E NE RG Y MAT TER S
FACT SHEET 11:
REDUCING LIGHTING COSTS
The basic principles of reducing energy
costs have been covered in Fact Sheet 8:
the following applies those ideas to
lighting.
The electrical energy used in lighting
(kWh) can be reduced either by reducing
the lighting power (kW) or by reducing the
time of use (hours). Since electric use is
measured and billed by kilowatt-hours,
reducing either reduces electric costs.
Keeping the lighting system working as
well as possible also helps.
1. Operating hours
If the light isn’t needed, why is it on? Belief
in two myths relating to switching on/off
of lights prevents energy users from saving
money.
Myth 1. “Continuously operating lights is
cheaper than turning them off for brief
periods”.
This myth is based on the idea that
switching on a lamp uses a lot of power
just to get it going.
There is a small element of truth here
for fluorescent lamps, but the startup
power spike is equivalent to about 5
seconds of running time use.
So if you intend to be out of the room
for less than 5 seconds, yes, leave the light
on!
For incandescent lamps, which are so
inefficient and use no extra energy in
starting up, there is no advantage to be
gained at all.
High-intensity discharge lamps do
take quite a while to start up, so it is more
a matter of convenience (waiting in the
dark) than saving energy with them.
Because they tend to be used in large
applications, time scheduling is used, so
they switch on a little while before needed
(eg street lights coming on before sunset).
Myth 2. “Turning lights off shortens
lamp life and increases maintenance
costs”.
We need to distinguish here between
operating life and calendar life.
Operating life is the number of hours that
the lamp is on before it fails. Calendar life
is the time between lamp changes, and
includes the time the lamp is off.
There is no doubt that the operating
life of fluorescent lamps particularly, and
incandescents less so, will be reduced by
the number of switch offs. However,
running them when they are not needed
uses up that operating life for no reason.
For example, a typical fluorescent
lamp operated continuously results in an
average operating (and calendar) life of
34,000 hours (3.9 years). Turning it off
once per day for 12 hours each day
decreases the average rated lamp life to
30,000 hours, but calendar life is extended
to 6.8 years!
For incandescent lamps, frequent
switching only mildly affects lamp life.
How to ensure the light gets switched off
Having dispensed with these myths, it is
now a matter of determining how the light
goes off when not needed:
•
switching – manual or timer
•
occupancy sensors
•
scheduling controls
•
photocells
Switches can be manual – they require a
person to switch them on or off – or timed
by a mechanism within the switch itself (as
opposed to remote control by a scheduling
system – see below). The timer could be a
push button which releases itself slowly or
a dial which allows the user to set a time
that the lights stays on or a fixed time that
the light stays on when the switch is
activated.
Occupancy recognition is the
strategy applied to intermittently occupied
areas to turn lights on when people are
present and automatically turn lights off
after the room is unoccupied (eg you might
have an outside light near your garage or
backdoor which comes on automatically
when you go near it, and then goes off a
short time later). The two principal
technologies used for occupancy sensors
are passive infrared (PIR) and ultrasonic.
Scheduling is the control strategy
employed to activate, extinguish, or adjust
lighting according to a predetermined
schedule and is best suited for facilities
where certain things happen at certain
times. Time clocks are the easiest way to
implement scheduling strategies. When
timers are used to turn off lights in
occupied areas, they should have a feature
that warns occupants when lights are
going to be turned off so occupants can
extend the on time.
EXERCISE 1
What aspects should be available for
programming in an ideal light scheduling
system?
2. Lighting Power
The simplest method of reducing lighting
power is design, either before construction
or as a part of alighting audit (see next
section). Lighting using the minimum
power for safe and effective functions
within the space being lit is used. If a 40W
lamp will do, then a 60W lamp is only
wasting energy.
More sophisticated systems which
including control mechanisms to provide
continuous control over lighting power
include:
•
dimmer controls
•
daylight harvesting – using light
sensors in conjunction with dimmer
controls
•
lumen depreciation compensation –
take into account the age and
cleanliness of the light, adjusting the
output accordingly
Ballasts typically have a long life;
therefore, replacing ballasts that are still
working can be one of the most costeffective energy improvements. Older
ballasts are incompatible with some new
lamps and will result in a decrease in lamp
life of up to 50 percent.
3. Maintenance
Maintenance means replacement of faulty
or blown lamps and cleaning of the optical
systems.
EXERCISE 2
How does maintenance of lighting systems
impact on energy saving?
Photocells are light-activated switches
used to turn off lights when daylight is
adequate for safety and task performance.
They are made of cadmium-sulfide cells
that reduce sensitivity over time. A delay
feature prevents rapid cycling during
cloudy days.
Energy Matters 11. Reducing Lighting Costs
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