Do compact florescents change maximum wattage allowed?

February 25, 2007 2:08 PM

Most of my lamps and light fixtures suggest a maximum wattage to use. I've always assumed this was related to heat output, not power consumption, so believe that if I switch to compact florescents, I can use higher wattage bulbs (if they're called that) in the fixtures. Am I right?
posted by EllenC to home & garden (10 comments total)

Sort of.

A watt is a watt is a watt, no matter what the device. However, compact fluorescent bulbs are generally lower-wattage than the equivalent incandescent bulb. You'll notice on the packaging for the CF bulbs that they are "equivalent to" a given wattage incandescent bulb.

So if your fixture has a maximum rated wattage of 40W, you're only going to want to put a maximum 40W bulb in there. Given that, though, a 40W CF bulb is a LOT brighter than a 40W incandescent bulb; a "40W equivalent" CF bulb is probably a 20ish watt device. You could conceivably put a "60W equivalent" bulb in there, for example.
posted by majick at 2:19 PM on February 25, 2007

Actually, a "40W equivalent" CF lamp draws about 9W. 20W CF lamps have about the same light output as 100W incandescents.

When you're figuring out what your fittings will cope with, use the CF's actual power rating, not its "incandescent bulb equivalent" rating.

To a first approximation, heat output and power consumption are the same thing, so you're safest to stick to your fixtures' rated maximum wattage regardless of your lighting technology.

Although a CF lamp does actually convert about five times as much of its input power to light as an incandescent of the same power rating, the vast bulk of the input power still ends up as heat for both lamp types.
posted by flabdablet at 2:43 PM on February 25, 2007

And keep in mind that the wattage rating of a lamp isn't (just) for the heat put out by an incandescent bulb -- it reflects the current that the wiring inside the lamp is able to handle, too.
posted by mendel at 3:01 PM on February 25, 2007

The wattage rating is for all the components up to and including the lamp. Putting a lamp that requires a higher wattage will drive more current through those components then what they are rated for causing them to heat up and fail or worse, start a fire.
posted by nickerbocker at 3:25 PM on February 25, 2007

Easiest thing to do is go down to the hardware store, and look for the wattage of incandescent bulbs that you normally use. Look on the packaging, and note the light output in lumens. Then get a fluorescent bulb with an equal or higher lumen output, and lower wattage.

Watts are power (electricity) consumption, and lumens are light output. Unfortunately, people are so used to talking about incandescent bulb brightness in watts, some fluorescent bulbs are marked in a kind of psuedo-unit that's "equivalent incandescent bulb wattage" but which has nothing to do with the bulb's actual power consumption.

Light output is really the key measurement when comparing incandescent to fluorescent bulbs, in order to find replacements.
posted by Kadin2048 at 3:48 PM on February 25, 2007

"in theory" most modern houses are wired so that any socket can support very high wattage, but this is an assumption, and one that I would not be comfortable with. The builders are free to use substandard methods for lower-rated sockets. Same deal with lamps — especially the cheaper ones. I would always respect the max-wattage. It's just not worth a wall fire.

But since compact-florecent bulbs are so efficient, it should not be a problem to keep to the advice above. Using less electrical wattage than the socket is rated for is just straight up sanity.

(In construction, you will frequently see 500-watt halogen bulbs run from electricity drawn from a basement light socket, for example. the wiring is robust enough in many cases. Just understand the risk of doing so is greater than the reward. )
posted by clord at 4:06 PM on February 25, 2007

There's a lot of weird, waffly, answers above.

The answer you're looking for is: yes, if your fixture says "60W max", you can put a 60W incandescent or a 60W compact fluorescent bulb in there. The largest compact fluorescents are usually about 23W - they generally say on the package "equivalent light output to a 100W incandescent" or some similar wording.

Basically, you can use the largest possible CFL bulb in any incandescent fixture. They only draw 23 watts or so, the fixture was designed to handle at least a 60 watt bulb... no problem.

The problem you'll actually find is physical size. Many CFL bulbs are larger in width and length than incandescent bulbs, and they won't fit in many enclosed light fixtures.
posted by jellicle at 5:29 PM on February 25, 2007

The vast majority of lamps with a wattage rating aren't limited by current (a 100W lamp draws less than an amp in the US, less than half an amp in the UK.) It's thermal issues that cause the limitations.

Incandescents are lousy light generators, but fantastic heat generators.

CFs draw less power, because they're more efficient at generating light, and they don't generate the large amounts of infrared light that an incandescent does (which adds to the heat problems in the lamp.) In fact, you could probably safely put a CF that drew 75W of power into a fixture rated for a 60W incandescent lamp.

However, that CF lamp would put out a *scary* amount of light -- close to the output of a 300-400W incandescent. So, you're more likely to put a ~30W CF, which isn't even going to have the heat load of a 20W incandescent.

The big question -- will it fit?

Note: Some CF are meant to burn *only* bulb up. Others can burn base up. Read the box, and choose correctly -- a base-down CF installed base-up can fail rapidly, because while there isn't nearly as much heat, there still is heat, and the lamp is counting on convection to keep the heat off the driver package in the base. Installing a base-down only lamp base-up means convection carries more heat to the driver package.

Read the box when you buy it, and you'll avoid that.
posted by eriko at 5:59 PM on February 25, 2007

Second jellicle.
posted by Mitheral at 6:14 PM on February 25, 2007

It is all about heat, as eriko says. However it gets complicated. A 60W fixture would be fine with 60W of CF bulbs in it, but the bulbs could easily overheat and fail.

The rating on a light fixture is for the fixture, not the bulb. Incandescent bulbs produce a lot of heat, but they are not at all temperature sensitive themselves, so a light fixture is going to fail before an incandescent bulb does. This is not true for compact fluorescents, which can overheat fairly easily. Obviously a bulb failure is less serious than a fixture failure, so experiment - within the constraint of the fixtures power rating, of course. If you notice that the bulbs are failing faster than you think they should, you know why - they can't take the heat.

More technically.. I don't know what the failure temperature for an incandescent would be, but it must be many 100s of degrees. Light fixtures are probably rated to about 85 degC, depending on details. CF bulbs could be rated as high as that, but most bulbs will be rated much lower, like ~50 degC, or less.

Here is a previous question that might be interesting:

Will I burn down my apartment?
May 26, 2005 3:33 AM

posted by Chuckles at 9:10 AM on February 26, 2007