Britain's power supplies enough to meet winter demand -National Grid

* Surplus supply margin seen at 5.5 pct this winter

* Should be enough to meet demand

* If Rough outage prolonged, still enough gas supplies (Adds detail on previous margins, BBL pipeline supply)

By Nina Chestney

LONDON, July 8 (Reuters) - Britain will have enough electricity supply to meet demand this winter after securing emergency back-up capacity last year, National Grid (LSE: NG.L - news) said on Friday.

Britain is forecast to have a surplus power margin of 5.5 percent this winter, similar to what was forecast last winter, National Grid said in its 2016/17 winter review and consultation.

"We believe the margin is manageable and that we have the right tools and services available, including extra power we can call on if we need it, for times of highest demand," said Cordi O'Hara, director of UK system operations at National Grid.

Concerns about power shortages have risen in Britain due to ageing nuclear power plants, a plan to close coal-fired power plants by 2025 and weak economic conditions for investment in new gas plants needed to replace them.

The electricity margin would have been just 0.1 percent if National Grid had not procured 3.5 gigawatts (GW) of back-up supply, the grid operator said.

In the winter of 2004/2005 the margin fell to just 0.7 percent but National Grid said factors such as increased wind generation and supply via interconnectors mean it does not compare years on a like-for-like basis.

National Grid's Supplemental Balancing Reserve (SBR) pays generators to provide back-up power over the winter months when demand is high.

From next year, this will be replaced by the government's so-called capacity auction. On Friday, the government said it would procure 53.8 GW of capacity under the scheme for winter 2017-18.

Gas supplies should be enough to meet demand for this winter and will come from a wide range of sources, including domestic production, imports from continental Europe and liquefied natural gas, the report said.

Britain will likely have record low levels of stored gas this winter as Rough, its largest storage site, is shut until at least August due for testing.

Britain depends largely on stored reserves to manage winter demand spikes while domestic stockpiles also help ensure supply. Reserves are typically replenished during summer months when demand and prices are low.

"If Rough is not completely filled before winter, or if it is not returned to normal operation, our analysis of available supplies suggests that there will still be sufficient capacity available to the market to meet demand," National Grid said.

It (Other OTC: ITGL - news) forecast that on a cold day supply from the Netherlands through the BBL pipeline at 40 million cubic metres (mcm) per day for winter 2016/17, the same level forecast for last winter.

This is despite a Dutch government decision last month to lower the cap on production at the Groningen gas field.

(Additional reporting by Susanna Twidale; editing by Jason Neely)