Capacity calculations for wind on the grid

Operators of the electrical grid have to know how much power capacity they can county on when they need it. This creates a problem for wind energy, because only the wind determines its production rate and time. It's largely chance that wind and demand might coincide in any dependable way. Therefore, wind is largely a sideshow, or even an added burden, to the real work of the grid.

Here is how different grid operators look at wind power's actual value in their planning, as compiled by the Utility Wind Integration Group ("Wind Power and Electricity Markets", information compiled through September 21, 2007). For "Capacity Calculation":

• PJM (Mid-Atlantic plus northern IN and IL) initially assigns a value of 20% of the rated capacity and then uses the average output over the previous 3 years ot 4, 5, and 6 o'clock p.m. from June through August.


• NYISO uses the average capacity factor between 2 and 6 o'clock p.m. from June through August and between 4 and 8 o'clock p.m. from December through February.


• ISO-NE (New England) uses the overall capacity factor.


• Ontario IESO uses 10% of the rated capacity for long-term (e.g., seasonal) forecasting and 0% (zero) for 1 to 34 days ahead.


• MISO (Midwest) gives wind a 15% capacity value for transmission planning purposes.


• SPP (KS, OK, TX panhandle) uses the level of output equalled or exceeded for 85% of the top 10% load hours.


• ERCOT (TX) uses 8.7% of the rated capacity in capacity reserve margin calculations.


• CAISO uses the average monthly output over the previous 3 years between 12 and 6 o'clock p.m. from May through September.


• Alberta Electric System Operator assigns a 20% value.

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