LONDON (Reuters) - Britain's National Grid said on Tuesday it expects low demand for electricity this summer as small-scale renewables output increases and warned there could be times when there is more generation than needed.

In its summer outlook report, National Grid said peak transmission system demand for high summer (June-August) was forecast at 33.7 gigawatts (GW) and the summer minimum at 17 GW.

"The increase in distribution connected generation, for example wind and solar photovoltaic, has contributed to this downward trend in demand," the grid operator said in the report.

Summer electricity demand from the grid has been falling over the past few years due in part to a rapid increase in the amount of solar power generation on people’s homes and factory roofs.

However, increased renewables output can create challenges as solar and wind output is variable.

"As a result, we may need to take more actions to curtail generation and possibly instruct inflexible generators to reduce their output in order to balance the system," the report added.

Lower overall electricity demand, along with increased renewables generation, means there is less need for gas-fired electricity generation.

Total UK gas demand for this summer is forecast at 35.7 billion cubic meters, slightly lower than last summer.

National Grid said it expects to see similar gas supply and demand patterns to last summer, with demand only around 30 percent lower than the winter peak.

However, National Grid said its expects one of the highest volumes of maintenance on the gas transmission system to date this summer.

The grid operator also forecasts that there will be 1.8 bcm of gas demand for storage sites, compared to demand of 2.5 bcm last summer.

The amount of gas which will be injected into gas storage sites this summer will depend on the level of stock at the end of winter.

Gas is traditionally stored during the summer months to be used during winter when demand and prices are higher. However the closure of Britain's largest gas storage site Rough means less gas can be stored.

Last year, the most significant injection into storage was when the InterconnectorUK pipeline to continental Europe closed for annual maintenance during the summer.

National Grid expects a similar trend this summer.

(Reporting by Nina Chestney; Editing by Susan Fenton and Adrian Croft)

By Nina Chestney