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The Economics of Lamp Switching, Guides to Energy Management

Should you let it Burn?

This article was posted on the University of Central Florida’s Website and is probably one of the best articles we’ve seen that best explains a very common question that many people have today in regard to energy conservation.

TURN OFF LIGHTING TO MAXIMIZE SAVINGS

Is it more economical to keep lights on continuously or to turn them off when not in use? Our experience indicates there are still people that aren't sure of the answer. Many past studies indicate to always shut off lighting when it is not being used. This article reinforces this conclusion to use lighting only when needed.

BACKGROUND

In the past, operating lighting systems continuously was justified because the cost of electricity used was less than the cost of increased lamp replacement caused by turning lamps off during breaks, lunches, or when people left the spaces.  In addition, lamps needed an increased surge of electricity to warm up the elements and the chemicals that allowed the lamp to light.  Electricity was much cheaper then and ballast and lamp technology have far surpassed that of even five years ago.

ENERGY EFFECT OF SWITCHING LAMPS ON

The fluorescent lamp is designed to burn for thousands of hours. However, its life is reduced when operated for short periods. When a lamp is switched on, the initial in-rush current flows for 1/120 of a second or half a cycle with a peak value of about five times the steady-state maximum. Since this in-rush current is of such short duration, it does not constitute a significant amount of energy. Therefore, fluorescent lamps need to be turned off for only one second in order to save the amount of energy expended when the lights are initially turned on.

LIFE RATING OF FLUORESCENT LAMPS

The life of a fluorescent lamp is dependent upon the filament's electron-emissive coating; which slowly evaporates during operation. The rate of coating erosion varies with gas fill, gas pressure, and cathode temperature. Also, every time a lamp is started, a quantity of coating is dislodged from the cathode resulting in shorter life. Manufacturers publish average life rating figures (where 50 percent of a given number of lamps failed under test conditions). These figures reflect the effects of both starting and on time. Any change in time on per cycle will affect lamp life Table 1 provides the expected life in hours for various operating cycles.

BURNING CYCLE CHARACTERISTICS

Lamp life is reduced with on/off cycling. However, on/off cycling lessens operating time. Thus, calendar lamp life is increased, and both replacement and electricity costs are reduced. For example, consider the calendar life and burning cycle for a single, 40-watt fluorescent tube found in a storage room This lamp operating for an average of 8 hours per day will last 9 1/2 years, with 27,740 hours of actual burn time. If that same lamp is switched off to operate only 4 hours per day, it will last 14 years, with 20,440 hours of actual burn time.

TYPICAL ENERGY SAVINGS

Research on a 100 lamp system, using 40-watt rapid start fluorescent lamps; shows that a typical daily energy savings of nearly 17% can be achieved by turning off a during break and lunch periods.  With increasing electricity, material, and labor costs, we recommend turning off incandescent and fluorescent lights regardless of the off time duration. The energy saved and the extended calendar life of the lamp will offset the reduction in lamp life caused by frequent switching.

NOTE: If you presently use standard fluorescents, you can realize extra savings by switching to energy saving fluorescents, which reduce power consumption while delivering a minimal reduction in light levels.

INCANDESCENT LAMPS

Since short operating periods have little effect on the life of incandescent lamps, they should always be turned off when not required.

HIGH INTENSITY DISCHARGE (HID) LAMPS

HID lamps, such as mercury vapor, high and low pressure sodium, and metal halide, require several minutes to warm up. In addition, when they are turned off, they need several minutes to cool off before the ballast will restart them. HID lamps should not be turned off unless the shut-off period is longer than 15 minutes. Ballast manufacturers are introducing solid state devices with instant restart capability. This allows HID lamps to be switched back on within a short period without an initial warm-up period. Contact your local HID lamp supplier for further details

BENEFITS OF FLEXIBLE SWITCHING

Often too many lights are connected to one circuit. This limits lighting control and results in energy waste. Multiple or flexible switching circuits will provide adequate light for safety and the capability of switching off lights in unused areas. Use labels beside appropriate switches as a reminder to turn off lights. When a number of switches are involved, consider color coding them. Occupancy sensors and time-switch controls are readily available for automatic switching of lighting systems. These energy savers eliminate the problem of occupants forgetting to turn lights off when not required. Although data for extremely short burn times is limited, manufacturers recommended that occupancy sensors be set for lighting to be on for at least 6 minutes when triggered to maximize total savings.

This article was adapted from Guides to Energy Management, "The Economics of Lamp Switching", produced by British Columbia Hydroelectric. For more information, contact NFESC Code 20, at DSN 551-1465 or commercial (805) 982-1465.