National Grid is calling on developers to bid for contracts to provide back-up power for a fee next winter.

It said margins between supply and demand “could be tighter” in winter 2015/16.

It has launched the second tender round for two balancing services to make sure there is enough power for the country during the winter when demand is high.

The first – called Demand Side Balancing Reserve (DSBR) – is looking to sign up large energy users who would be willing to lower their electricity use between 4pm and 8pm during weekdays.

The other would be a contract for energy reserves from power plants that would otherwise be closed or mothballed – called the Supplemental Balancing Reserve (SBR).

National Grid is seeking up to 1GW of extra capacity as a precautionary measure if power supplies are tight. The first round secured 700MW of extra capacity, which included three gas-fired power plants.

National Grid paid around £30 million, including setup and admin fees, for reserving the extra capacity in 2014/15 but were not needed due to “good levels of generator reliability, consistent continental imports, high levels of renewable output and generally milder weather conditions”.

It added less than 50p on the average domestic electricity bill, the grid operator said.

Cordi O’Hara, National Grid’s Director of UK Market Operation said: “We already know that the margin between supply and demand next winter could tighten further. We want demand side providers and power station operators to come forward and ensure a competitive tender. We’re particularly keen to see large energy users come forward to join this voluntary scheme.

“Our first tender round secured 700MW of additional reserve and this second tender round will help build on this as the outlook for next winter becomes clearer.”

The contracts for the second round will be offered in May this year.