The NESO is worried about a lack of spare capacity in the grid from 4pm until 7pm due to “system constraints”.
The body, which is in public control having been part of National Grid until last autumn, said in an update that it was seeking 1,200 megawatts (MW) of power as part of the so-called system margin notice.
Such notices are a call for a greater safety cushion between power demand and available supply.
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The NESO was at pains to point out that it does not signal that blackouts are imminent or that there is not enough generation to meet current demand.
The number of such power stations held in reserve was gradually drawn down under efforts to reduce the country’s carbon footprint.
Ratcliffe-on-Soar power station shut down in September.
The UK has reciprocal arrangements with neighbouring countries to draw power via so-called interconnectors if and when required to help keep the lights on.
National Grid data showed that more than 50% of the UK’s power was being generated through natural gas.
Renewables accounted for just 16% while France and Norway were helping provide 10% of output, with nuclear and Biomass accounting for the bulk of the balance.