Loft insulation will save you up to £180 a year, says the government – and a new boiler will cut as much as £310 off your bills. But the first official study on the true impact of energy efficiency measures installed in British households indicates that the financial savings have been around half the amount promised.

In a devastating blow for the government's controversial Green Deal programme, an analysis of the figures by a leading environmentalist suggests that the annual savings from loft insulation on 21,000 homes tracked by the Department of Energy and Climate Change (DECC) have been around £15.50 a year, compared with the "up to £180" figure promoted by government bodies.

The Energy Saving Trust, part funded by the government, has told consumers that a new boiler will save between £105 and £310 a year. Its figures are the gold standard in the energy industry, and have been widely used in adverts by companies such as British Gas to persuade householders to spend thousands of pounds replacing an old boiler. But the DECC figures suggest that the typical annual saving on bills has, in reality, been closer to just £70 a year.

In 2009 the Labour government launched its £200m boiler scrappage scheme which gave £400 off a new boiler. Around 125,000 homes replaced their often working boiler with a new one, encouraged by what now look like discredited figures.

The Energy Saving Trust says it is reviewing its financial guidance and will, later this month, downgrade the savings it tells consumers they can expect to earn from installing energy efficiency measures. It could come as the final blow for the Green Deal programme, which relies on estimates of future financial savings to encourage consumers to take out loans to pay for energy-saving home improvements.

The figures are contained in the National Energy Efficiency Data-Framework (NEED) published by the DECC late last year. It examined "observed" savings from loft insulation in 21,000 homes, new boilers in nearly 14,000 homes, and cavity wall insulation installed in 16,000 homes, and compared them with properties that did not have the measures installed. It found that on average, loft insulation decreases home gas consumption by 1.7%, cavity wall insulation by 7.8% and a new boiler by 9.2% (median figures were slightly higher). Installing all three produced greater savings.

It concluded that: "There are significant savings from installations of all the energy efficiency measures considered in this report, and provides further support for the value of installing each individually or in combination."

But environmentalist Chris Goodall, author of How to Live a Low Carbon Life and who stood as a Green Party candidate in the 2010 general election, says that while the measures undeniably cut energy consumption, they come nowhere near the financial savings claimed by the energy industry. "Home insulation measures deliver half the savings that are claimed," he says, adding that households that take out the government's Green Deal, far from saving money, will actually be worse off by around £200 a year. His analysis is published on his blog, carboncommentary.com.

Goodall took the NEED figure for the average number of kilowatt hours of gas that the 21,000 households in the study saved through loft insulation – 400kWh – and multiplied it by the 3.874p per kWh, which is the cheapest tariff offered by any of the big six energy companies at the time, in mid-December.

The saving works out at just £15.50. Even with all the measures added together, he calculates that the households have made a saving worth just £139.46 off their gas bills – half the Energy Saving Trust's headline estimate of £270.

In one of Goodall's most extraordinary findings, he notes how in some properties gas bills actually went up after the installation of energy efficiency measures. "Perhaps 40% of homes with new insulation experienced increased bills compared to the control group. This might be because the insulation was installed badly – a depressingly common phenomenon – or because the occupants decided to heat their house to a higher temperature as a result of the better insulation."

We showed Goodall's findings to both the Energy Saving Trust and the DECC. David Weatherall, energy efficiency expert at the trust, said: "The NEED data is the first large-scale data on energy saving and we are looking at what the numbers mean, and releasing updated figures. There will be a reduction in the average savings figures we use, based on the NEED data." But he added that the data "is not really comparing apples with apples" and that "we are definitely not in the business of overstating the savings that can be achieved".

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.

The implications for the Green Deal, introduced by the government in January 2013, are serious. At the time deputy prime minister Nick Clegg said: "The Green Deal will help thousands of homes stay warm for less. Those people will benefit from energy saving improvements – and their energy bills will fall."

But the Green Deal relies on a mathematical calculation that sets the cost of installing items such as new boiler against the projected savings, and lets households obtain a loan if the savings outweigh the costs. The programme has so far failed to take off (see right) and once the Energy Saving Trust reduces its estimates for future savings, even fewer households will qualify for a loan.

Existing households who have taken the Green Deal may also feel misled. Goodall says: "When is DECC going to get sued for not telling people trying their best to save money that the Green Deal will typically cost families hundreds of pounds a year?"