Energy savings in UK households could be up to 30% lower than previously thought, jeopardising efforts to cut the nation's carbon dioxide emissions.
Rebound effects may take the shine off energy saving figures
The UK Energy Research Centre (UKERC) blamed the miscalculation on "rebound effects" from energy-saving measures.
As people cut their bills by using more efficient devices, they tend to spend the extra money buying additional goods that cancel out some of the savings.
The government has set a binding target of cutting CO2 levels by 60% by 2050.
The UKERC's research director, Jim Skea, said the introduction of the legally binding CO2 target in the Climate Change Bill increased the need for accurate measurements.
"If the government's feet are going to be held to the fire over carbon emissions, then it really needs to understand what the consequences are for the policy interventions it is going to make," he said.
"It is absolutely critical that it gets its sums right in estimating the implications of energy efficiency measures.
"There is clear evidence from this report that some of the policy appraisals conducted in the past did not take full account of all of the impacts," Professor Skea observed.
Less is more
The report's author, Steve Sorrell, said that there were two main forms of rebound effects - direct and indirect.
He used the example of someone buying a fuel-efficient car as an example of a direct rebound effect.
"It should mean that I use less petrol, that's the assumption," he said. "But because I am using less petrol, my running costs are less.
"As a result, I may choose more often, I may choose to drive to the shops rather than take the bus.
"In the end, I may drive further because driving is cheaper and that will offset some of the energy savings. The energy consumption per mile may be less, but I am driving more miles."
To illustrate an indirect effect, Mr Sorrell said that if he did not drive more miles then he would save money, which he could spend on other goods and services.
"To take an extreme example, I could put some of that money towards an overseas holiday.
"In which case, some of that money is being spent on kerosene, so there is energy being used elsewhere in the economy as a result of me saving money on my car travel."
Mr Sorrell admitted that the concept had always been controversial, ever since it was first used back in 1865, when it was applied to the efficiency of steam engines.
"Experts have continually failed to come to any agreement on how important these effects are and how much is actually saved through energy efficiency," he said.
The UKERC's report assessed the existing research and case studies on the impact of rebound effects on energy savings to see if there was evidence of an economy-wide effect that could jeopardise the UK's climate policies.
Mr Sorrell said measuring rebound effects was "notoriously complex" and the report found that there were wide discrepancies in previous studies.
But he added that it would be a mistake for government ministers and officials to ignore them.
Professor Skea said the legal requirement to meet future targets meant that the government was likely to pay close attention to factors such as rebound effects in the future.
He said: "One of the recommendations of this report is that the government should be allowing a little bit of headroom in its carbon targets in order to get the assurance that they are actually going to be met."