Why green power has left us all in the dark

These problems are recognised by the Department for Environment, Food and Rural Affairs, which advises people with green tariffs that 'the renewable energy you are using has already been accounted for in the UK energy mix' and gives even supposedly 100 per cent-renewable tariffs a carbon rating. Defra does, however, urge people to sign up for such tariffs because they increase demand for clean energy - an argument also put forward by campaigners, who like the idea of green tariffs but are troubled by their complexity.

This argument does not seem to stack up either, though, because demand already far exceeds supply. While only a few households are on green tariffs, British businesses pay a 'climate change levy' of 0.43p for every kilowatt hour of electricity - except on renewable energy, which is exempt. This tax, and rising consumer pressure, mean big businesses have rushed to buy renewable supplies - overwhelming the less than 5 per cent of power available from these sources.

Electricity suppliers, though, defend their eco-tariffs. Eon, a big supplier with a green tariff that does not, overall, meet its RO minimum, argues that if enough customers sign up to its tariff, it would push the company to exceed the minimum. Good Energy, the only UK supplier to offer 100 per cent renewable energy, agrees buying green does push up demand. That this is not yet resulting in an increase in supply is the fault of other policies, particularly planning constraints, it says.

Meanwhile, many campaigners are reserving judgment - afraid to deter consumers from doing something to help, but worried that if green tariffs are seen as a sham there could be a backlash.

'It certainly shouldn't be one of the first things people do,' says Keith Allott, head of climate change for WWF. 'If you have only time to do two or three things, I'd improve the energy efficiency of your home, think seriously about personal transport and how frequently you fly.'

Some believe the premium charged for green power will be overtaken as the cost of coal and oil drives the price of 'conventional' electricity above the current level of eco-tariffs. But Doug Stewart of supplier Green Energy thinks renewables customers are not safe from rising fossil-fuel prices yet: 'If old "brown" energy rises and green energy stays where it is, the market will pile into green energy until the price equalises.' The turning point will come, he says, when there is enough green energy to satisfy demand - then, oil's influence over the price of electricity will start to diminish.

·greenelectricity.org; electricityinfo.org; ofgem.gov.uk