EU7. Energy for Lighting

cover for gss book Energy Use

Chapter 7

{ Energy Use Contents }

The continental United States viewed from space at night reveals the tremendous amount of energy we use for lighting. Where is your hometown?

View of the United  States at night
Source: U.S. Air Force satellite. Update April 2019: LightTrends, a light pollution web application ( for examining changes in nighttime light emissions (nearly) worldwide, from 1992 up until last month. Data from two satellite sensors: Operational Linescan System of the Defense Meteorological Satellite Program (DMSP; 1992-2013). and the Day/Night Band of the Visible Infrared Imaging Radiometer Suite instrument (VIIRS DNB); 2012-present).

An astronomical observatory near Tucson, Arizona, found the quality of its observing conditions seriously impaired by the street lights of a small neighboring community. 

The lights shone into the sky and illuminated dust particles in the air. Over the years, as the town added more street lights, the quality of images the astronomers obtained through their telescopes declined until they could no longer see the faintest stars against the glare of the background sky. 

Rather than giving up, the astronomers decided to find some constructive solutions to their problems. They realized the city lights could not simply be turned off—citizens required the lighting for public safety. However, the astronomers recognized that, from their point of view, there is “good light” and “bad light.” “Good light” goes downwards where people need it. “Bad light” goes upwards into the sky. They calculated nearly 50% of the light from the street lights went upwards.

The astronomers determined they could reduce the amount of light going into the sky by nearly 90% by adding an inexpensive metal reflector around each street light. They further calculated the city would need less electrical energy to provide the same amount of illumination, since most of the light generated would be pointed in a useful direction, rather than being lost to space. In fact, the city’s energy savings would, within just a few years, completely pay for the one-time cost of installing the metal reflectors on the street lights. This last point was so convincing, the city agreed to implement the astronomers’ plan. It was a “win-win” solution. The astronomers were delighted with darker skies and the city maintained safe street lighting at a lower cost.

Can you and your household “win” this way? How about your school? What choices do you make when you use electric lighting? Would you like more light shining on this page? Sometimes we are not conscious of our energy-wasting habits. Is there light shining somewhere that you are not using? What qualities would you look for if you were designing a lighting system? 

A wealth of information about issues of “good” and “bad” lighting may be found on the website of the International Dark Sky Association at


EU7.1. Investigation:
Comparing Light Sources

Build an oil-smudge photocomparator so you can compare the brightnesses of different types of lights

I. Efficiency, Costs, and Savings

Suppose you find that a 60-watt incandescent bulb is just as bright as a 15-watt compact fluorescent light. They give similar light and fit the same fixture. For energy efficiency the one with lower wattage is best. But what about the total cost of the light? A 60-watt light bulb costs about 60¢ and lasts for about 1000 hours. A 15 watt compact fluorescent bulb costs about $12 and lasts for 10,000 hours. Electricity costs, for example, about 10 cents per kilowatt-hour. On the next page are two ways to use this information to compare the costs of the lights. Try them both to see which is the better bargain.

Paybacks from Changing A Bulb

Assortment of different kinds of lights (incandescent and compact fluorescent)

Imagine yourself in the hardware store buying a bulb to replace one that burned out. You see the regular incandescent bulbs and you see the compact fluorescent bulbs. You’ll also find LED lights (light emitting diodes), though none of that type are shown in this photo.

Some cost less than others initially, but then some are more energy efficient. 

  • Which one will you choose? 
  • What difference will your choice make?

Payback To Households

Suppose you select the more expensive compact fluorescent bulb. There will come a time when the savings on the electricity bill exactly matches the extra money you paid. That is the payback time. At the payback time you have neither gained or lost, as far as money is concerned—except for one thing: The energy efficient light bulb is now paid for, and still working for you. From this point on, all energy savings are also money savings.


EU7.2. Investigation:
Which Light is Most Cost Effective?

Method 1: Life Cycle Cost

The life cycle cost of a light bulb includes the purchase price and the cost of energy use during its entire lifetime. Since a compact fluorescent bulb lasts 10 times longer than an incandescent bulb, let’s compare one compact fluorescent bulb to 10 incandescent bulbs over a 10,000 hour cycle. 

