What does electricity cost?

By Marcus Wilson 21/05/2013

 I was at a local intermediate school this morning, talking to a group of students about energy. It’s a pretty broad topic, and they were very enthusiastic, meaning I only got through about half of what I wanted, but that’s OK. If it inspires them to go and find things out for themselves, then that’s a positive result.

I talked a little bit about measuring energy, and the unit of the ‘joule’, ‘kilojoule’, ‘kilowatt-hour’ and so on, as well as what power is (the rate of change of energy). At the end I asked the students how much they thought that a ‘unit’ (a kilowatt-hour) of electricity cost. That’s the cost of having a kilowatt of stuff running for one hour.  

Now, I didn’t really think that the students would have much idea. I mean, they aren’t the ones forking out every month for their power bills (ahem! that should be energy bill – remember a 100 W light bulb running for one hour will cost you the same as a 50 W light bulb for two hours – it’s energy that you pay for, not power). Estimates ran from 2 cents a kWh to ten dollars, mostly weighted to the several dollars end of the spectrum. I’m rather glad they’re not in charge of the power companies if they wish to charge that price!  I suggested that they ask their parents to look at an electricity bill (and compare a summer bill with a winter one). 

But I was a little surprised that the two teachers in the room had no idea either (or, if they did, they weren’t going to air it in front of their students). They knew what they paid roughly a month, but not what it cost, for (say) a light bulb for eight hours. One of them said that her parents told her that leaving a light bulb on for a few hours was equivalent to her week’s pocket-money (probably a good way of getting her to turn it off). I think they were being rather harsh on this one…

An incandescent light bulb, at 100 W (a bright one), will go through 800 watt-hours in eight hours (e.g. if you go out for the day). That’s 0.8 kWh, or 0.8 units. At current prices, of around 30 cents a unit (depends on what sort of contract you have of course) that gives about 24 cents cost. Withdrawing a week’s pocket money for this offence is a bit unreasonable! Leaving all the lights of the house on, while you go on holiday for a week, however, is a slightly different story. 

21/6/13 The observant ones among you will notice that I mucked up the units in my light-bulb calculation in my original attempt. Units really are slippery things.