Incandescent Lights
Fluorescent Lighting
Energy Savings
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
Lights
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
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.
Email Us for more energy efficiency information to speak with an Energy Advisor call, 1-800-562-1482.
Source:
Energy Efficiency
and Renewable Energy (EERE), Department of Energy