Britain to hold winter 2020/21 back-up power auction on Dec.6

* Auction for winter 2017/18 set for Jan 31 (Updates with details, background)

LONDON, Nov 15 (Reuters) - Britain will seek almost 52 gigawatts of back-up electricity for the winter of 2020/2021 in a capacity auction on December 6, National Grid (LSE: NG.L - news) said on Tuesday.

Britain began capacity auctions in 2014 looking to head off future winter power shortages as coal plants close and low power prices dissuade investors from building new ones.

Power plant owners are paid to make available back-up electricity at short notice.

Back-up electricity is tapped when supply is too low to meet demand, for instance during cold snaps, when renewable energy sources fail to produce enough power, or when thermal power plants have failures.

National Grid said it would seek almost 54 gigawatts of back-up electricity for winter 2017/18 in an auction to take place on January 31, 2017.

The first auction was held in December 2014 for back-up power for winter 2018/19. Last December an auction for winter 2019/20 and cleared at 18 pounds per kilowatt per year.

As wholesale electricity prices have fallen further since, operators are suffering more and will likely need bigger incentives in future auctions to keep capacity available, analysts say.

(Reporting by Susanna Twidale; editing by Jason Neely)