LONDON (Reuters) - Technology enabling the UK power grid to control the charging of electric cars could correct only 6 percent of the additional imbalance in supply and demand by 2020 caused by rising use of the cars, a study said on Tuesday.

The technology could also provide a vehicle owner with a modest annual financial return of 50 pounds, National Grid estimated in a report published together with engineering company Ricardo.

Utilities and grid operators fear that the large-scale introduction of electrical cars could cause supply problems when millions of car users charge their vehicles during peak demand hours after returning from work.

With a system in place, the grid operator would be able to interrupt or vary the charging of an electric vehicle according to grid imbalances during peak and off-peak times.

"This mode of balancing provision requires zero investment from the vehicle owner but is clearly limited in the extent of service that can be provided to the grid," the report said.

"Using demand side management, the projected fleet of plug-in electric vehicles in 2020 would be able to provide an average of 6 percent of daily GB network balancing service requirements. This rises to a maximum of 10 percent in the evening and overnight," the study said.

The UK government said on Monday that it expected 60 percent of all new cars registered in 2030 to be electric.

Another option considered by the industry is to invest in vehicle-to-grid (V2G) technology that also enables transfers from cars back to the grid, known as reverse charging.

The study said vehicle owners with V2G technology could reap annual revenues of 600-8,000 pounds, depending on the vehicle's size.

But it also said that "very significant capital cost and balancing limitations ... would serve to render the fleet scale roll-out of the V2G balancing service uneconomic" if extended to owners of single cars.

V2G operation "may be attractive for owners of captive vehicle fleets such as industrial or local delivery vehicles ... where interface costs might be shared across multiple vehicles or battery packs," the report said.

Despite the limited balancing possibilities shown in the report, Ricardo's chief technology and innovation officer, Neville Jackson, said the company "believes that the energy storage capacity of a future electric vehicle fleet needs to be viewed as an integral part of the power system."

He also said that "if operational synergies at both a grid and distribution network scale are exploited, some of the obstacles to the mass roll-out of electric vehicles will be tackled."

National Grid's strategy development manager, Mike Edgar, said that developments in the electric vehicle sector, "as it matures, may lead to a position where the balancing service contribution it can provide is both commercially viable and practical."

(Reporting by Henning Gloystein, editing by Jane Baird)