How many watts is a burning candle?

03 February 2008

Question

How many watts is a burning candle?

Answer

We had an answer on our forum from Bored Chemist. He pointed out that the original standard candle burned a waxy substance called spermaceti. It's called that because it comes from sperm whales.

The best that we have today is, of course, petroleum based wax. The standard candle, he says, would burn 120 grains of spermaceti an hour. That's 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 - in joules - per second of the candle.

Assuming that spermaceti is similar to a typical type of fat or oil, it's about 37 kilojoules (kJ) 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 isn't as bright as an 80W light bulb is because it's really inefficient. Most of that 80W is actually given out as heat rather than light. So it's not like the energy-efficient light bulbs.

Most of the stuff coming off candles is heat. Only about 0.05% of the energy - so not very much at all - comes out as light.

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