Posted by: Li Ling Young | July 7, 2013

Energy Units

It started with this question.., “Why don’t we use the dryer anymore?”  This is my son asking, and it’s May.  Both of our kids are pretty used to my long-winded answers by now, but this one had a short answer: “Because the dryer uses 5 kWh for each load!”  It was the next question that really got me started, and I didn’t really stop until sometime in the middle of school the next day.

“What’s a kilowatt-hour anyway?”

Now, I’ve answered this question dozens of times, with varying degrees of success, but here was my chance to try my answer out on someone who a) is pretty sharp math-wise, and 2) doesn’t think they already sort of understand it.  Ready to give it a whirl?

A kilowatt-hour, abbreviated “kWh” is an amount of energy.  It’s sort of like a “gallon” (of milk), or a “cord” (of wood), or an “acre” (of strawberry plants).  What it’s not is a rate, like “gallons-per-day” (a good cow produces over 5 gallons/day), or “cords-per-year” (3: that’s how quickly we burned wood last year: way too fast!), or acres-per-hour (a crew of strawberry pickers might harvest a few acres/hour).

It’s confusing because a kWh has the word “hour” in there, which is sort of like miles-per-hour, or something else with a time unit (see all the rates I made up in the previous paragraph).  But it’s absolutely nothing like that because it’s not “kilowatts-per-hour”; it’s just “kilowatt-hour”.  Mathematically the difference is a rate is divided by some unit of time, and a kWh is multiplied by time: big difference.  Never say “kilowatt-per-hour”.  That’s mostly nonsense and it’s really bad science literacy.  If you have to correct someone who has said it incorrectly, be kind.

The reason a kWh categorically is not a rate is because it’s an amount of something: energy in this case.  A rate never tells you how much, only how fast.  As an example, say you’re running to get in shape.  Every day you run, and over time you gradually increase the distance you run each day so you can continue to challenge yourself and get stronger.  You probably also get a little faster as the weeks of consistent training start to pay off.  But really you’re keeping track of how many miles you run every day, because that’s how you measure progress.  Week one you run 2 miles each time you go out.  But by week five you’re running 4 miles a day, and by week eleven you’re up to a 7-mile run.  Sharp readers will have noticed I slipped a rate in there, but you look and feel awesome: science literacy is not a burning issue in your mind.

Nonetheless, how much sense would it make if instead I told you that on week one you ran four miles-per-hour (hereafter abbreviated in the normal way), week five 4.2 mph, and week eleven 3.8 mph?  First off, you’re getting slower, and b) how far did you run?  THAT is the important difference between a rate and an amount.  The rate, “mph”, is not enough information to know how far you ran.  To know that you have to multiply the rate (in this case miles-per-hour) by the duration.  On week one we know you ran for about 30 minutes.  After that you only ran for about 10 minutes per outing, stopped working out consistently, and went to a few parties.  By week eleven you’d gained weight and developed a bad back from sleeping on friends’ couches.  Rate tells you nothing when what you want to know is “how much”.

Back to kWh…  A kWh is a “how much” of energy.  There is a corresponding rate, and it’s called a “kilowatt”.  Very confusing because there is no time unit in there, yet it is a rate, just like balloons-per-day, inches-per-year and revolutions-per-minute.  You just have to take my word for it.  If you really want to know, it’s 1000 joules-per-second, but I’m not going to talk about joules (though the sharp reader will notice that the time unit has made a reappearance, like a proper rate).  So, just like with any rate, you multiply the rate by the duration and you get the amount of something.  I just thought of a familiar corollary that helps explain having the time unit where it doesn’t belong: a light-year.  A light-year sort of seems like a rate (maybe light/year?), but really it’s a unit of distance: the distance light travels in one earth year.  Just so, a kWh is the amount of energy needed to power a 1 kilowatt device for one hour.  The “per” has nothing to do with it!  “Per” means “each”, whereas light-year and kilowatt-hour mean “during”.

And now, the dryer!  I told my son that 5 kWh for a dryer load is crazy because we only use about 10 kWh/day on a normal, non-laundry day.  A couple loads of laundry could double our day’s electricity consumption?  Wow!  To paraphrase an old farm saying, “Do laundry when the sun shines.”  Starting in May we can consistently hang our laundry to dry if we keep an eye on the weather and only wash on sunny days.

One last thing…  We haven’t talked about the electrical demand of the dryer: the rate at which the dryer uses electricity.  By using our energy monitors I know a dryer load uses about 5 kWh, and draws 6 kilowatts (kW) while it’s running.  As a test to see whether my son really absorbed the whole long answer, I challenged him to explain the difference between kW and kWh to his math teacher and then pose this question to him to measure his comprehension and my son’s explanation:

How long does a dryer load take?  Give it a whirl.


Responses

  1. A good way to explain what 5 kwh means is to note that a person can generate about 100 watts pedaling on an exercise bike, so 5 kwh would take 50 hours of pedaling! 1 kwh is like a person pedaling all day on a bike – and we get that for less than 15 cents. It’s like slave labor.


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