RE Government plans to build up to 7,000 giant offshore wind turbines by 2020 - the equivalent of one every half mile of UK coastline (Echo, Dec 10).

This will require two of the 2,000- ton turbines to be commissioned every working day until 2020.

Denmark, the world's largest offshore wind generator, never managed to build more than two a week.

The turbines will have a capacity of 33 gigawatts (GW). However, as wind blows intermittently, they only generate on average a third of the time. Back-up sources, that can come on and off line at a moment's notice to balance supply and demand, are needed.

The proposed UK turbines will require a grid system capable of withstanding power swings of up to 33GW. Unfortunately, the only outside back-up on which our island grid can depend is a 2GW connector to (nuclear-powered) France.

Denmark exports more than 80 per cent of its wind-generated electricity to prevent its grid being swamped when the wind blows.

When it is not, Denmark needs to reimport power from Sweden and Germany. The Danes learned their lesson and decided in 2002 to build no more turbines. We British have still to learn that wind power is not a viable large-scale alternative.

Councillor Stephen Allison, UK Independence Party, Hartlepool Borough Council.

BUSINESS Secretary John Hutton's idea that 7,000 giant wind turbines sited around the UK coastline will give us clean electricity and boost British industry (Echo, Dec 10) is woolly thinking. It won't. Wind turbines only work at 33 per cent efficiency. They don't work when it's very windy and don't work when it's calm.

The private companies who will commission them will buy them "off the peg" from Germany. They won't wait for British turbines as we are 20 years behind the Continental turbine industry.

Britain has the longest coastline in Europe. Every day, billions of tons of water move in and out of our estuaries, rivers and shallow bays.

The technology is simple - the French built a tidal barrage 40 years ago on the River Rance, which still works, as new, today.

These tidal barrages do not have to be huge. The estuaries in Devon and Cornwall could supply all the electricity for those counties and could be up and running in five years. You need a half-mile long dam (filled with household rubbish perhaps), into which you drop several generators. Devon and Cornwall - sorted.

Other European nations look at our tidal power potential with envy.

We ignore it.

Dave Hodgson, Richmond, North Yorkshire