Note: The graph above shows MW demand over the last hour. Please note that the graph refreshes itself every 15 seconds, but may need manually refreshing if left open for a prolonged period.
Demand over last 24 hours
Demand over last 8 days
Note (1): The 24 hour and 8 day graphs above do not auto-refresh in place.
Note (2): This is the total instantaneous MW demand being supplied across England, Wales and Scotland. It is the sum of the output of all the generation connected to the GB Transmission System plus imports from External Interconnectors i.e. from France-GB and Ireland-GB (Exports to France and Ireland are subtracted). The generation from pump storage is treated in the same way as other directly connected generation and hence is included in the instantaneous demand calculation. The pump storage demand or the station transformer demand are not netted off (i.e. they are not subtracted from the total instantaneous demand).
AC Frequency variation over last 60 minutes
Note: The normal system frequency is 50Hz. As electricity cannot be stored, the instantaneous generation must match the demand being taken from the system. If the instantaneous demand is higher than the generation, the system frequency will fall. Conversely, if the instantaneous generation is higher than the demand, the frequency will rise. System frequency will therefore vary around the 50 Hz target and National Grid has statutory obligations to maintain the frequency within +/- 0.5Hz around this level. However, National Grid normally operates within more stringent 'operational limits' which are set at +/- 0.2Hz.
For a lot more information see http://www.nationalgrid.com/uk/Electricity/Data/