If you are an average American, lighting makes up about 15 percent of your electricity bill, reports "Good Housekeeping." You’ll often hear that turning lights off when leaving a room is a good way to save energy and, thus, lower that bill. However, it’s not quite that simple. The cost-effectiveness of turning off lights actually depends on the type of lights and the price you pay for electricity, according to the U.S. Department of Energy.
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There are many reasons the type of light is important in calculating the energy savings for turning it off or leaving it on. Lights have an operating life, meaning the total number of hours they’ll provide light. This operating life is affected by how often they are turned on and off. The more frequently they get switched on and off, the lower their operating life, according to the DOE. As a general rule, the more energy-efficient a bulb is, the longer you can leave the light on before it becomes cost-effective to turn it off.
The DOE recommends turning incandescent lights off whenever they are not needed. Incandescent lighting is the most common lighting type in homes, accounting for about 85 percent of household illumination. These are the least expensive to buy, but due to their short life spans and relative inefficiency, they are more expensive to operate. Only 10 to 15 percent of the electricity that incandescent lights use results in light. The rest turns into heat. The value of the energy saved via not having such lights on is far greater than the cost of replacing these bulbs.
Fluorescent lights are more expensive than incandescent lights to buy. Their operating life also is affected more by the number of times they are switched off and on. This leads to a cost trade-off between saving energy and money via turning such lights off frequently and having to replace the bulbs more often, advises the DOE. You also need to consider the environmental impacts, including energy demand due to their production and disposal, advises "Scientific American." The easiest rule of thumb to remember is to turn fluorescent lights off if you will leave a room for more than five minutes, Francis Rubinstein, a staff scientist from Lawrence Berkeley National Laboratory's environmental energy technologies division, tells John Matson of "Scientific American." The DOE advises that in places with low electricity rates, this time frame may be extended to 15 minutes.
You can calculate the value of energy savings that you’ll achieve by turning a light off, according to the DOE. First, determine how much energy the light consumes when it’s on. Look at the watt rating printed on the light. Then find out what you pay for electricity per kWh. This will be detailed on your utility bill. It may be different during peak versus general periods. Multiply the rate per kWh by the amount of electricity you will save to gain the value of the savings. For example, if the bulb’s rating is 40 watts, and you leave the bulb on for one hour, it consumes 0.04 kWh. In contrast, if it’s off for one hour, you save 0.04 kWh. If your electric rate is 10 cents per kWh, your energy savings is 0.4 cents. The value of the savings increases with watt ratings on bulbs, advises DOE.
People frequently forget to turn lights off in high-traffic areas such as hallways, stairways or garages, notes "Good Housekeeping." Fortunately, there is a way to circumvent this forgetful behavior and get the lights switched off. Simply install motion sensors that turn lights on and off for you. They sense the motion when you enter the area and flip the light on. When motion is no longer detected, they shut the light off. You can also extend light bulb life and save energy by using dimmers.