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Up to half of the savings gained from energy-efficiency measures could be confounded by "rebound effects", according to experts, because people having invested in saving energy may then use more energy than they save by using their "energy-efficient" car more.
Scientists argue that the effects, ignored by governments until now, could have major implications for future climate change targets.
Rebound effects happen when people change their behaviour as a result of using energy more efficiently.
A more energy-efficient fridge would be cheaper to run but a consumer might end up choosing a bigger model and end up using just as much electricity.
Steve Sorrell, a senior research fellow at an energy research centre, said many reviews of future energy use have ignored rebound effects.
"Because we have ignored (it), it's likely that we have over-estimated the potential of the amount of energy that could be saved through energy efficiency policies," he said.
"If we're not saving energy from improvements we may fail to meet our carbon targets."
Experts have known about rebounds for more than a century, but they have failed to come to any agreement about how important the effects are.
In a recent study Dr Sorrell reviewed more than 500 studies of the effect as it applies to different sectors.
He found that the direct rebound effects of driving more efficient cars, for example, was less than 30 per cent. The indirect effects on the economy as a whole are harder to calculate but could be even higher.
"A number of modelling studies suggest that, particularly in the case of energy-intensive industries (such as steel and aluminium), these effects could take back more than half the energy savings. But it does vary widely from one circumstance to another."
