FAQs

There are a number of significant differences between the operational System Warning messages and Capacity Market Notices, these are:-

• Trigger - Capacity Market Notices are issued based on an automated system margin calculation using data provided by market participants, whereas System Warnings are manually issued by the NESO control room using engineering judgement based on experience, skills and knowledge of managing the electricity transmission system.
• Threshold - Capacity Market Notices are triggered where the volume of available generation above the sum of forecast demand and Operating Margin, is less than 500MW. The 500MW threshold is taken from the Capacity Market Rules. System Warnings are triggered by varying volumes, for example a EMN may be issued where NESO has less than 500MW of its required Operating Margin. There can therefore be a 1,000MW+ variance between these two discrete alerts.
• Constraints - The Capacity Market Notice calculation does not take account of any transmission system constraints that may be preventing capacity from accessing the network. System Warnings however do take such constraints into account.
• Lead time - Capacity Market Notices are initially issued four hours ahead of when the challenge is foreseen, whereas System Warnings can be issued much closer to real time.