Energy-saving bulbs use up to 80% less electricity than traditional bulbs
Energy-efficient light bulbs lose on average 22% of their brightness over their lifetime, a study has found.
In some cases they emit just 60% as much light as traditional models which are being phased out of shops, it says.
The study in Engineering and Technology magazine concluded that consumers were being misled by the bulbs' packaging.
Of the 18 energy-saving bulbs tested over 10,000 hours by the Institution of Engineering and Technology, three stopped altogether.
The magazine's editor, Dickon Ross, told the BBC that packaging claims about the power of the bulbs did not live up to what they delivered in terms of people's perceptions of light.
He said: "It may not be deliberate, but because of the standards set, you end up with figures that are exaggerated compared to what people really experience."
Traditional bulbs lose no more than 7% of their brightness by the time they burn out - equivalent to about 2,000 hours from first use.
But the new energy-saving models, known as compact fluorescent bulbs, use up to 80% less electricity than traditional bulbs and could save up to £37 a year on energy bills.
Critics, however, claim they can trigger migraines, make skin conditions worse and lead to other health problems.
EU Countries started the mandatory phase-out of 100W and frosted incandescent light bulbs earlier this year.