Watts do matter when it comes to paying your electric bill, of course. That’s because a watt is a unit of power. The number of watts you use in a month are added up, and that’s what you’re charged for.
Watts also matter when you’re choosing a light bulb to use in a fixture that has a maximum wattage limit, or you’re replacing one of the bulbs in a multi-bulb string, such as your Christmas tree lights.
When Watts Don't Matter
When it comes to how much light you get from a light bulb, watts don’t matter. Light isn’t measured in watts. It’s measured in foot-candles or lumens. One foot-candle, as the name implies, is the equivalent of the light from one candle, measured at a distance of one foot from the center of the flame. One lumen is the amount, or flow, of light which distributed uniformly, produces an illumination of one foot-candle on every point across over one square foot of surface area.
For years now – basically ever since electric lights replaced gas lights – we’ve talked about light in terms of bulb wattage, especially when we’re talking about incandescent lights. We’ll say, for example, “I want a 60-watt bulb in that lamp,” when what we’re really thinking is, “I want the light from that lamp to be as strong as what I’m used to seeing from a lamp with a standard 60-watt incandescent bulb in it.”
It’s become a habit, for most of us, to think of the amount of light that way. We’re so used to conceptualizing and describing light amounts in watts that most of us don’t even stop to think about it. That’s why, in order to give us a frame of reference for the amount of light we’ll get from an alternative source such as a compact fluorescent (CFL), halogen, or LED light bulb, the manufacturers will often say that their newer bulb is a “60-watt equivalent.”
What they’re really saying is “The bulb in this package produces approximately the same lumens as a standard 60-watt incandescent bulb.” That’s all, but it’s important. The amount of light we’re going to get from the bulb is the most important thing we look for when we’re shopping for a replacement. If it’s going to give us too little or too much light, it’s not going to work for us in that location. If we’re satisfied with the amount of light and the color temperature of the bulb, we’ll probably enjoy having and using it.
Understanding the Labels
What the company isn’t telling us in that statement about equivalency is any real data. You’ll have to look for that elsewhere on the package or, sometimes, on the bulb itself.
- Lumens. The lumens the bulb actually emits should be printed somewhere on the package. For reference, most old-fashioned 60 watt bulbs produced between 750 and 1,049 lumens, with the average being 800 to 850 lumens. The lumens they list for the one you’re looking at can be compared to those numbers to give you a sense of how much light you’ll be getting from the new bulb.
- Watts. If the bulb isn’t an incandescent bulb it won’t actually draw 60 watts. The watts it will draw will be printed somewhere on the package. For most CFL bulbs, the actual power draw is about 25 percent of the “equivalent” incandescent bulb. The 60-watt equivalent Bright from the Start™ CFL light bulb from GE, for example, uses exactly 15 watts per hour.
- Efficiency. The efficiency of a light bulb – any light bulb – is the amount of light it emits compared to the amount of power it uses. It is the lumens divided by the watts or the lumens per watt. Once you’ve got the lumens and the watts, you can calculate the efficiency. It probably won’t be printed on the package.
- Color Temperature. The color temperature of light its temperature in degrees Kelvin. Without putting too fine a point on it, a “soft white” light is usually about 2700K. A “bright white” light might be around 3500K. “Natural light” and “Daylight” can range from 4100K up to 6000K. This is the number that will tell you how red (yellow) or blue the light will appear, and the one you want is just the one you think you’ll be most comfortable with.
Sometimes watts matter. When it comes to the amount of light you’ll get, though, they don’t.