How to Calculate Efficacy

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In science, efficacy is a quantity of measurement for light sources and is described as the ratio of luminous flux to power (lm/W). This is important since it essentially tells us how much light is being given compared to the amount of electricity used. Why is this useful? The typical household spends 30% of their electricity bill on lighting! You can save a lot of money by using the most cost efficient lighting option in your home. Use the formula below to calculate the efficacy of various light sources and compare them. The higher the efficacy, the more money you'll save.

    Make a note about the formula you will use. The equation will be efficacy = luminous flux / power. The SI unit for the result will be lm/W.

    Gather the information about the wattage and luminous flux for the light source you wish to analyze. Most bulbs will have this information on the box it came in or on the bulb itself.

    Input the gathered data into the formula. For example, a 50-watt light bulb usually produces 700 lumens. So, efficacy = 700 lumens / 50 watts

    Complete the calculation. The efficacy of the 50-watt light bulb is 14 lm/W.

    You can repeat this calculation with any light source as long as you have the data pertaining to its power and luminous flux.

    Things You'll Need

    • Calculator
    • Data from a light source
    • Paper and pencil (optional)


    • Efficacy can be useful when you are shopping for light bulbs. Most boxes will have a bulb's wattage and lumens produced. Use the above calculation method to see how cost efficient a bulb will be for your home.