Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Will a CFL light bulb and an incandescent light bulb, in separate respective closed systems, produce exactly the same amount of overall temperature increase over time?

Assume you have two identical closed systems with gray walls, with a system input of 20 watts of power each.

EDIT added for clarity: (On the packaging of the CFL light bulb the large print equivalent wattage is irrelevant... the input current of both bulbs is a consistent 20 watts of power each. The comparison wattage vs. the actual wattage of the CFL is off subject.)

One has a CFL, one has an ordinary incandescent light bulb. Will both systems increase in heat the exact amount, every hour?

Due to conservation of energy it shouldn't matter if one light source is more efficient, right?... it's the same amount of energy input. One light makes more heat one makes more light, but the light when it hits the gray wall is converted to heat, right?

There is no such thing as loss of energy... it's just converted to another form of energy... and light is converted to heat, right?

The back story of this question is my wondering that if my wife leaves an incandescent light bulb on in the winter time it's not so bad because even though no one is in the room it's still heating up the room. On the other hand if she leaves on a CFL it's more efficient but it should still add heat to our "system," i.e., our home.

share|cite|improve this question

No they won't

if you're reading the "watts" off the package.

That is because for an incandescent bulb, the "60 W" is a measure of the actual electric power it dissipates. Most of that power is thermal (i.e. heat) from the get go, and every last Watt ends up heating in the room eventually (when the light energy thermalizes).

On the other hand, CF lamps and LED lamps are labeled with "light output equivalent" wattage, because the first thing people care about when buying a bulb is how much light they're going to get and we've grown used understanding that in terms of an incandescent bulb's total power dissipation (I suppose that in twenty years this will look like a really stupid convention to the kids). If you read the package closely you'll see that high efficiency lamps generate about the same amount of light, but much less heat. The relationship is not linear in equivalent wattage, but runs about 1/7 for CF bulbs in the 60--120 "Watt" range.

Yes they will

if you really mean, that each bulb dissipates 20 Watts.

However, that's a really dim incandescent bulb suitable for little more than a night light, but a pretty good CF bulb suitable for keeping a medium sized room well lit and allowing you to read comfortably.

share|cite|improve this answer
Putting the "no they won't" for some rather obscure technicality is a little confusing. CFLs that I've bought list the true wattage pretty clearly. The first thing that popped up on Google images was unambiguous – Mark Eichenlaub Feb 27 '11 at 15:23
@Mark: On the packaging of the ones I bring home, the real power dissipation is in small type, and you have to go looking for it, which the incandescent equivalent is a great big number. Maybe things are different in other parts of the world, or maybe it's changing, but none of my boxes are as clear as the picture you linked to. – dmckee Feb 27 '11 at 15:25
Sorry about being clear. I just edited the question to add this: (On the packaging of the CFL light bulb the large print equivalent wattage is irrelevant... the input current of both bulbs is a consistent 20 watts of power each. The comparison wattage vs. the actual wattage of the CFL is off subject.) – Pete Alvin Feb 28 '11 at 11:43

The relevant comparison is between bulbs with similar light power.

Let's assume for sake of argument that a 20 W CFL light generates as much light as a 100 W incandescent bulb. For instance, both generate 5 W of light. If you leave the CFL light burning in a room, you get 15 W of heat and 5 W of light. If the room has no windows, no light energy leaves the room, and the room heats up with a rate of 15 + 5 = 20 W. In the same room, the incandescent light gives 95 W of heat plus 5 W of light, for a total of 100 W of heating power.

It is generally not a good idea to heat up your home with incandescent bulbs (or any other form of electrical heating) as it takes much more energy in the form of hydrocarbons to generate a given amount of electrical energy. Efficiency wise, is better to heat up your home by burning the hydrocarbons locally.

And where the difference between CFL and incandescent bulbs really starts to matter is during summer when each Watt of power dissipated in your home needs to get pumped to a hot outdoor environment using an AC.

share|cite|improve this answer
This answer doesn't make sense. He clearly asked about two lightbulbs, both 20W. Why did you answer by talking about two lightbulbs of different wattages? – Mark Eichenlaub Feb 27 '11 at 15:25
Well, the question as stated made little sense: the relevant comparison is between bulbs that generate similar amounts of light. So made that explicit in my answer. – Johannes Feb 27 '11 at 15:32
Added a starting sentence to my answer that should make clear where the answer is going... – Johannes Feb 27 '11 at 15:39
I stated in the original question that exactly 20 watts of power are being input to both systems... it doesn't matter what the stated wattage of the bulbs are... just that one is more efficient than the other... energy is transformed in different ways between the bulbs. I'm guessing that the overall temperature would have to increase at exactly the same rate but I don't know. – Pete Alvin Feb 28 '11 at 11:52

The answer is yes, of course both will produce the same amount of heat energy. Any electrical load with an input of 20 real watts will output 20 watts, in some form or other. In a perfectly insulated box, all of that energy will eventually be converted into heat because no energy conversion is 100% efficient.

Doesn't matter if the load is an incandescent bulb, a fluorescent bulb, a motorized egg whisk, or an air conditioner. If it uses 20 W, it will eventually produce 20 W of heat.

Reactive power is a different story, since the energy is temporarily stored in the load and then returned to the power line.

As for the rate of temperature increase, I'm not sure, since the ratio of different forms of energy in in the box will be different, even though the total amount of energy in the box will be the same. Does light count towards temperature? Temperature is a measure of the kinetic energy of particles. Does a perfectly mirrored box with light bouncing around but no particles inside have a temperature?

share|cite|improve this answer
To answer the questions within your answer: yes, the light bouncing around inside the box does have a temperature. It won't in general be equal to the temperature of the air in the box and its walls, but if it's emitted by a tungsten filament it will have the same temperature as the filament. "Photon gas" and "radiation temperature" are good search terms if you'd like to know more. However, the amount of energy in the box in the form of light will be tiny, since each photon will only bounce of the walls a few times before being absorbed. ... – Nathaniel Jan 14 '12 at 18:39
... The rate at which light is converted into heat must equal the rate at which light is emitted by the bulb, hence all the light produced by the bulb is very rapidly converted to heat. So the rate of temperature increase caused by both bulbs will indeed by the same. – Nathaniel Jan 14 '12 at 18:41
@Nathaniel: Yes, but if you do the experiments side-by-side, in separate boxes, and start both at exactly the same time, is the temperature of both boxes always the same, or is there a delay in the CFL box because the light takes a small amount of time to be converted into heat and the CFL produces a greater proportion of energy in the form of light? – endolith Jan 14 '12 at 19:27
Each photon will only bounce off the walls a few times before being absorbed. Light travels at around $3\times 10^8$ m/s, so if the box is 1m in size then the delay due to the conversion of energy into light and then heat will be of the order $10^{-8}$ seconds. This is far too small to measure in any normal situation. In fact, I guess if you did the experiment you might be able to detect the box's walls heating up faster with the CFL bulb, because the light reaches them faster than the convective currents induced by the hot tungsten bulb. – Nathaniel Jan 15 '12 at 19:55

no they will not because cfl bulbs consumes less energy

share|cite|improve this answer
Welcome to Physics.SE! You may find that such minimal answer do not attract a very positive response. Further, I'd suggest that you read the clarification in the questions carefully. – dmckee Jan 14 '12 at 15:47

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.