Comments on: Like math? Check this out

By: Cindy Joffrion

Cindy Joffrion — Wed, 05 Dec 2007 04:04:05 +0000

This looks correct to me.

By: Baylink

Baylink — Sun, 25 Nov 2007 01:40:27 +0000

Science!

It’s too new!!

:-)

Yes; energy is sold and priced in kWh, so you have to get the dimensional analysis right.

2.3kW is about right, based on the nameplates I’ve read.

Almost all driers run for a set time, commonly a minute Down Here

3 million dryers times 100 usages a day — which seems a high mean, to me, but we’ll take it to compare apples to apples — is 300 million operations daily.

If each one is a minute, then 60 is an hour, and therefore 60 is 2.3kWh.

300M / 60 is 5 million hours of dryer use, per day.

This is where it gets dicey.

In Florida, electric power is about 10cents/kWh.

So the money shot (sorry) is

.10 * 2.3 * 5M = $1,150,000 per day.

Jesus y Maria, that’s a lot of money.

But the question is, did they get the kWh right?

Back the .10 per kWh out, and you get (well, *I* get) 11.5M kWh.

That’s twice what she got, and a bit, so I suspect she left 2.3 out somewhere.

At least, I’ve shown my work, which will make it easier for someone to come along and correct it. :-)

By: Cindy Joffrion

Cindy Joffrion — Sat, 24 Nov 2007 05:00:44 +0000

This is interesting.

By: Charlene

Charlene — Fri, 23 Nov 2007 06:25:31 +0000

I also wonder at the estimate that the average hand dryer is used 100 times a day. Is that estimate from an official source, or did the reporter just pull that out of her hat?

By: R.M. 'Auros' Harman

R.M. 'Auros' Harman — Thu, 22 Nov 2007 21:24:28 +0000

Problem: They list the hand dryers’ “wattage”, or instaneous rate of power consumption, in kWh, a measure of total energy consumed over some period of time. If the driers run at 2.3 kW, they would indeed consume 2.3 kWh in an hour, but that doesn’t mean you measure wattage in kWh…

By: Steve Klein

Steve Klein — Thu, 22 Nov 2007 13:32:02 +0000

They’re going to have to post a correction to their correction.

The correction implies that she used a formula that assumes the average hand dryer is used for only 1 second per use!

I get that from this:
“Clarren multiplied the average hand dryer’s wattage, 2.3 kWh, by 0.0083 (how long a second is expressed in hours), and came up with 0.019 kWh. Then she multiplied that by the 3 million hand dryers that exist nationwide, X 100 uses per day, and came up with a total of 5.7 million kWh used by hand dryers every day.”

I checked the math, and it turns out they made a mistake.

Since there are 60 seconds in a minute, and 60 minutes in an hour, there are 60*60 seconds in an hour, or 3600 seconds.

To convert from seconds to hours, therefore, we divide by 3600.

1 second divided by 3600 equals 0.000277778 hours.

I guessed that the average dryer was probably used for 30 seconds, and did the math:

30 seconds divided by 3600 equals 0.008333333.

Hmm, where have we seen that number before?

So it seems that the author, Rebecca Clarren, did her math right, but the correction itself is in error.

I’ll be sending them a copy of this note.