Incandescent light bulbs comparison.
Comparison of efficacy by power (120 Volt lamps).
The kilowatt-hour is the usual unit of electrical energy purchase. The residential cost of electricity in the United States ranges from $0.07 to $0.18 per kilowatt-hour (kWh) in most states, but can be as high as $0.36 per kWh in certain areas.
The desired product of any electric lighting system is light (lumens), not power (watts). To compare incandescent lamp operating cost with other light sources, the calculation must also 
consider the lumens produced by each lamp. For commercial and industrial lighting systems the comparison must also include the required illumination level, the capital cost of the lamp, the labor cost to replace lamps, the various depreciation factors for light output as the lamp ages, effect of lamp operation on heating and air conditioning systems, as well as the energy consumption.
Overall cost of lighting must also take into account light lost within the lamp holder fixture; internal reflectors and updated design of lighting fixtures can improve the amount of usable light delivered. Since human vision adapts to a wide range of light levels, a 10% or 20% decrease in lumens still may provide acceptable illumination, especially if the changeover is accompanied by cleaning of lighting equipment or improvements in fixtures.
Comparison of efficacy by power (120 Volt lamps)
| Power (W) | Output (lm) | Efficacy (lm/W) |
| 5 | 25 | 5 |
| 15 | 110 | 7.3 |
| 25 | 200 | 8.0 |
| 35 | 350 | 10 |
| 40 | 500 | 12.5 |
| 50 | 700 | 14.0 |
| 55 | 800 | 14.5 |
| 60 | 850 | 14.2 |
| 65 | 1000 | 15.4 |
| 70 | 1100 | 15.7 |
| 75 | 1200 | 16 |
| 90 | 1450 | 16.1 |
| 95 | 1600 | 16.8 |
| 100 | 1700 | 17.0 |
| 135 | 2350 | 17.4 |
| 150 | 2850 | 19.0 |
| 200 | 3900 | 19.5 |
| 300 | 6200 | 20.7 |
Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light. Many light sources, such as the fluorescent lamp and the light-emitting diode offer higher efficiency, and some have for convenience been designed to be retrofitted in the already existing fixtures.
Luminous efficacy of a light source is a ratio of the visible light energy emitted ( the luminous flux) to the total power input to the lamp. It is measured in lumens per watt (lm/W). The maximum efficacy possible is 683 lm/W for monochromatic green light at 555 nanometres wavelength, the peak sensitivity of the human eye. (The lumen is defined in terms of the power of light as perceived by the human eye, not in terms of the absolute power output by the light.) For white light, the maximum luminous efficacy is around 240 lumens/watt. Luminous efficiency is the ratio of the luminous efficacy to this maximum possible value. It is expressed as a number between 0 and 1, or as a percentage. However, the term luminous efficiency is often used for both quantities.
A closely related idea is the luminous efficacy of radiant energy, also measured in lumens/watt. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy is a measure of how well the distribution of energy matches the perception of the eye.
Unfortunately, the spectrum emitted by a blackbody radiator does not match the sensitivity characteristics of the human eye. Tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid (below 3683 kelvins / 3410°C / 6,170°F). Donald L. Klipstein explains it this way: "An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous efficiency is 95 lumens per watt." No known material can be used as a filament at this ideal temperature, which is hotter than the sun's surface. An upper limit for incandescent lamp luminous efficacy is around 52 lumens per watt, the theoretical value emitted by tungsten at its melting point.
For a given quantity of light, an incandescent light bulb produces more heat (and consumes more 
power) than a fluorescent lamp. Incandescent lamps' heat output increases load on air conditioning in the summer, but the heat from lighting can contribute to building heating in cold weather.
High-quality halogen incandescent lamps have higher efficacy, which will allow a 60 W bulb to provide nearly as much light as a non-halogen 100 W. Also, a lower-wattage halogen lamp can be designed to produce the same amount of light as a 60 W non-halogen lamp, but with much longer life.
Alternatives to standard incandescent lamps for general lighting purposes include: * Fluorescent lamps, and Compact fluorescent lamps * High-intensity discharge lamps * LED lamps
None of these devices rely on incandescence to produce light. Instead, all these devices produce light by the transition of electrons from one energy level to another. These mechanisms produce discrete spectral lines and so are not associated with the broad "tail" of invisible infrared emissions produced by incandescent emitters, which is energy not usable for illumination. By careful selection of which electron energy level transitions are used, the spectrum emitted can be tuned to either mimic the appearance of incandescent sources or else produce different color temperatures of white for visible light.
Efforts to improve efficiency
Due to the measures noted above, there have been recent efforts to improve the efficiency of incandescents. For example the consumer lighting division of General Electric announced that they are working on a "high efficiency incandescent" (HEI) lamp, which they claim could ultimately be as much as four times more efficient than current incandescents, although their initial production goal is to be approximately two times more efficient.
The U.S. Department of Energy is also currently developing a filament lamp at Sandia National Laboratories with improved efficiency from 5% to 60%. Prompted by U.S. legislation mandating increased bulb efficiency by 2012, new "hybrid" incandescent bulbs have been introduced by Philips. The "Halogena Energy Saver" incandescent is 30 percent more efficient than traditional designs, using a special chamber to reflect formerly-wasted heat back to the filament to provide additional lighting power.
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