Consultancy is not about knowing all the answers; rather it is about
asking the right questions.
DETERMINING THE NEED
Before any step is taken to work towards conservation of lighting energy, it
should first be ascertained whether such an activity should at all be
undertaken.
The following questionnaire should help in determining the need for
lighting audit.
a) Has the function of buildings remained the same since they were
constructed?
b) Have you developed a lighting schedule for indoor and outdoor lighting?
c) Have you converted from incandescent lighting to fluorescent?
d) Have you converted from standard fluorescent to more energy efficient
compact fluorescent lamps?
e) Have you converted from conventional magnetic to energy efficient
electronic ballasts?
f) Do you use metal halide, high pressure sodium, or low pressure sodium
lamps for outdoor lighting?
g) Do you have controls in your lighting system to provide increased flexibility
and energy savings?
h) Has the lighting system been modified to accommodate rearrangement of
desks and workstations?
i) Have employees been exposed to awareness programmes regarding energy
conservation in lighting?
j) Was the building lighting system designed with energy efficiency in mind?
k) Have you investigated rebate programs, if any offered by your local utility or
the government?
IF ANSWER TO ANY OF THE ABOVE QUESTIONS IS A "NO",
THEN YOUR ESTABLISHMENT NEEDS A LIGHTING AUDIT.
Procedure for Assessment of Lighting Systems
To Determine the Minimum Number and Positions of Measurement Points:
LxW
Calculate the Room Index: RI=
Hm( L+W )
Where L = length of interior; W = width of interior; Hm = the mounting height, which is the
height of the lighting fittings above the horizontal working plane. The working plane is usually
assumed to be 0.75m above the floor in offices and at 0.85m above floor level in manufacturing
areas.
It does not matter whether these dimensions are in metres, yards or feet as long as the same unit is
used throughout. Ascertain the minimum number of measurement points from Table10.1.
Table 10.1
Determination of Measurement
Points
To obtain an approximately “square array”, i.e. the spacing between the points on each axis to
be approximately the same, it may be necessary to increase the number of points.
For example, the dimensions of an interior are:
Length = 9m, Width = 5m, Height of luminaires above working plane (Hm) = 2m
9 ×5
Calculate RI= =1.607
2×(9+5)
From Table 10.1 the minimum number of measurement points is 16
As it is not possible to approximate a “square array” of 16 points within such a rectangle it is
necessary to increase the number of points to say 18, i.e. 6 x 3. These should be spaced as shown
below:
Therefore in this example the spacing between points along rows along the length of the interior
= 9 ÷ 6 = 1.5m and the distance of the 'end' points from the wall = 1.5 ÷ 2 = 0.75m.
Similarly the distance between points across the width of the interior = 5 ÷ 3 = 1.67m with half
this value, 0.83m, between the 'end' points and the walls.
If the grid of the measurement points coincides with that of the lighting fittings, large errors are
possible and the number of measurement points should be increased to avoid such an
occurrence.
Calculation of the Installed Load Efficacy and Installed Load Efficacy Ratio of a General
Lighting Installation in an Interior
STEP 1 Measure the floor area of the Area = -------------------- m²
interior:
STEP 2 Calculate the Room Index RI = --------------------
STEP 3 Determine the total circuit Total circuit watts =
watts of the installation by a -------------
power meter if a separate
feeder for lighting is
available. If the actual value is
not known a reasonable
approximation can be
obtained by totaling up the
lamp wattages including the
ballasts:
STEP 4 Calculate Watts per square W/m² = ------------
metre, Value of step 3 ÷ value
of step 1
STEP 5 Ascertain the average Eav.maint. = ------------
maintained illuminance by
using lux meter, Eav.
Maintained
STEP 6 Divide 5 by 4 to calculate lux Lux/W/m² = ------------
per watt per square
Metre
STEP 7 Obtain target Lux/W/m² lux Target Lux/W/m² =
for type of the type of
interior/application and RI
(2):
STEP 8 Calculate Installed Load ILER =
Efficacy Ratio ( 6 ÷ 7 ).
Table 10.2 Target lux/W/m² (W/m²/100lux) values for maintained illuminance on horizontal
plane for all room indices and applications:
Ra : Colour rendering index
The principal difference between the targets for Commercial and Industrial Ra: 40-85 (Cols.2 &
3) of Table 10.2 is the provision for a slightly lower maintenance factor for the latter. The targets
for very clean industrial applications, with Ra: of 40 -85, are as column 2.
ILER Assessment
Compare the calculated ILER with the information in Table 10.3.
Table 10.3 Indicators of Performance
ILER Ratios of 0.75 or more may be considered to be satisfactory. Existing installations with
ratios of 0.51 - 0.74 certainly merit investigation to see if improvements are possible. Of course
there can be good reasons for a low ratio, such as having to use lower efficacy lamps or less
efficient luminaires in order to achieve the required lighting result –but it is essential to check
whether there is a scope for a more efficient alternative. Existing installations with an ILER of 0.5
or less certainly justify close inspection to identify options for converting the installation to use
more efficient lighting equipment.
Having derived the ILER for an existing lighting installation, then the difference between the
actual ILER and the best possible (1.0) can be used to estimate the energy wastage. For a given
installation:
Annual energy wastage (in kWh)
= (1.0 - ILER) x Total load (kW) x annual operating hours (h)
This process of comparing the installed load efficacy (ILE) with the target value for the Room
Index and type of application can also be used to assess the efficiency of designs for new or
replacement general lighting installations. If, when doing so, the calculated ILE (lux/W/m²) is less
than the target value then it is advisable to ascertain the reasons. It may be that the
requirements dictate a type of luminaire that is not as efficient as the best, or the surface
reflectance are less than the normal maxima, or the environment is dirty, etc., whatever the
reasons, they should be checked to see if a more efficient solution is possible
Example of ILER Calculation
STEP 1 Measure the floor area of Area = 45 m²
the interior:
STEP 2 Calculate the Room Index RI = 1.93
STEP 3 Determine the total circuit Total circuit watts = 990 W
watts of the installation by
a power meter if a separate
feeder for lighting is
available. If the actual
value is not known a
reasonable approximation
can be obtained by totaling
up the lamp wattages
including the ballasts:
STEP 4 Calculate Watts per square W/m² = 22
metre, 3 ÷1 :
STEP 5 Ascertain the average Eav.maint. = 700
maintained illuminance,
Eav. Maintained (average
lux levels measured at 18
points)
STEP 6 Divide 5 by 4 to calculate Lux/W/m² = 31.8
the actual lux per watt per
square Metre
STEP 7 Obtain target Lux/W/m² Target Lux/W/m² = 46
lux for type of the type of
interior/application and RI
(2):(Refer Table 10.2)
STEP 8 Calculate Installed Load ILER = 0.7
Efficacy Ratio ( 6 ÷ 7 ).
Referring to table 3, ILER of 0.7 means that there is scope for review of the lighting system.
Annual energy wastage
= (1 – ILER) x watts x no. of operating hours
= (1 – 0.7) x 990 x 8 hrs/day x 300 days
=712 kWh/annum
