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?
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.
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.
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.