I bought a reading lamp that on the box says: Max 1x40W
The shop also sold me an LED light bulb that says: 9W=75W
So my question is, can I use light bulb or not? I am confused between the 75W and the max of 40W mentioned for the lamp

  • 5
    Possible duplicate of Can I use a 23 W CFL in a fixture that's rated for a 60 W incandescent? Commented Mar 15, 2016 at 13:02
  • 2
    I think this question is just barely unique enough since the proposed duplicate is asking about CFL's. While the answers are very similar, there's a lot of discussion on the behavior of CFL's when they get warm that may not apply to LED's.
    – BMitch
    Commented Mar 15, 2016 at 15:47
  • The answer to both is the word equivalent and is addressed by Aaron's comment on Wolf's answer.
    – Mazura
    Commented Mar 15, 2016 at 18:41
  • You “can” but depending LED pattern, CCT temp and CRI >92 you may not like it for reading . Commented May 15, 2018 at 1:56

6 Answers 6


Yes. The fixture is concerned with heat. Your LED light bulb makes about 9 watts of heat. Your fixture is rated for a bulb that makes 40 watts of heat.

With an incandescent bulb, 98-99% of its wattage turns directly into heat. Even LEDs, 90% of their wattage turns into heat. For thermal planning, it's not worth pulling out the calculator. You're "close enough" just pretending it all becomes heat.

On the LED bulb, when it says "75 watt equivalent" it's trying to give ordinary consumers a general sense of how bright the bulb is - i.e. about as bright as an old style 75 watt incandescent. The more proper way to say that is "950 lumens", which is a unit of measure for describing brightness to human eyes.

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    The problem is that lightbulbs used to be measured in popular parlance by wattage (energy), not by light output (lumens) So people went shopping for a 100W bulb not knowing how much light output they were looking for. Since nowadays we have CFL and LED lamps that produce the same amount of light on much less wattage, they helpfully tell you what the equivalence is for the light, in this case a 9W using (energy) will produce the same amount of light as a 75W conventional bulb
    – Aaron
    Commented Mar 14, 2016 at 18:46
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    So it can handle a 9W bulb because the lamp is rated to dissipate up to 40W of heat energy, but you wouldn't want to install an old-style 75W as the lamp could overheat?
    – BAR
    Commented Mar 14, 2016 at 19:56
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    @Jim: That's not really right. The meaning of 9W=75W is that the LED lightbulb consumes only 9W of electric power to produce the same light output (luminous flux) as would be produced by a 75W tungsten-filament incandescent lightbulb. I find it best to use lumens to compare lightbulb brightness. Commented Mar 14, 2016 at 20:02
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    @BAR that is correct. Go by the actual wattage (energy consumption) of the light. Commented Mar 14, 2016 at 20:20
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    @SkipBerne - No. LEDs in commercial applications do not have 'current limiting resistors' - they are driven by constant-current power supplies. The bulk of the heat from the entire unit still comes from the LED itself as real LEDs are still nowhere near 100% efficient. Take a commercial LED fixture apart and you'll find its the the LED that is mounted on a big heatsink, not the driver electronics.
    – brhans
    Commented Mar 15, 2016 at 20:03


LONG ANSWER: The maximum wattage limits are largely a function of heat. For incandescent lights, more wattage means more heat. And too much of a heat buildup could result in shorter bulb life and even risk of fire, especially in a fixture in which the bulbs are fully enclosed.

While lighting has traditionally been sold by wattage, what you are really buying is the amount of light, actually measured in lumens. For years, wattage gave us a good idea of how much light came from a bulb. A 100 watt bulb was much brighter than a 40 watt, and a bit brighter than a 75 watt.

As different types of bulbs came into use, lumens started to be listed. For a given lumen level, LED bulbs draw much less wattage than an incandescent bulb. For example a 75 watt incandescent bulb gives off between 700 and 1100 lumens (depending on the type). But LED bulbs that give off about 900 to 1100 lumens only use between 9 and 15 watts. Since they are similar to a regular (incandescent) 75 watt bulb in the amount of light output, they are called 75 watt equivalent.

A 15 watt LED bulb generates as much light, but much less heat than a 75 watt incandescent bulb. In fact it probably generates less heat than a 25 watt bulb. But it does generate some heat. And some bulbs are marked not for use in fully enclosed fixtures.

So long as the bulbs are not marked for limited use, you can generally use an LED bulb that is rated much higher than an incandescent bulb. In general, there is no problem using a 75W equivalent LED bulb in a fixture marked 40W maximum. You are only drawing about 15 watts and generating much less heat than a 40 watt bulb.


