Grid frequency
Electricity in Great Britain alternates 50 times a second, which is 50 hertz (Hz). The frequency shows, second by second, whether supply and demand are in balance.
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What the frequency tells you
When more power is being used than generated, the frequency drops. When more is generated than used, it rises. So a reading below 50 Hz means demand is slightly ahead of supply, and above 50 Hz means supply is slightly ahead.
The National Energy System Operator (NESO) must keep the frequency within 1% of 50 Hz, which is between 49.5 Hz and 50.5 Hz. It pays for services, many of them batteries, that react within seconds to push the frequency back towards 50 Hz. It also changes what power stations and wind farms produce through the day.
A big, sudden drop usually means a large power station or an interconnector has tripped off. Most of the small wobbles in the charts are normal.
Source for the limit: NESO, Frequency response services. Last checked: 11 November 2026.