I picked up a light fixture at a reuse store and it says max 40w incandescent or 11w CFL. It doesn't say anything about LEDs. Thoughts?


3 Answers 3


Two issues

Can the fixture handle the heat?

With all lights now on the market, quite close to 100% of the energy turns into heat. That means essentially we can assume the actual-watt rating of the bulb equates to its heat output.

The fixture is rated to handle the heat of a 40W bulb. So it will not be damaged if the CFL/LED is less than 40 watts.

Can the bulb handle the heat enclosed in the fixture?

This varies wildly, because it's all about how air moves through the fixture.

While an incandescent works great inside an oven, CFLs and LEDs both dislike excessive heat. So it's a question of how well heat can leave the fixture, or alternately, how well air moves through the fixture... and that varies wildly depending on fixture design. It's pretty much a matter of trial and error.

It helps a lot to use quality screw-in LED "bulbs" with well-built power supplies and overbuilt heat sinks -- as opposed to the built-for-price cheapies often found in the big box and dollar stores.

  • Thanks for the answer. The good news is it's not an enclosed fixture. It's the same type of light as Pixar uses in their logo. Also, the bulb is a Philips 435016 15 Watt Indoor/Outdoor PAR38 so I feel it's a better bulb than generic ones.
    – TwisterMc
    Sep 26, 2017 at 12:34

I would be willing to bet you are all set. but, err on the side of caution, and do not exceed the 11W for the base of the socket.

CFL's generate a lot of heat in the base, so its a better measure of that socket's rating to compare the heat the LED bulb generates to that fixture.

An incandescent generates most heat at the filament, which is not centrally located in the base of the bulb, unlike a CFL.

This is why you see many cooked CFL's. Here is a comparison chart to relay the idea (thanks Google and Milwaukee): enter image description here

Another goodie from Google: enter image description here

  • Love the visuals, this helps a lot.
    – TwisterMc
    Sep 26, 2017 at 12:36

40 Watts is the amount of Power that the fixture can handle! So to be clear on my answer here : The 40W -11W CFL are generalizations to equate heat , but that is really not what those numbers represent..

You can put anything in that socket that uses LESS than 40 Watts - so long as the heat of the unit does not exceed that of a 40 Watt Incandescent- if it says 11W CFL - 11W CFL is the rough equivalent wattage required to get the same level of light output as a 40 Watt incandescent bulb and you can be assured the heat of the unit in the fixture will be in that range as well.

It is a generalization because of the amount of heat dissipated - this is the real concern for your light socket and generally any LED you are going to put into that socket should be using less than 10 Watts - most likely around a 5 Watt LED (40 Watt Equivalent) to a 10 Watt LED (100W equivalent).

It would not be advisable to install a 100Watt equivalent .. a decent 60 Watt equivalent (800 lumens) should use about 6 to 8 watts max. The heat dissipated depending on brand may vary greatly .. also your lamp enclosure itself may dictate a different bulb.

Unlike Incandescent bulbs; LEDs should not be enclosed because of their requirement to dissipate heat.


  • Are we not answering the same question, arriving at the same result, via a slightly different approach on how to view the bulbs specifications and how to interpret it? Except that to be clear, the fixture itself is not in and of itself necessarily rated for 40W of heat. If it was, then your own rationalization of 11 ~= 40 would be a misnomer because as indicated, the wattage listed is purported by the bulb manufacturer towards equivalent light output. Thus why the socket listed the two number separately. Thus the socket will not care, the overall fixture will
    – noybman
    Sep 26, 2017 at 4:02
  • This is really no different than reading 600VAC on the side of a socket. But do we ever put that much through it?
    – noybman
    Sep 26, 2017 at 4:02
  • @noybman - I did not say the socket was rated for 40W of heat. I said the wattage was a generalization of light output. The heat generated by the bulbs rated as equal would fall into a rough equivalency..not because of the power consumed - as in a heating element - but as a matter of function of the bulb.. Rating the socket at 600 vac does not tell you much - for example 480 VAC at 1000 Amps draw would probably be a bit much for that socket even though it meets the voltage requirement.
    – Ken
    Sep 26, 2017 at 5:09
  • 1
    Agreed. I miss read it. In fact I see an error in my own response. And I was initially thinking op post on fixture was the socket not the entire assembly. I'll revise it to be more clear. Ultimately as you pointed out those wattages are generalizations. The importancrs are current draw (which isn't much of an issue here), and primarily the heat generated and where it is generated. that's what the fixture assembly sees, temperature.
    – noybman
    Sep 26, 2017 at 16:59

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