The 98% demand rule in GB and Ireland


The previous post showed that GB wind power at 2016/17 levels would save around 1GW of the highest peak demand on non-wind generators, for repeats of the weather of the most recent nine winters. The 1GW figure means that non-wind generators would be required to generate 98% of peak demand (i.e. around 50-55 GW), on at least one day per winter, unless the would-be high demand days fall on weekends or holidays.

The 98% result for recent GB winters is very close to figures quoted by EirGrid for the all-island Irish electricity grid, in the following document extract:


Source of the extract above:

The 98% figure for GB comes about from a combination of the differences in demand between breezy and calm cold days, and the wind speeds on the calmest cold days. Ireland and GB probably have similar weather and consumption patterns, so the similarity of the demand percentage figures is understandable.

Ireland has a greater percentage of wind power in its generator mix, which suggests that both GB and Ireland have reached saturation in terms of the impact of wind power on the highest non-wind generator demands. The following figure for GB shows that once wind capacity gets to the level needed to deal with any demand excess of breezy cold days over calm cold days, adding more wind capacity has very little effect on demand percentage:


This 98%-of-peak-demand rule may be a better (or a complementary) basis for assessment of capacity margin adequacy than the use of a winter average “capacity credit” for wind power.

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