Energy companies' green tariffs have been criticised for not living up to their environmental claims.
A report by the National Consumer Council says that the firms are failing to provide enough information for users to make a proper choice.
It also says the different types of green tariffs on offer vary greatly in their environmental benefits, with some delivering poor value for money.
Households are responsible for just under a third of the UK's emissions of carbon dioxide (CO2), believed to be the most important gas contributing to global warming.
Heating and powering the average home causes the release of about six tonnes of CO2 a year, enough to fill 34 double-decker buses.
Nationally, just 5% of electricity is generated from renewable sources such as wind turbines or hydro-electric facilities.
Well over 70% is produced by burning CO2 emitting fossil fuels, mostly coal and gas.
Over 70% of the UK's power is produced from fossil fuels
To counter this, many power companies offer green tariffs.
However, their aims and means vary, with some investing in green energy projects, while others promise to supply only electricity generated from renewables.
A third type buys 'carbon offsets' to attempt to 'neutralise' CO2 emissions.
Lord Whitty, chairman of the National Consumer Council says more needs to be done to clarify how much difference the tariffs actually make.
"The Information provided is not clear and not a basis on which consumers can make an informed choice," he told Working Lunch.
The consumer watchdog wants the industry to introduce a code of practice to govern standards of information for consumers.
In the survey, British Gas Green Electricity and Ecotricity were named as the two deals which delivered the least environmental benefits.
However a British Gas spokeswoman said:
"We are the only company to buy enough green electricity to cover our renewable programme."
Good Energy's Green Supply tariff was awarded the highest overall score.
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