Scottish Power is planning to double the size of its hydro-electric power plant which creates and stores energy.

It says the UK will need much more energy storage capacity as renewables increase on the National Grid.

It says it can add 400 megawatts (MW) of on-demand electricity by building a new dam in front of the existing dam.

But it says the £300-£400m cost is prohibitive unless it can get a guaranteed floor price for its use from the government.

In return, Scottish Power says it will accept a cap on profits.

The Cruachan plant near Oban in Scotland pumps water 400m uphill at night when energy prices from wind farms are cheap.

It then lets the water flow downhill in daytime in order to generate electricity when demand peaks and power prices are expensive.

Scottish Power's Neil Clitheroe told BBC News: "Pumped hydro is ideal because it's relatively cheap, it's virtually instant, and it provides power at scale. We will need much more of this sort of thing when we get more wind power on the system."

Ministers have recognised that energy storage in the UK needs to be radically improved as renewables produce more and more power.