Explainer: Capacity Factor

News stories about renewable energy installations -- or other power plants, for that matter -- often mention building a certain number of megawatts' worth of power plant. For instance, take the headline of this February, 2009 piece from the LA Times business section, describing plans to build a whole lot of mid-sized photovoltaic installations across the state:

Or take this headline from a 2011 Bloomberg story covering solar company BrightSource's proposal to build a huge solar plant south of Blythe:

You might sensibly read statements like this as meaning that once these projects are built, California's power grid will have an additional 500 or 750 megawatts more electrical power available to it. Sadly, that's not precisely right.

When news stories or press releases refer to a power plant's megawattage, they are almost always citing what is called the "nameplate capacity" of the plant: the maximum amount of power the plant can safely put out on a sustained basis.

But power plants don't always run at peak performance. Anything from unplanned outages to scheduled maintenance and refueling can reduce a power plant's output all the way to zero for a time. Some kinds of power plants spend more time below peak production, or even offline, than others.

In order to judge a power plant's actual expected output over time, those predictable outages must be taken into account. Utility and grid planners quantify this by expressing the expected output of a plant as a percentage of theoretical maximum output.

Let's use the example in the first headline to illustrate what this means. That 500 megawatts of solar, if it generated power at full capacity for a year, would produce about 4,383,000 megawatt-hours of electrical power. (That's 500 megawatts, times 24 hours in a day, times 365.24 days in an average year.) But we know that solar cells don't put power out at peak capacity 24/7. For one thing, there's this pesky time period each day in which the sun goes down and no longer shines on the solar cells. Even when the sun is up, sometimes clouds can cut down the amount of light hitting the solar cells. Dust and leaves and occasional repairs each eat into the total amount of power produced in a year.

500 megawatts of PV will more likely produce something like 800,000 megawatt-hours of power in an average year. That's around 18 percent of maximum possible output for a 500 MW power plant. And that 18 percent is what the engineers call the PV installation's "capacity factor," the percentage of maximum possible output actually produced by the plant.

Different types of power plants will have different capacity factors. 

• Coal plants in the US tend toward very high capacity factors, in the 75-80 percent range.
• Nuclear power plants can theoretically reach capacity factors in the 90s, but seem in practice to run about equal with coal.
• Hydroelectric plants have capacity factors that vary wildly depending on their design and the size of the water source that powers them. Many dam operators regulate power output to help store water or preserve downstream flow levels for environmental reasons, introducing variability in power production. Small dams on large rivers can run at capacity factors of 99 percent, but somewhere between 40-50 percent is more common for large dams.
• Wind turbine installations, given variability of wind as a power source, generally have capacity factorsbetween 20-40 percent.
• In sunny areas, solar generally has a capacity factor of just under 20 percent.
• Geothermal offers the possibility of very high capacity factors compared to other renewables, in the neighborhood of 85-90 percent.