How many watts is a burning candle?

03 February 2008

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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.

Comments

Re: "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"

Um, light bulbs are extremely inefficient also, and give out most of their energy as heat. And who is using an energy-efficient light bulb that is 80 Watts! That would burn out your retina.

I've got some high wattage energy efficient lamps; 80W is a perfectly reasonable rating.

But what is the equivalent light wattage? Maybe someone wants to use a candelabra. Your answer isn't very helpful

It tells you that the cancle is converting about 80W ("The candle was burning 2 thousandths of a gram each second which gives us a power of about 80 watts.")

Next it tells you "Only about 0.05% of the energy - so not very much at all - comes out as light."

Therefore you can find 0.05% of 80W to find the visible light "wattage" for the candle... 

 

I agree. I'm trying to find a light bulb that is the same brightness as a candle. I found a GE vintage style amber glass 5 watt bulb at Lowes that is good. How hard is it to just get an easy answer for those of us wanting a light bulb?

... and our grandma in the old Country would put a tall, clear glass cover over the oil lamp flame and the light would quadruple in size instantly. Enough to have light in the entire room. And we would all gather around those flames and tell stories and laugh. Talking about quality family time!! Interesting stuff!

About 100 watts of heat is about right. It's interesting to note that is about the same as the heat from a human body or the heat that is needed to keep a cup of coffee hot.

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