The discussion of wind power’s capacity factor and intermittency often brings up other terms that bear defining.


Modern commercial wind power plants are fairly reliable, which is to say, they are not shut off for maintenance or repairs very much of the time. “Dispatchability” is not synonymous with “reliability”.


Dispatchability is the ability of a power plant to be turned on quickly to a desired level of output. Wind power plants are not dispatchable.


All power plants must be taken down for maintenance, both scheduled and, at times, unscheduled maintenance. The percentage of time that a wind power plant is not down for maintenance and is able to operate is called it availability. Modern wind turbines may have a guaranteed availability of 95% while under warranty.


Wind power penetration is the amount of energy produced by wind power, as a percentage of total energy used, in a given region. In the US as a whole, the wind power penetration is a small fraction of a percent.

Marginal emissions

Each year the operator of our electric grid, ISO New England Inc., analyses and reports on the marginal emissions rate for our region. “Marginal” means the change in emissions that would occur if one more or one fewer MWh were generated. These figures are specifically intended to be an indicator of the value of conservation, efficiency, and renewable energy.
For instance, the annual marginal average emissions rates for 2002 were:


A major impact of this pollutant

Marginal emissions rate


Acid rain

3.27 lbs/MWh


Smog, asthma

1.12 lbs/MWh


Global climate change

1337.8 lbs/MWh

So for instance, one wind turbine rated at 660 kW with a 28% capacity factor (i.e. about 1.5 million kWh/year) eliminates the production of about:


Emissions avoided


5,300 lbs


1,800 lbs


1,100 tons

These numbers are the annual average; see the full ISO New England report for a more complete discussion of regional, seasonal, and time-of-day variations.

References & Resources