What per cent of a 100 watt light bulb’s output is heat as compared to light?
I’ve heard that it’s about 90% heat but don’t have a cite.
The equivalent light output for a compact florecent bulb is 25W so it is safe to say that a standard bulb produces at least 75% heat. C.F. bulbs are not totally efficent either though as they do get warm if used for a while so the 90% doesn’t sound that far off.
When operated at its rated voltage, I believe your typical incandescent light bulb has an efficiency of around 5%. (95% heat, 5% light.) But it must also be remembered that, if the light bulb is in a closed room, nearly 100% of its energy is turned into heat. This is because the light energy is eventually absorbed by everything in the room, and thus contributes to heat.
Also, if you’re a bit looser in your definition of “light”, the efficiency goes up, since most of the “heat” produed is infrared light.
According to the Illuminating Engineering Society (IES) 1984 Reference Handbook "If the energy in any light source could be converted without loss into yellow-green light (555 nanometers) the efficacy of the source would be 683 lumens per Watt (100 percent of the theoretical maximum). A typical 100 Watt bulb with 750 hours of life kicks out 1750 lumens, so is 17.5 lumens per watt. This is 2.56% efficient. Longer life bulbs put out less lumens per watt(2500 hours - 14.4 lumens/Watt, 4000 hours - 12.9 lumens/watt) due to the sturdier fillament. Fluorescent lamps are all over the map, and you have to include the ballast factor to be fair. The worst could be as bad as 15 lumens per watt, with the best being over 110.
I was just looking this stuff up last month in the quest for the perfect bicycle illumination system. I thought that LEDs would be very efficient, but it turns out only red and yellow LEDs are much more efficient than incandescent bulbs. But I digress.
This article in Physics Today discusses solid-state lighting and includes comparisons to traditional light sources.
This graph sums it up quite nicely - Incandescent bulbs are at best about 10% efficient, fluorescents are around 25%, and red LEDs can be close to 50% efficient.