One of the fundamental specifications to consider when evaluating any lighting system is the
power consumption, as it forms a basis of its economic viability. Since there will generally be
some form of power converter (sometimes known as a ballast) between the mains input and
the lighting fixture, the end user must be clear in specifying that wattage will be measured at
the wallplug and not at the fixture. Furthermore, there may be local regulations regarding the
luminous efficacy of the installed system that stipulates a minimum lumens per watt value. In this
case, the end user must be careful in specifying the lumen output of each fixture to confirm that
the standard is met. Simply accepting the figure advertised by the lighting fixture manufacturer
on a datasheet is not advisable and it has been shown that manufacturers commonly exaggerate
the lumen output2. This misrepresentation is not always wilful but can arise through ignorance
of the technology. Typically an errant manufacturer will simply take the LED manufacturer’s
data for a single LED and multiply it by the total number of LEDs to arrive at the lumen output
figure, failing to take into account losses due to lenses, covers, reflectors, side walls, etc. These
can easily cause a 30 to 40% loss in lumen output. Luminous efficacy is dependent on lumen
maintenance, so as the output degrades over time so does the efficacy.
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