Sunny, Windy Sundays

By Daniel Drew

Throughout the day National Grid (the system operator of the electricity network in Great Britain) must ensure there is a balance between the demand for electricity and the amount generated. Historically this has involved forecasting the level of demand based on meteorological conditions and human activity and adjusting the generation from conventional power stations accordingly. However, the dramatic growth of wind and solar power capacity in recent years makes things more complicated as there is now a need accurately to forecast the renewable generation as well.

National Grid has a licence obligation to keep the system frequency between 49.5 and 50.5 Hz. Any imbalance between supply and demand leads to a change in the frequency of the network. The rate at which the frequency changes following an imbalance between supply and demand is dependent on the system inertia – higher levels mean it takes longer to reach a new steady state. System inertia is the stored rotating energy of all the machines directly connected to the network, it is therefore a measure of resistance in the network to changes in frequency. The growth of renewable generators such as solar panels and wind turbines reduces the amount of system inertia. This presents a challenge to National Grid, particularly on days where renewables provide a large proportion of demand. It is therefore important to have a clear understanding of the proportion of electricity provided by renewables throughout the year.

During the calendar year of 2016, wind and solar power contributed approximately 15% of UK electricity. However, for individual 30 minute periods this proportion can be a lot higher. Figure 1 shows that the contribution of renewables to electricity demand exceeded 25% for approximately 5% of the year. In general, the highest penetrations are observed on sunny, windy and warm days, when the electricity demand is relatively low and the generation levels of wind and solar are both relatively high. If these meteorological conditions happen to fall on a Sunday the proportion of renewables is amplified as electricity demand is highly suppressed.

Figure 1. The cumulative distribution of the 30 min proportion of electricity demand provided by wind and solar power for 2016. Derived from data from

Given the short time period for which the turbines and solar panels have been installed, the distributions shown in Figure 1 are based on a limited number of meteorological conditions. It is therefore unclear what proportion of demand could be provided by renewables based on the current capacity of wind farms and solar panels. We are therefore currently working with National Grid to extend the dataset taking into account the full range of meteorological conditions which could occur in the UK.


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