Suppose I've got a basement that kept heated to 20 degrees Celsius using an electric space heater with thermostat. There are no windows in this basement (light is not escaping the room in any significant amount, and I doubt sound is either). Am I correct in thinking that leaving the lights on 24/7 causes zero additional electrical cost due to the fact that the light is converted into heat, reducing the burden on the space heater by the exact same amount (it is on a thermostat)?

ie, should I leave the lights on?

  • $\begingroup$ Depends on whether the lights will mostly heat the room (rather than the ceiling space that many types of recessed lights are mounted in). If you want to save energy, the best thing to do is to lower the temperature during the night. As for cost, lights have a finite lifetime and are probably more costly to replace than the heater. I would turn them off. $\endgroup$
    – CuriousOne
    Oct 8, 2014 at 0:18
  • $\begingroup$ It lowers the peak current draw unless the heater has the ability to be partially on (not all do). $\endgroup$ Oct 8, 2014 at 0:54

2 Answers 2


Your basic intuition that anything that uses power will heat the room and reduce the consumption of the space heater is a good one. To first order it is correct. One can quibble that the heat is released in a different place, so may not heat the thermostat as effectively, causing a (slightly) higher room temperature and more total consumption, but that is not in the spirit of your question and probably small. As long as you absorb the light it becomes heat. The sound from your stereo likewise.


Incorrect. The heat produced by lighting device is very low (also depend on device type: LEDs, bulbs, etc) because it is made for produce light and heat is a sign of inefficiency. For example, the most inefficient lighting device is light bulb with 75% efficiency. It means that only 75% of the power converted into visible light, and another 25% converted into another form of energy including heat which is very low compared to the size of the room (I assume it's big one). You can try to measure how much is the temperature difference by turning the light on alone (turn off the thermostat) with thermometer on your room.

And it's better to turn the light off. I hope this help, and CMIIW. :)

Add: Sorry I must add something, hehe. It depend on what kind of lighting device you use. Some kind with high efficacy? Or low one? Because they produce different amount of heat.

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    $\begingroup$ Doesn't all the visible light get converted to heat once it hits surfaces in the room? In effect making it 100% efficient at creating heat. $\endgroup$
    – tom
    Oct 8, 2014 at 0:40
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    $\begingroup$ No they don't, because the surface in the room will absorb and reflect the light that comes from lighting device. Altough the contact between photon and surface can produce heat but that is very very small amount. And the heat that produced by the lighting device is naturally comes from the device itself, not from the light. $\endgroup$ Oct 8, 2014 at 0:55
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    $\begingroup$ @tom - Some of the light goes out the window. (But this is a basement; it might not have windows.) Discounting that escaping light, you are correct. Master Mujahid, you are wrong. $\endgroup$ Oct 8, 2014 at 2:51
  • $\begingroup$ oh, I thought heat mostly come from the lighting device not from the light. Thermal energy produced by filament in light bulbs, junction in LEDs. Except, you buy a lamp that produce more heat in purpose such as reptile/aquarium lamp. :) $\endgroup$ Oct 8, 2014 at 3:25
  • $\begingroup$ I'd like to see a light bulb with 75% efficiency! Most incandescents convert only about 2% of the energy into light. So, regardless what happens to the light, it's the 98% wastage that heats the room. LEDs are around 6x more efficient, i.e. they still waste almost 90% as heat. $\endgroup$
    – hdhondt
    Jan 3, 2015 at 9:18

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