Definition

Capacity factor is the ratio of the actual energy produced in a given period, to the hypothetical maximum possible, i.e. running full time at rated power.

Example: Suppose you have a generator with a power rating of 1500kW x (365 x 24 hours) = 13,140,000 kWh in one year. Suppose that in fact I made 3,942,000 kWh in one year. Then in that year, the generator operated at a:

capacity factor that year.

What are common values for capacity factor?

All power plants have capacity factors and they vary depending on resource, technology and purpose. Typical wind power capacity factors are 20-40%. Hydro capacity factors may be in the range of 30-80%, with the US average toward the low end of that range. Photovoltaic capacity factors in Massachusetts are 12-15%. Nuclear capacity factors are usually in the range 60%-100%, and the US national average in 2002 was 92%. The capacity factors of thermal plants cover a wide range; base-loaded thermal power plants (e.g. large coal) may often be in the range of 70-90% and a combined cycle gas plant might be 60% depending on gas prices, whereas power plants in the role of serving peak power loads will be much lower. One might expect a new biomass thermal plant to have an 80% capacity factor.

Is capacity factor the same as efficiency?

No, and they are not really related. Efficiency is the ratio of the useful output to the effort input – in this case, the input and output are energy. The types of efficiency relevant to wind energy production are thermal, mechanical and electrical efficiencies. These efficiencies account for losses, most of which turn into heat in the atmosphere and water. For instance, the average efficiency of the US electricity generation infrastructure is about 35% - this is because in most thermal plants about two thirds of the input energy is wasted as heat into the environment. The mechanical conversion efficiency of commercial wind turbines is a fairly high, in the range of 90%.

Wind power plants have a much lower capacity factor but a much higher efficiency than typical fossil fuel plants. A higher capacity factor is not an indicator of higher efficiency or vice versa.

Is a higher capacity factor “better”?

Within a given technology or a given plant, yes, you can generally say that a higher capacity factor is better and in particular, more economical. But it does not make sense to compare capacity factors across technologies, because the economics of both production and capacity are so different from one technology to next – the capacity factor is just one of many factors in judging if a power plant is feasible. Instead, more useful is to compare the cost of producing energy among the various technologies.

References & Resources

• http://www.umass.edu/windenergy/publications/published/communityWindFactSheets/RERL_Fact_Sheet_2a_Capacity_Factor.pdf