The Energy Saving Trust says that its figure for a £180 saving from loft insulation only refers to “virgin” homes that have never had insulation before, while the figure for a £310 saving on a boiler is for a G-rated boiler (the most energy inefficient) being replaced by an A-rated boiler. Many of the households in the NEED data will have been upgrading for D or even C-rated boilers, and may have only been adding some additional insulation, rather than fitting it for the first time.

DECC disputed the accuracy of Goodall’s figures. “The Carbon Commentary comparison of costs and savings is inaccurate, whereas DECC has developed a methodology — approved by independent experts on the Committee on Climate Change — to calculate savings from energy efficiency measures. No one can borrow more than they will pay back in energy savings, so no one will lose money by taking out a Green Deal loan.”

The Energy Saving Trust declined to say the scale of the reductions it will make for its energy saving estimates, but said new figures will be issued at the end of the month. They are likely to have a significant impact on the insulation industry, which uses the trust’s figures extensively in advertising. British Gas, for example, advertises loft insulation with the claim that it will “Help reduce your energy bills by up to £180 a year,” sourcing that to the Energy Saving Trust.