Incandescent bulbs have historically been marketed by wattage. Energy price increases and pending rules in the United States that will outlaw incandescent bulbs is leading the public to a changeover to compact fluorescent bulbs (CFL) and LED bulbs. But these bulbs are marketed by lumens, not watts. Lumens are a measure of perceived brightness on a finite area. Watts provided are a measure of the energy in, not the light output. So we don’t have a direct conversion.
With some assumptions, we can make some headway. We are trying to buy a light bulb after all, not a high precision laser. So what are some rules of thumb to help you buy the proper brightness bulb? I am assuming the incandescent bulb you are replacing was a standard white bulb.
|100 watt||1700 lumens|
|60 watt||850 lumens|
|40 watt||490 lumens|
So using this chart, if you want to replace a 100 watt incandescent bulb, get a replacement that outputs about 1700 lumens.
A CFL uses approximately 1/3rd to 1/5th the incoming wattage to generate the same lumens as an incandescent bulb.
An LED bulb at about 100 lumens per incoming watt uses approximately 1/5th to 1/8th the incoming power of an incandescent bulb.
At electric rates of 15 cents per kilowatt hour (kWh), we can estimate the payback period. A 100 watt incandescent bulb left on for 10 hours will have used 1 kWh and cost 15 cents. So if you are saving 2/3rd the cost with a CFL bulb, you will be saving 10 cents in 10 hours, or 1 cent an hour. So if the CFL cost an extra $1, you will need to wait 100 hours to break even.
If the CFL really lasts 10 times as long as an incandescent as the photo above claims, then you will have to buy 10 incandescent bulbs for every CFL bulb. 10,000 hours will create $100 in energy savings, and reduce the extra cost to a tenth of $1. Keep in mind however that the 10,000 hour claim is based on median lifetime, not average. So half the light bulbs will last longer, but the other half won’t last as long.