When a lamp lists a maximum wattage, its doing that based on the heat an incandescent bulb of that wattage will put off. The 9W=75W on the LED bulb is telling you that "even though this bulb only uses 9W of power, it puts out the same amount of light as a 75W incandescent". Lamps are rated on heat and power usage not light output.

LED bulbs do get hot, and some get so hot that you can't use them in an enclosed fixture because their internal electronics will overheat, but it won't hurt the lamp. If the lamp is rated for the heat of a 40W bulb, an LED that uses only 9W of power just can't get that hot even if it was nothing more than a 9W heating element.


The bulb will not pose a safety or fire hazard.

Those fixture ratings are primarily concerned with:

  1. How much current the fixture is designed to safely supply to the bulb.
  2. How much heat the fixture is designed to accept from the bulb.

The bulb itself is rated 9W. While it has an equivalent rated light output of 75W, for the purposes of heating and current carrying the actual wattage - 9W - is all you need to pay attention to.

Therefore, this fixture will accept and run the lightbulb you've selected.

That said, please note that unlike incandescent bulbs, the newer LED and CF bulbs incude circuits that typically don't like to get hot. An enclosed fixture, or a fixture where the bulb hangs lower than the base, may overheat these components.

If this fixture is enclosed or hangs the bulb from the base, then you will need to make sure the selected bulb is rated for this use. If you don't, it won't present a safety hazard, but the bulb life may be affected.


A fixture's rating indicates how much power it can dissipate without things getting so hot as to damage the fixture or its surroundings. The rating is thus affected by two factors:

  1. How hot things can get without damage to the fixture or its surroundings.

  2. How much power the fixture can generate for each degree of temperature rise relative to ambient.

Incandescent bulbs can operate perfectly happily at temperatures which would cause most kinds of fixtures to melt or catch fire, so a fixture which has a heat-resistant surface may be able to tolerate a powerful bulb even if it can't dissipate much heat. LED bulbs, however, will degrade rapidly if operated at temperatures far below those that typical incandescent fixtures can handle without damage. A 9W LED bulb may only generate 1/4 as much heat as a 40W incandescent, but that's still a significant amount of heat. If an incandescent bulb would have reached 150C when operating at a 30C ambient temperature, a 9W LED might reach 60C. A fixture which can withstand temperatures of 150C or even 200C without difficulty won't be damaged at 60C, but LED lifetime may be greatly degraded at such temperatures.


The answer is always YES. An LED is about 96-98% efficient. The issue is heat. You can hold a '60' Watt equivalent light output LED bulb in your hand forever. Try that with an incandescent and you will burn yourself in 2 minutes. Even a CFL will get too warm to hold in 5 minutes.

The MAXIMUM ratings for fixtures are based on the heat that an incandescent bulb will generate. LEDS give off virtually none, and they last a very long time!

I actually use LEDs for my aquariums and have silver Mylar laying right on top of them: '400' Watts of light and only warm to the touch. Phillips only though.

  • 2
    LEDs are nowhere near that efficiency. Wikipedia has a nice table showing the luminous efficacy of various light sources. As you can see, a bare white led has a maximum effiency of 22%, and complete LED lighting fixtures are generally around 10% efficient. This is better than the ~2% of incandescents, but miles away from your claim.
    – marcelm
    Commented Mar 16, 2016 at 11:10
  • the table is bogus ... it shows that a high pressure sodium lamp is more efficient than an LED. You hold that and I will hold the LED. see who lets go first. LEDs require power supplies. Phillips flat bulbs don't. my 75 watt bulbs are only warm to the touch. Crees will burn you because they use an inefficient regulator. I use AAs to power my sail boats LED navigational lighting array, replacing a car battery that could barely run 1 night.
    – SkipBerne
    Commented Mar 16, 2016 at 13:11
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    The bulb itself is not 96-98% efficient, the SMPS might be but even that is a stretch. In terms of energy output by LED's as light vs heat that chart is correct. You find a LED that outputs the same light as a high pressure sodium or vice versa I'll take your bet.
    – Sam
    Commented Mar 16, 2016 at 14:18
  • HPS vs LED cool to the touch. ... we have them in Cambridge MD. illuminating the city offices façade.. drive up, park, and touch. It is the only decision in 400 years that Cambridge got right. In fact they work better/ brighter . Its all in the regulators. Phillip's yes the others and no doubt the chart's ... no.
    – SkipBerne
    Commented Mar 16, 2016 at 14:42
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    I'm not going to explain all of electrical engineering to you or basic physics. If you need a current limiting resistor, you are using the wrong SMPS. Led's should be driven by a current source. "Quantum emission" has literally nothing to do with it. Led's become vastly less efficient when driven at or near max power, since cree's "thing" is "max brightness" they drive their LEDs near max power which is less efficient.
    – Sam
    Commented Mar 16, 2016 at 15:02

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