The Transmission System Demand Forecast is made by the System Operator.

The System Operator Transmission System Demand Forecast is based on historically metered generation output for Great Britain. This value  includes Interconnector flows and demand from station transformers and pumped storage units.

The  Transmission System Demand Forecast is comparable with the Initial Transmission System Demand Out-Turn (ITSDO). All the forecasts are unrestricted, i.e. any notified customer demand management is included in the history.