Energy-saving light bulbs lose as much as 22 percent of their brightness over their lifetime, according to a report from the Institution of Engineering and Technology.
The Institution surveyed evidence including tests undertaken by the German consumer organisation Warentest for the European Commission, and official US Department of Energy tests.
Warentest examined just 18 compact fluorescent bulbs (CFBs), but found that after 10,000 hours - the advertised lifetime of the bulbs - three had stopped working and the rest showed an average 22 percent reduction in brightness.
In the US tests, 28 percent of the bulbs had stopped working after just 2,400 hours.
It's a pretty poor performance compared with traditional incandescent bulbs, which lose no more than seven percent of their brightness by the time they stop working, after about 2,000 hours.
The authors of the report reckon that many manufacturers exaggerate the brightness of their CFLs, claiming that 11-14W CFLs are the equivalent to a 60W traditional bulb. But the EC says a 15W CFL is required to match the output of a 60W incandescent bulb.