Can you trust the energy saving claims?

Confused? You will be if you are considering installing energy-saving measures in your home, following the publication of figures from the Energy Saving Trust, which works closely with the government.

This week it revised the savings that can be achieved as a result of measures such as loft insulation or a new boiler.

It had been expected to lower the amounts, but has mystified green experts by upping some of the figures substantially.

Three weeks ago Guardian Money revealed how a study of 21,000 homes tracked by the Department of Energy and Climate Change had suggested the savings claims made by the EST were seriously flawed and in no way borne out by real-life conditions.

An analysis of DECC's own data suggested the annual savings from installing loft insulation have been around £15.50 a year, compared with the "up to £180" figure promoted by the EST.

The figures are important because they have been widely used in adverts from companies such as British Gas to persuade householders to spend thousands of pounds on replacing an old boiler or paying for other energy efficiency measures. They also form the basis of the government's Green Deal.

At the time the EST told Money it was revising the quoted savings in light of the study.

As expected, it has slashed the prospective savings for those installing internal and external wall insulation from £460-plus to just £270. These were the home improvements ministers hoped the Green Deal would provide in a new cost-effective way.

However, the EST has amazed some observers by claiming that those replacing a band D gas boiler with an energy efficient A-rated model will now save £160 a year – rather than the £105 it used to quote.

Those ripping out a band E boiler will see their savings rise by £190 a year, it now claims, up from the 2013 figure of £155.

The EST, the official body to whom the energy industry looks for its figures, has downgraded the savings for those replacing the least efficient grade G boiler with an A-rated one – but only by £5. It still claims households can save £305 a year, assuming a typical three-bed semi.

Money's own analysis suggests that these levels would be virtually impossible for most families. Three-bed households replacing D-rated boilers would have to be spending £1,600 a year on heating and hot water alone to make the kind of savings the EST claims.

Chris Goodall, a keen environmentalist and author of How to Live a Low Carbon Life, says the EST's figures are still about twice the energy savings actually recorded over the past decade.

"It seems to be using figures which are at the top end of what is conceivably possible and aren't adjusting adequately for the indifferent quality of most cavity fills and the remaining problems of air leaks and 'bridges' that conduct heat through the external wall," he says.

The EST defended the increases, saying the figures included savings made by installing thermostatic radiator valves in each room, and a thermostat.

When Money pointed out that few homes with D-rated boilers would not have a thermostat, we were told these figures were "the industry standard and have been used for many years".