INCANDESCENT: Material cost of ten 60 W incandescent bulbs = $______________

Energy used over life cycle is 60 W x 10,000 hrs = ______________ watt-hours 

divided by 1000 watt-hours/kilowatt hour = ______________ KW-hours

Operating cost over lifecycle = ______________ KW-hours 

times $0.10/ kilowatt hours = $______________

Total Cost (material cost plus operating cost) of 

ten 60-watt incandescent bulbs = $______________

COMPACT FLUORESCENT LIGHT (CFL): Material cost of one 15-watt CFL = $______________

Energy used over life cycle is 15 W x 10,000 hrs = ______________ watt-hours

divided by 1000 watt-hours/kilowatt hour = ______________ KW hours

Operating cost over lifecycle = ______________ KW-hours

times $0.10/ kilowatt hours = $______________

Total Cost (material cost plus operating cost) of one 15-W CFL = $______________

Method 2: Payback Time

Buying the energy efficient light bulb means spending more money at the start, but it also means saving a little money every time you have the light on. A time will come when the savings are enough to pay back the extra cost of the energy efficient light. That time is called the payback time. 

To find the payback for choosing a compact fluorescent bulb over an incandescent bulb, make a table showing the total cost for each kind of bulb month by month as time goes by. The payback time comes during the month when the total cost of using an incandescent light bulb catches up to the cost of using the energy efficient compact fluorescent bulb.

At the beginning of the first month the cost is simply the price of the light bulb. For each month after that add on a month’s worth of energy costs. Assume the light is on for four hours a day. Use the wattage of the bulb and the energy cost (10¢ per kilowatt-hour) to calculate the monthly energy cost. Whenever the operating time of the bulb reaches the bulb’s lifetime, add in the cost of a new light bulb. Make a graph of cost versus time. Plot the data for both light bulbs on the same graph. Where the two lines cross is the payback time.

Payback To the Utility Company

Your electric utility company provides electricity to your home as long as someone pays the bills. The more energy people use, the more money the electric company makes—right? Not necessarily! What people really want and need are the services this energy provides. Light, heat, entertainment, and communication are a few of the services people expect to receive from their electricity. 

Power plant parts

With improved energy efficiency, energy utility companies can charge more per unit of energy while consumers pay less for their light, heat, and the rest. That is one reason why electric companies encourage energy savings. They have even offered money-saving deals on compact fluorescent lights. 

There is another important way electric companies benefit from energy saving efforts. The demand for energy services increases as the population and the economy grows. There are two ways to meet the growing demand. The power company can spend hundreds of millions of dollars to build new power plants, or, by promoting energy conservation, it can provide more services with the energy from the power plants it already has. 

Payback to the Country

US map

According to a 1990 study by the Lawrence Berkeley National Laboratory, incandescent light bulbs in the United States use about 200 billion kilowatt-hours of energy each year. If half of the incandescent bulbs were replaced with compact fluorescent bulbs that gave the same amount of light, the savings would be about 75 billion kilowatt-hours, since most compact fluorescent lights are about four times as efficient as incandescent lights. At 10 cents per kilowatt-hour, this would save people in the United States eight billion dollars on energy bills!

Eighty billion kilowatt-hours of energy per year is the output of 16 large power plants, which could be retired and not replaced. Since the cost of a large power plant is approximately $1.5 billion, the overall savings to the power industry (and ultimately the American consumer) would be $24 billion. 

Payback to the Planet

Earthrise photo from the Moon

Over its lifetime, a single compact fluorescent light bulb saves in electrical energy the equivalent of 450 pounds of coal, or 40 gallons of gasoline. Ten compact fluorescent light bulbs operating continuously, 24 hours per day, save an amount of energy equivalent to the amount of gasoline needed to run a new car for a year.

By saving that energy, not only would it save money for consumers, it would also:

  • Reduce damage to the planet from mining and drilling.
  • Reduce the amount of fuel transported, thus reducing the number of oil spills.
  • Reduce air and water pollution.
  • Reduce the amount of carbon dioxide injected into the atmosphere, thus reducing the risk of global warming.

II. Conclusion

It’s not just light bulbs. There are other ways to reduce the electricity bill in your home or school or community. Just as there are paybacks for saving electricity, there are paybacks for saving every other form of energy. The next chapter deals with a form of energy that is vital for human survival and comfort. It is also a form of energy involved in almost every energy transformation—heat. 

See Stay current for this chapter.