Capacity Factor

What is the capacity factor (CF)?
The capacity factor measures the performance of a power source as a percentage of its total power potential.  It can be looked at as the percentage of power it is operating at compared to its maximum operating power and for what percent of the time it is operating.  The higher the CF the more stable the grid is.  If a power plant has a high CF it rarely strains the grid because it is constantly putting out power thus other parts of the grid don't need to pick up its "slack" when it is not producing power.

Various CF's from nuclearfissionary.com:

• Combined Cycle Natural Gas Plant–11.4%
• Oil–13.4%
• Hydroelectric–36.3%
• Renewables (Wind/Solar/Biomass)–40%
• Coal–73.6%
• Nuclear–91.8% 

 The two most important numbers here are renewables and nuclear.  Renewables low CF means they are unreliable and this is a major reason there must be other power sources.  It is because they are not as reliable as nuclear for example.  Nuclear's high CF is an important factor as well.

Sources:
http://nuclearfissionary.com/2010/03/05/why-capacity-factor-matters-to-energy-production/