UK's National Grid secures backup electricity for winter 15/16

* SSE (LSE: SSE.L - news) , Centrica (LSE: CNA.L - news) among winners

* Tender cost 36.5 mln pounds (Updates with detail, background)

LONDON, June 3 (Reuters) - Britain's grid operator has secured an extra 2.56 gigawatts (GW) of electricity to cover potential demand during winter 15/16, National Grid (LSE: NG.L - news) said on Wednesday.

The reserve capacity will cost about 36.5 million pounds, or 50 pence per year on the electricity bill of an average customer.

"Our competitive tender has secured the reserve we need to do our job, as early as possible, while providing market certainty and keeping costs down for consumers," Cordi O'Hara, Director of Market Operation, National Grid, said in a statement.

A series of closures at coal-fired plants due to environmental regulations and the mothballing of some uneconomic gas plants has shrunk Britain's generating capacity over the last few years, forcing National Grid to take measures to ensure the country's lights stay on even in a cold winter.

Some 665 MW of the capacity was secured last year while 1.9 GW was secured in a second tender, National Grid said.

Coal and gas-fired power plants owned by SSE, Centrica, GDF Suez (Swiss: GSZ.SW - news) (now ENGIE (Other OTC: GDFZY - news) ) and Uksmouth Power were winners of the tender.

"These contracts are positive developments for SSE, and demonstrate confidence that our generation units can reliably provide backup service at short notice if ever required," Jim Smith, SSE Energy Portfolio Management managing director said in a separate statement. (Reporting by Susanna Twidale; editing by David Clarke)