FAQ - How do we calculate LED efficiency?

One of the light characteristics we measure is efficiency. It is calculated as the ratio of total luminous flux (lumens) to total power consumed by the source (watts), and thus a measure of how efficiently a light source converts electrical power into visible light. It is often referred to by manufacturers as a source's luminous efficacy however, as it is still a measure of efficiency, for the sake of the layman we have chosen to use the more familiar concept of efficiency to describe it.

Efficacy: The Relationship between LED Light Output and Wattage

In contrast to incandescent and halogen lighting, the expected light output of an LED light cannot be directly gauged by it's claimed wattage. In general, LED efficiency and efficacy (lumen output per consumed Watt), far exceeds that of incandescents and halogens. The most lumens per Watt observed in our product testing was achieved by Philips' 10W A19 LPRIZE-PRO, with an efficacy of 96 lumens/Watt, compared to a typical efficacy of 15Lm/W for incandescent/halogens and 70Lm/W for CFLs. The potential efficacy of LED light sources are not, however, utilised consistently across all products as many factors come into play in the conversion of electrical power to light output. As the efficacy of LED products can vary widely, the wattage alone, nor even the wattages of the incandescents or halogens a light claims equivalency to, is enough to know how an LED product will perform, particularly when it comes to light output.

While an SSL product's wattage can serve as a preliminary indication of how much light and heat it will produce; the ratio of useful light to waste energy depends entirely on the efficacy of the system. A 10W light may not necessarily produce more light than a 5W. This becomes apparent upon comparing various lights tested in our database. OSRAM's Parathom Pro MR16 35 Advanced paired with OSRAM's Redback electronic transformer produced an output of 295 lumens with a total wattage of 11.7W, yielding an efficacy of 25Lm/W. In contrast, iGlobe's 5W GU10 CW Downlight produced an output of 357 lumens for a total wattage of just 4.3W, resulting in a much higher efficacy of 83 Lm/W.