When to Turn Off Your
Lights The cost effectiveness of when to turn off
lights depends on the type of lights and the price
of electricity. The type of light is important for
several reasons. All types of lights have a nominal
or rated operating life, which is the total number
of hours that they will provide a specified level or
amount of light. However, the operating life of all
types of light bulbs is affected by how many times
they are turned on and off. The more often they are
switched on and off, the lower their operating life.
The exact number of hours that switching lights on
and off reduces the total operating life depends on
the type of light and how many times it is switched
on and off.
Incandescent Lighting
Incandescent lights (or bulbs) should be turned off
whenever they are not needed. Nearly all types of
incandescent light bulbs are fairly inexpensive to
produce and are relatively inefficient. Only about
10%–15% of the electricity that incandescent lights
consume results in light—the rest is turned into
heat. Turning the light(s) off will keep a room
cooler, an extra benefit in the summer. Therefore,
the value of the energy saved by not having the
lights on will be far greater than the cost of
having to replace the bulb.
Fluorescent Lighting
The cost effectiveness of turning fluorescent lights
off to conserve energy is a bit more complicated.
For most areas of the United States, a general
rule-of-thumb for when to turn off a fluorescent
light is if you leave a room for more than 15
minutes, it is probably more cost effective to turn
the light off. Or in other words, if you leave the
room for only up to 15 minutes, it will generally be
more cost effective to leave the light(s) on. In
areas where electric rates are high and/or during
peak demand periods, this period may be as low as 5
minutes.
Fluorescent lights are more expensive to buy, and
their operating life is more affected by the number
of times they are switched on and off, relative to
incandescent lights. Therefore, it is a cost
trade-off between saving energy and money by turning
a light off "frequently" and having to replace the
bulbs "more" frequently. This is because the
reduction in usable lamp life due to frequent on/off
switching will probably be greater than the benefit
of extending the useful life of the bulb from
reduced use. By frequent we mean turning the light
off and on many times during the day.
It is a popularly held belief that fluorescent
lights use a "lot" of energy to get started, and
thus it is better not to turn them off for "short"
periods. There is an increase in power demand when a
light is switched on, and the exact amount of this
increase depends on the type of ballast and lamp.
The ballast provides an initial high voltage for
starting the lamp and regulates the lamp current
during operation. There are three basic types of
ballasts: magnetic (of which there are
energy-efficient and not so energy-efficient types),
cathode-disconnect, and electronic. All types can
operate two or more lamps simultaneously. There are
three main methods that are used in a lamp's ballast
to start the lamp: preheat, rapid-start, and
instant-start.
In any case, the relatively higher "inrush" current
required lasts for half a cycle, or 1/120th of a
second. The amount of electricity consumed to supply
the inrush current is equal to a few seconds or less
of normal light operation. Turning off fluorescent
lights for more than 5 seconds will save more energy
than will be consumed in turning them back on again.
Therefore, the real issue is the value of the
electricity saved by turning the light off relative
to the cost of relamping a fixture. This in turn
determines the shortest cost-effective period for
turning off a fluorescent light.
The value of the energy saved by turning a
fluorescent light (or array of lights) off depends
on several factors. The price an electric utility
charges its customers depends on the customer
"classes," which are typically residential,
commercial, and industrial. There can be different
rate schedules within each class. Some utilities may
charge different rates for electricity consumption
during different times of the day. It generally
costs more for utilities to generate power during
certain periods of high demand or consumption,
called peaks. Some utilities can charge commercial
and industrial customers more per kilowatt-hour
(kWh) during peak periods than for consumption
off-peak. Some utilities may also charge a base rate
for a certain level of consumption and higher rates
for increasing blocks of consumption. Often a
utility adds miscellaneous service charges, a base
charge, and/or taxes per billing period that could
be averaged per kWh consumed, if these are not
already factored into the rate.
Energy Savings
To calculate the exact value of energy savings by
turning a light off, you need to first determine how
much energy the light(s) consume when on. Every bulb
has a Watt rating printed on it. For example, if the
rating is 40 watts, and the bulb is on for one hour,
it will consume 0.04 kWh, or if it is off for one
hour, you will be saving 0.04 kWh. (Note that many
fluorescent fixtures have two or more bulbs. Also,
one switch may control several fixtures—an "array."
Add the savings for each fixture to determine the
total energy savings.)
Then you need to find out what you are paying for
electricity per kWh (in general and during peak
periods). You will need to look over your
electricity bills and see what the utility charges
per kWh. Multiply the rate per kWh by the amount of
electricity saved, and this will give you the value
of the savings. Continuing with the example above,
let us say that your electric rate is 10 cents per
kWh. The value of the energy savings would then be
0.4 cents ($ 0.004). The value of the savings will
increase the higher the watt rating of the bulb, the
greater the number of bulbs controlled by a single
switch, and the higher the rate per kWh.
The most cost-effective length of time that a light
(or array of lights) can be turned off before the
value of the savings exceeds the cost of having to
replace bulbs (due to their shortened operating
life) will depend on the type and model of bulb and
ballast. The cost of replacing a bulb (or ballast)
depends on the cost of the bulb and the cost of
labor to do it.
Lighting manufacturers should be able to supply
information on the duty cycle of their products. In
general, the more energy-efficient a bulb/light is,
the longer you can keep a light on before it is cost
effective to turn it off.
