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Lighting and Electricity Concepts

Wattage, Lumens and Efficacy

Wattage is a measurement of the electrical energy used by an electrical device, such as a light bulb, but it is not a measurement of the amount of light being produced. The measurement of light output from a lamp is the lumen. All light bulbs have a lumen rating, and it is the relationship between the lumens being produced and the wattage being consumed that can provide us valuable information about the energy-efficiency of a light bulb.


The relationship between lumens and wattage is called efficacy. This is the ratio of light output from a lamp to the electric power it consumes and is measured in lumens per watt (lpw).

Incandescent lamps have an efficacy range from 15 lpw to slightly over 20 lpw. Fluorescent and LED lamps have an efficacy range from 60 lpw to almost 100 lpw. As you can see from these numbers, the choice of light bulb can have a significant impact on electrical energy consumption.