of 50%. To match the gas plant's output, the wind farm would need to be about 143 MW, since 438,000 / (8,760 x 0.35) is roughly 143. These are illustrative figures.

How to model it in Machinations

Model the plant as a Source whose output per step is set by a Register: capacity times an availability value you can vary. A Pool records energy delivered, and a second Register divides it by the maximum possible to give the capacity factor. Make the availability and maintenance downtime Levers, then Play and chart the factor across the year.