In my last column I
reviewed some of
the details of lighting
with the intent of
applying these details
to the replacement of
traditional tungsten filament lamps with
fixtures containing solid-state lighting engines.
I suggested that filament lamps in a typical
broadcast studio are never operated
at rated voltage and, as a result, would exhibit
a much longer lamp life than is usually
applied in payback studies.
I would like to conclude this discussion
by describing my approach to this
The method is quite simple. We will
compare the yearly operating cost of two
equivalent fresnel-lensed instruments: A
traditional unit with a tungsten filament
lamp and one with an LED source.
I will assume we know the replacement
LED instrument will be much less
expensive to operate and that we can
easily calculate the operating savings—
the difference in operating expenses between
the two instruments.
To offset this gain we will have the
initial cost of the new instrument. The
Payback Period (in years) is this initial
cost divided by the yearly savings due to
be realized because of the more efficient
performance of the new LED instrument.
As we are retrofitting, we assume that
the tungsten filament instrument is already
part of the studio’s lighting complement.
The yearly studio usage was arbitrarily
set at a reasonable 2,912 hours.
FINDING A REPLACEMENT
To facilitate the review of the elements
of the method, the pertinent information
and calculations are outlined in the table
in Fig. 1. I selected a current well-designed
LED fresnel fixture that is comparable to
the popular 650-watt tungsten fresnel
You will note that the candlepower of
the two instruments (line 4) is the same
order of magnitude. The candlepower of
the tungsten unit is still a little greater
than the LED unit even when operated at
the suggested under-voltage value of 108
volts. This operating voltage was suggested
last month as a good guess as to the
average dimmer setting in a studio (about
8.5 points), a very conservative guess so
as not to exaggerate the expected life.
The effect of operating at this voltage
is reflected in the adjusted values of input
power (line 6) and adjusted lamp life
LAMP REPLACEMENT COST
The current purchase cost of the tungsten
lamp has been entered in the table
on line 9. Using the assumed studio yearly
hours, two simple calculations are made:
one determining the number of lamps to
be used (line 10) and the second, calculating
the total yearly lamp replacement
cost (line 11). No replacement lamp cost
is in the LED column as the estimated
instrument life of the LED fixture is so
great—50,000 hours or 17 years at the assumed
As it is expected that the Payback Period
will be appreciably shorter than this lamp life,
LED lamp replacement will not be a factor.
|Fig. 1: A comparison for replacing a tungsten fresnel with an LED fresnel
There are two significant items relative to
a lighting fixture’s energy requirement. One
is the direct power demand of the unit itself—
expressed in watts—that the electrical
power source will supply for the instrument’s
operation. Line 6 shows these values for each
As the lighting output of each unit is approximately
equal, this shows the higher
efficacy of the LED unit. The yearly energy
amount is then calculated by multiplying the
operating wattage by the total yearly hours
and is expressed in Kilowatt-Hours (kW-h).
Line 12 displays this result.
In addition, we must consider the cost of
removal of the heat generated by our lighting
instruments. Calculation of this cost is slightly
more complicated than the calculation of direct
Line 13 shows estimates of what percentage
of the input power is produced as heat
for each unit. The LED does produce less heat
than the tungsten filament. Continuing, we
produce the yearly energy heat load utilizing the
yearly hour total (line 14).
Line 16 reflects the effectiveness of the air-conditioning
system in removal of heat by applying
something called the Coefficient of Performance
(line 15). This educated estimate of 2.0 means that
for every 1 kW-h supplied to the HVAC system, 2.0
kW-h of heat will be removed. It should be noted
that, for simplicity, no reduction has been applied
in attempting to accommodate for the number of
regional cooling days.
METER READING AND TOTALS
Eleven-and-a-half cents per KW-h has been assumed
to be the electrical rate. Adding the two
power requirements determined above and adding
the lamp cost per year (yes, $0.00 for the LED)
and multiplying by this rate, gives the total electrical
cost (line 20).
As anticipated, the operating cost of the tungsten
lamp is appreciably more than the LED fixture.
As was indicated in the first article, the labor
cost of lamp replacement is not considered a significant
issue and, as a result, has not been added
to the operating cost of the tungsten instrument.
THE BOTTOM LINE
The Yearly Savings, the difference between
each fixture type, is noted on line 21. The Payback
Period is then calculated by dividing the purchase
price of the LED fixture by the Yearly Savings. This
is the time that the savings would need to accumulate
to equal the cost of the retrofit.
The Payback Period is estimated to be 11.9
years. If the adjustments for lower-voltage operation
of the tungsten unit were not applied, the
Payback Period would be 6.2 years. The interpretation
of these results is left to the reader.
Bill Klages would like to extend an invitation
to all the lighting people out there to give him
your thoughts at firstname.lastname@example.org