Not all kilowatts of generating capacity are created equal: some generate more electricity than others over the course of a year. The capacity factor (CF) is defined as the ratio between the amount of electricity a plant actually generated in a year and the amount of electricity it could have generated operating at full power for the entire year. For example, a solar photovoltaic system on your roof with a nameplate capacity of 1 kW-ac has the potential to generate 8,760 kWh if it runs at maximum output over the entire 8,760 hrs of the (non-leap) year. But your system will generate no electricity at night, and probably only reach its peak at solar noon in the summer months. At the end of the year, your system may have generated something close to 1,400 kWh, making its capacity factor equal to the ratio 1,400 / 8,760, or 16%.
The capacity factor of each resource varies significantly, and is dependent on both intrinsic characteristics of the resource as well as how the grid operator uses the resource. Your PV system, for example, will have its capacity factor limited by the availability of sunlight, as described above, but also by how often the grid operator disconnects your system because of grid outages or required maintenance on your distribution feeder. Similarly, a conventional resource, such as a coal plant, has limited output over the year due both to 1) forced and planned maintenance outages at the plant, and 2) how often the grid operator dispatches the coal plant. If demand is below usual and less power is needed, the coal plant may be dispatched only to partial capacity for much of the year, dropping its annual CF. But even if the grid operator dispatches a coal or nuclear plant to maximum power all the time, it is nearly impossible to have a 100% CF over the entire year, because thermal plants simply cannot run all year without shutting down for planned or emergency maintenance.
This table shows the capacity factors for various resources (averaged across the entire U.S. grid) in 2050 in Reinventing Fire’s Transform scenario. The capacity factors for renewable resources are mostly resource-limited, because grid storage, demand response, and better forecasting and operations allow almost all of the renewable energy to be used when it is generated. The capacity factors for conventional resources in this scenario are mostly grid-limited. The high adoption of renewable resources and the must-take attitude towards renewable energy means that conventional generators are often throttled up and down as variable demand and renewable supply fluctuate throughout the day. And the 2050 Transform scenario has no coal capacity, making coal’s CF irrelevant.
For comparison, Rocky Mountain Institute’s modeled CFs for conventional resources in 2010 are 90% for nuclear, 53% for gas combined cycle plants, and 80-85% for coal. These are very representative of CFs on today’s grid.