Cost of lighting
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. The initial cost of an incandescent bulb is small compared to the cost of the energy it will use.
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.