Britain was exporting power to Ireland even as British households were asked to cut their usage on Monday night, export flows show.
Traders sent electricity via undersea cables to Northern Ireland and the Republic while thousands of British households avoided activities such as running the washing machine to save electricity in Britain.
Exports to the Irish single electricity market were on course to take place on Tuesday afternoon as well, as British households were asked by National Grid to cut their usage for a second time.
In both cases, households were asked to turn down hours before export trades were booked, amid British market forecasts which turned out to be too pessimistic.
The figures highlight the growing interconnectedness of Britain’s electricity market at a time of stressed supplies.
On Sunday, National Grid, which is in charge of keeping the lights on in Britain, asked households in Britain to cut their electricity usage between 5pm and 6pm on Monday, amid concerns about low wind and high demand.
It marked the first use of its new “demand flexibility service” under which households who voluntarily sign up can receive payments to cut their electricity usage when needed to help National Grid manage the system.
Data from market specialists EnAppSys show that the cables were exporting power from Britain to Ireland between 5pm and 6pm on Monday at similar capacity to that cut by British households.
Auction timings mean that traders would have booked the exports after the call for households to cut usage had been made.
National Grid had to decide on Sunday whether to ask households to cut usage on Monday, as it needs to give 24 hours' notice.
It accepted offers on Sunday from suppliers whose customers were willing to use less power, committing National Grid to follow through on Monday.
Monday’s market outlook looked worse on Sunday than it turned out to be, with wind power holding up reasonably well. During the period, Britain was importing heavily from France and other markets.
Households were asked on Monday to cut usage for a second time, between 4.30pm and 6pm on Tuesday. Exports to Ireland have also since been booked for this period, data from EnAppSys shows.
Ireland’s electricity market has been through periods of tightness. In September 2021, the operators had to freeze exports to Britain to prevent shortages in Ireland.