Science Questions

How many watts is a burning candle?

Sun, 3rd Feb 2008

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Question

Jůhannes Gunnarsson asked:

How many watts is a burning candle?

Answer

We had an answer on our forum from Bored Chemist.  He assured us the original standard candle will burn a waxy substance called spermaceti.  Itís called that because it comes from sperm whales.  The best that we have today is, of course, petroleum based wax. Weíve got to compare for that.  But the standard candle, he says, would burn 120 grains of spermaceti an hour. Thatís 8g.

Bored Chemist worked out for us that this means that it burns 2.16mg or about 2 thousandths of a gram of spermaceti every second.  If we know how much energy is in a gram of this stuff we can work out its wattage which is essentially energy per second of the candle.  Assuming that spermaceti is similar to a typical type of fat or oil we can see that it gets about 37kj per gram.  The candle was burning 2 thousandths of a gram each second which gives us a power of about 80 watts.

The reason it isnít as bright as an 80W light bulb is because itís really inefficient.  Most of that 80W is actually given out as heat.  So itís not like the energy efficient lightbulbs.  Most of the stuff coming off candles is heat with only 0.05% of it, not very much at all, coming out as light.

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