System frequency is a continuously changing variable that is determined and controlled by the second-by-second (real time) balance between system demand and total generation. If demand is greater than generation, the frequency falls while if generation is greater than demand, the frequency rises.

National Grid has a licence obligation to control frequency within the limits specified in the 'Electricity Supply Regulations', i.e. +/-1%; of nominal system frequency (50.00Hz) save in abnormal or exceptional circumstances. National Grid must therefore ensure that sufficient generation and / or demand is held in automatic readiness to manage all credible circumstances that might result in frequency variations.

System frequency is measured in cycles per second or Hertz (Hz).

Note that whilst the graph updates approximately every 2 minutes, the underlying data is shown at 15 second granularity.