The Power of Bulbs
Lamp or bulb power is measured in watts. One thousand watts equals one kilowatt, and a kilowatt-hour is equal to 1000 watts used for 1 hour. Bulbs operate within a set range of watts. The maximum wattage defines the bulb: 40, 60, 75, or 100 watt for typical incandescents.
Fluorescent lights are also measured in watts, but they're so energy efficient that you'll get much better light output and longer lamp life, and still use less wattage. For example, a common 75-watt incandescent bulb has an average lamp life of 750 hours, compared to between 5,600 and 11,100 for halogens and 7,500 to 20,000 for fluorescents. Be careful with halogens, however: They must be used correctly or can be a fire hazard.
The light output or amount of light produced by a bulb is measured in lumens. “Efficacy,” a lighting term for optimal bulb efficiency, is measured by comparing the amount of output in lumens with the number of watts used to get there. It is called the lumen-per-watt ratio and is a very important factor in judging and selecting bulbs for your fixtures. The higher the ratio, the more efficacious the bulb. For example, a 50-watt floodlight will put out 410 to 420 lumens per bulb and a common 40-watt incandescent frosted bulb will put out 460 to 505 lumens. A 40-watt fluorescent, however, can put out as much as 2,600 lumens, making it by far and away the most efficacious.
“Efficiency” is the lighting term for rating fixtures. Efficiency is what you need to look for because it explains what percentage of the bulbs' output a fixture actually uses. So, if a fixture has one fluorescent lamp or bulb that emits 1,000 lumens, but the fixture only emits 900 lumens, then it will have an efficiency rating of 90 percent. The idea is to choose fixtures with the highest efficiency ratings. Fixture manufacturers publish this information in their specifications. Before purchasing a fixture, ask your local lighting retailer to show you its specifications for.
Soon information on energy efficiency and savings will be published right on the bulb or fixture label. For now, you'll need to do your research.
Making the Best of Light and Energy
When you hear about the efficiency of a fixture, it refers to the percentage of light that actually leaves through the bulbs. In an inefficient fixture, energy is wasted because the light is absorbed by the fixture itself or emitted into the ceiling cavity. It's critical that you choose a lamp that is compatible with your fixture and that you direct the light correctly, to maximize the energy efficiency of the fixture. In addition, dirty, dusty, insect-laden globes, diffusers, reflectors, or shades have an enormous effect on fixtures' light production and efficiency is enormous. So, if you're aiming for efficiency, get up there and clean them off!