Stop by a lighting display store and ask to see a lamp catalog from one of the major manufacturers. There are hundreds of lamps available, and many high-efficiency lines are on the market, too. A close inspection shows a wide variety of lamps that have nearly the same light output, measured in lumens. Replacing an existing lamp with one having a similar lumen rating does not affect the light level in the space.
Industrial consumers of electricity are usually billed a demand rate for any electricity used during the peak hours of the week, i.e., during regular business hours. This is a monthly charge that’s on the order of $10/kWh used during periods of peak energy use. If the fluorescent lamp in the example is used during this period, there will be more energy to be saved by replacement with a more efficient lamp.
If the retrofit is carried out as a maintenance item—changing out the lamps as existing lamps burn out—the effective cost is negligible. (If this method is done, though, be sure the replacement lamp has the same color characteristics as the existing lamps, so the new ones do not stand out badly.)
If the retrofit is done as a single short-term project, the savings are immediate, often with a payback of a year or less. After that time, the money saved can be rolled back into the energy-conservation program, to help pay for some projects that are more capital-intensive and with a longer payback.
Simple payback can be defined as
Payback = cost of project/annual savings in utility bills
This is the determining factor in carrying out an energy-conservation project. Conservative administrators try to limit the payback to 5 years or less, with the possible exception of major equipment, with an expected usable lifetime of over 20 years. A longer payback of 8 to 10 years may be accepted for such long-term projects.
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Excerpt from Energy Conservation Projects.