Capacity Factor and Reality

Capacity Factor is key to understanding the ineffectiveness of the most popular renewables, wind and solar.

Power generation equipment, from steam turbines to wind turbines, are assigned nameplate ratings.

The nameplate rating defines the amount of electricity a unit, be it a gas turbine, steam turbine or wind turbine, can produce, if operated continuously.

For many reasons, all power generation installations are not able to operate consistently, all the time. There are maintenance issues, cycling issues, efficiency issues and several other issues that cause units to not generate electricity at their nameplate rating.

The theoretical amount of electricity that can be generated by a unit over one year can be calculated by multiplying the nameplate rating, by 365 days and then by 24 hours, with the answer expressed in kilowatt-hours, kWh.

Taking the total amount of electricity actually generated during the year and dividing it by the amount that could theoretically be generated, gives the Capacity Factor.

The capacity factor of a nuclear power plant is typically 90% or slightly more. This means that an investment in a nuclear power plant can be expected to generate 90% of the electricity that could theoretically be generated based on its nameplate rating.

Wind turbines have a capacity factor of around 30%. This reflects the fact that they don’t generate electricity when the wind isn’t blowing or when the wind blows too hard, say over 55 mph. It also reflects that between the units lowest operating speed, say 5 mph, to its most efficient operating speed, say 35 mph, it operates at different efficiencies. Wind turbines are least efficient at low wind speeds; say 5 mph, with efficiency  gradually increasing as wind speed increases, until the wind turbine is operating most efficiently.

An investment in a wind turbine generates only 30% of the electricity that could theoretically be generated based on its nameplate rating.

Typical capacity factors are:

  • Nuclear 90%
  • Coal 85%
  • Natural Gas Combined Cycle (NGCC) 85%
  • Wind 30%
  • Concentrating Solar (CCS) 22%
  • PV Solar 16%

It’s been said that electricity from nuclear, coal and NGCC power plants is more valuable than electricity generated by wind, CCS or PV power plants.

The American Wind Energy Association (AWEA) actually misleads the public when it announces that “x” megawatts (MW) of wind turbines have been installed during the year.

Comparing the MW of installed wind with the MW of installed nuclear, coal or NGCC, overstates the amount of electricity that can be produced by wind turbines.

It compares apples with oranges, and misleads people into thinking that equal investments in MW of wind or NGCC power plants produce equal amounts of electricity.

Wind also has the disadvantage of generating electricity at night when it isn’t needed. The NY Times noted this past August that the investment in wind energy couldn’t provide electricity during the afternoon when it was needed to meet the air conditioning load.

Understanding capacity factor helps to understand why wind and solar are so expensive and why they can’t produce enough electricity to meet our needs.


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