Does turning a light off save electricity in a room heated by an electric space heater? 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?
 A: 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.
A: 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.
