It was also revealed that the number of such units in use could be increased to nine million by 2020 if the government introduces measures to encourage their use.
Malcolm Wicks, the energy minister, said: "Microgeneration has the potential to make a significant contribution to overall energy use in the UK and, combined with energy efficiency measures, will help towards reducing our carbon emissions.
"The concerned individual can take an active role in the battle against climate change."
Writing on the Guardian newspaper's Comment is Free blog, prime minister Gordon Brown recently stated that the development of a low-carbon economy in the UK is "an environmental imperative".
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