Estimate wattage from a lumen output and the source efficacy.
Typical LED efficacy is 80–120 lm/W. Worked example: 800 lm ÷ 100 = 8 W.
Lumens measure how much light a lamp produces, and watts measure the power it uses. To find the power needed for a given amount of light, divide the lumens by the efficacy in lumens per watt. An 800 lumen lamp at 100 lumens per watt needs about 8 watts, which replaces a 60 watt incandescent bulb.
Common equivalents help when you shop. About 450 lumens is the light of a 40 watt incandescent bulb, 800 lumens a 60 watt, 1,100 lumens a 75 watt, and 1,600 lumens a 100 watt. An LED that produces the same lumens saves most of the power.
The more efficient the lamp, the fewer watts it needs for the same light. Efficacy depends on the product, and warmer colors and high color rendering lamps are sometimes slightly less efficient. Use this estimate for planning and rely on the label of the product you buy for the real wattage and lumen output.
Handy conversions at 100 lumens per watt: 400 lumens needs about 4 watts, 1,000 lumens needs about 10 watts, 2,000 lumens needs about 20 watts, and 5,000 lumens needs about 50 watts. At a lower efficacy of 70 lumens per watt, the same amounts need about 43 percent more power.