Energy Star products in 2003–04, one quarter of tested CFLs no longer met their rated output after 40% of their rated service life. Energy efficiency Because the eye's sensitivity changes with the wavelength, the output of lamps is commonly measured in lumens, a measure of the power of light as perceived by the human eye. The luminous efficacy of lamps is the number of lumens emitted for each watt of electric power used. The luminous efficacy of a typical CFL is 50–70 lumens per watt (lm/W) and that of a typical incandescent lamp is . Compared to a theoretical 100%-efficient lamp (), CFL lamps have lighting efficiency ranges of 7–10%, versus 1.5–2.5% for incandescents. Because of their higher efficacy, CFLs use between one-seventh and one-third of the power of equivalent incandescent lamps. Of 2010 world total lighting sales, 50 to 70 percent were incandescent. Replacing all inefficient lighting with CFLs would save per year, 2.5% of world electricity use. In the US, it is estimated that replacing all incandescents would save 80 TWh yearly. Since CFLs use much less energy than incandescent lamps (ILs), a phase-out of ILs would result in less carbon dioxide () being emitted into the atmosphere. Exchanging ILs for efficient CFLs on a global scale would achieve annual reductions of 230 Mt (million tons), more than the combined yearly emissions of the Netherlands and Portugal. If a building's indoor incandescent lamps are replaced by CFLs, the heat emitted due to lighting is significantly reduced. In warm climates, or in office or industrial buildings where air conditioning is often required, CFLs reduce the load on the cooling system when compared to the use of incandescent lamps, resulting in savings in electricity in addition to the energy efficiency savings of the lamps. However, in cooler climates in which buildings require heating, the heating system must replace the reduced heat from lighting fixtures. In Winnipeg, Canada, it was estimated that CFLs would only generate 17% savings in energy compared to incandescent bulbs, as opposed to the 75% savings that could have been expected without space heating considerations. Cost While the purchase price of a CFL is typically 3–10 times greater than that of an equivalent incandescent lamp, a CFL lasts 8–15 times longer and uses two-thirds to three-quarters less energy. A US article stated "A household that invested $90 in changing 30 fixtures to CFLs would save $440 to