Efficacy

Efficacy is the measurement of the amount of useful light output per unit of electricity,and is measured in Lumens per Watt.  This is calculated by taking the total lumens released from the fixture by the total wattage consumed by the light fixture.   

Efficacy example:  Downlight


Lamp Lumens = 6007 Lumens

Fixture Lumens= 2066 Lumens

Fixture Wattage= 50 Watts

Fixture Efficacy= 41.32 Lumens per Watt 

Efficiency is calculated by dividing the fixture lumens by the total fixture wattage consumed.  2066 lumens/50 watts = 41.32 lumens per watt.   This is NOT good efficacy.  Most modern LED fixtures are over 100 lumens per watt efficacy with some exceeding 150 lumens per watt!

Efficiency vs. Efficacy


Efficiency

Efficiency is calculated by comparing the amount of light produced by the light source to the total amount of light coming out of the fixture. Efficiency is measured in percentage.   This is important to understand, since many types of fixtures trap the emitted light inside the fixture. Many downlights, wall packs, ceiling troffers, and other fixtures have low efficiency, requiring much more lumens from the lamp than actually makes it outside the fixture to provide useful light output.

Efficiency example:  Downlight


Lamp Lumens = 6007 Lumens

Fixture Lumens= 2066 Lumens

Fixture Efficiency= 34.4%


Efficiency is calculated by dividing the fixture lumens by the lamp lumens.  2066/6007= .344 = 34.4%. This means almost 66% of the light is trapped inside the fixture, wasting electricity!