Inexpensive Home Building

Cut through the jargon and nonsense of home building and house construction by starting from zero dollars and trying to figure best-value bang-for-your-buck when choosing construction methods or construction materials. My research might answer some of your questions and at other times perhaps you have the knowledge or experience to post the answers to my questions and thereby help others too. The goal is an affordable and sustainable home for all.


Thursday, February 22, 2007

The Difference between Energy and Power: Do Not Confuse


"Work" Preface: A joule is used as either a unit of energy or a unit of work. Technically, energy is the ability to do work (effort or input for potential work) while work is the measured effect of applied energy (accomplishment or result or output); directional mechanical work = force * distance and the motively similar thermodynamic work = energy transferred. Your electric bill measures what reached your meter (the work/accomplishment from the power company's view), not how much benefit you wrung out of it.

  • energy = work (technically, the ability to do work)
  • power = work rate (energy per time)
  • joule (J) = energy (work)
  • watt (W) = power (rate of work, energy per time)
  • watt-hour (Wh) = energy (work)
  • energy = J
  • power = J/s

The trick is that the word "watt" includes a built-in time component:
  • 1 watt = 1 joule per second (Jps or J/s)
So, a watt is a transmitting speed, or rate per time, like miles per hour (mph) or bytes per second (bps). You do not see the "ph" or "ps" at the end of "watt" because "watt" replaces "Jps" or "J/s."
  • 1,000 watts = 1,000 joules per second
You can think of watts like a speed but it is never "watts per second," it's either "watts" or "joules per second." Solar panels and generators usually are rated at their maximum "speed" of watts or kilowatts (not kilowatt-hours, not kWh).
  • 12,000 joules PER SECOND = 12,000 watts = 12 kilowatts
You can drive a car 60mph but whether you did 60mph for 1 second or for 12 hours determines how far you traveled. That's where watt-hours or kilowatt-hours (kWh) enter the picture. A watt-hour (Wh) is a sum of all joules used over time, a cumulative energy total like miles driven on an odometer.
  • 1 watt-hour (Wh) = 1 joule per second for an hour (3,600 seconds) = 3,600 joules (energy)
  • 1kW=1,000W (power)
  • 1kWh=1,000Wh (energy)
If you did a "speed" of 0.5kW for 24 hours, then you totaled 12kWh.
If you did a "speed" of 1kW for 12 hours, then you totaled 12kWh.
If you did a "speed" of 2kW for 6 hours, then you totaled 12kWh.

A final definition will allow you to make sense of many consumer electronic labels:
  • watts (W) = volts (V) * amperes (A)
High-school algebra works perfectly on all these letters:
  • W * h = Wh
  • 2W * 3h = 6Wh
Algebraic division works perfectly on "joules per second" and other time rates.
  • J/s = 1 joule divided by 1 second
Now you can use algebra to transform units when comparing information that appeared apples-oranges at first.

Good luck.

2 Comments:

At 2:07 PM, Anonymous Ed Davies said...

Well said - a lot of people are confused on this stuff and you've explained it pretty well. You might have seen the one or two rather lecturing posts from me in Yahoo groups you list on your sidebar about it.

Also, well done for correctly spelling the names of the units which are named after people (James Watt and James Prescott Joule) in all lower case. However, the ISO convention is that symbols for units named after people should be written in upper case, so they should be J, W, kWh, etc.

This sort of thing gets to be important when you want to remeber how to distinguish things like a Nm (newton·metre, a unit of torque) from an nm (nanometre, a small distance).

 
At 10:19 AM, Blogger J said...

Ed,

Thank you, I recently had to sort this out for myself not long ago, so maybe I learned from you.

Thank you for the reminder about capitalization. I intend to fix the old posts eventually.

Thank you for the visit and stop by anytime.
J

 

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