It’s cold today, and my new house has one of those annoying pay-as-you-go electricity meters with a horribly expensive energy company that I’m pretty much stuck with until I get it all changed to my preferred energy supplier.

It’s all electric heaters here, and loath to guzzle more electricity, I was doing ok in the cold until I realised one of my pet rats had started hibernating. That’s worrying, because rats don’t naturally hibernate. Something had to be done.

Now, I do have enough on the meter to get us through this cold patch, but I also have rather a lot of candles – a whole box full in fact – lying around since the move. And in my realisation that the candles could be placed on the coffee table far closer to my pets’ cage than the electric wall heater, lo and behold, a scientific experiment was born.

It’s a known fact to everyone who has stuck their hands too close to a fire that fires generate heat as well as light. So I wanted to know how much I could heat the room by burning candles. And I want to know the output in terms of Watts, so I can compare it to the power of my electric heater.

To start with, it is helpful to know what a Watt is. It’s a SI unit for power, measured in Joules per second.

Tea lights weight roughly 20 grams and burn for around 5 hours – that’s four grams an hour.

6 inch dinner candles weigh 60 grams and burn for around 6 hours – that’s ten grams an hour.

3 inch high pillar candles weigh roughly 300 grams and burn for around 40 hours – that’s 7.5 grams an hour.

So it seems that the tea lights burns the least grammage per hour and the tapers the most grammage per hour. But how does that convert to heat? Which is better?

I am going to approximate that all my candles are made from paraffin wax. Now, paraffin wax burns at about 43kJ per gram of material, or 43000 J per gram.

For my tealights, at 4 grams an hour, that’s 4 x 4300 = 172,000 J per hour

For my tapers, at 10 grams an hour, that’s 10 x 43000 = 430,000 J per hour

For my pillar candles, at 7.5 grams an hour, that’s 7.5 x 43000 = 322,500 J per hour.

I have 21 tealights, 3 tapers, and 3 pillar candles.

(21 x 172000) + (3 x 430000) + (3 x 322500) = 5 869 500 Joules per hour.

But remember, Watts are Joules per second. So we need to convert this.

There are 3600 seconds in an hour, so we divide our answer by this to get the Joules per second.

5 869 500 / 3600 = 1630.41667

That’s ~ 1630 Watts for my 27 candles.

Now, the average candle emits light at only around 0.05% efficiency. So the most significant part of that wattage output is as heat (infra-red) rather than visible light, and therefore it’s really negligible to try and calculate how much of that total radiative power is emitted under the visible spectrum rather than the infra-red so I’m just going to leave it out.

My electric heater produces 2000W (in other words 2000 Joules per second).

1630W for my 27 candles really isn’t all that bad in comparison. I know that the amount of joules per second will vary as the candles will run out at different times, and I know that this is a very sketchy exercise in a field I’m no expert in, but it’s still a pretty decent amount of energy being emitted.

For now, though, I am going to whack the heater on full blast in addition to burning the candles. And Porkchop, my wee pet that started this entire exercise, is certainly benefiting from this double output of heat.

Fun facts I didn’t know before I started this exercise: The temperature in the centre (blue bit) of a candle can get up to 1000 degrees Celsius! And light is measured in lumens, the SI unit of luminous flux (or the portion of radiative power falling in the spectrum of visible light).