Welcome again to the wonderful but sometimes weird world of wireless comms, written by Joel Young, CTO of Digi International
How did we ever end up with something called a kilowatt-hour? The kilowatt-hour has always been, at least for me, one of the more puzzling of all the units of energy.
I know, for many of us we are continually embroiled in the metric vs. imperial system debate, this is a debate that I understand - miles vs. kilometres for example, both seem reasonably arbitrary for me.
However, the kilowatt-hour is an abomination, created by someone who forgot what they learned in physics. Ever since we changed our first light bulb, we have been familiar with the almighty watt. We might not have completely understood the nuances of the watt, but it didn't take us long to figure out that light bulbs with more watts, are brighter and hotter. The bill payers in our house were noticeably irritated when we left the lights with big watt numbers turned on.
How did we ever end up with something called a kilowatt-hour? The kilowatt-hour has always been, at least for me, one of the more puzzling of all the units of energy.
I know, for many of us we are continually embroiled in the metric vs. imperial system debate, this is a debate that I understand - miles vs. kilometres for example, both seem reasonably arbitrary for me.
However, the kilowatt-hour is an abomination, created by someone who forgot what they learned in physics. Ever since we changed our first light bulb, we have been familiar with the almighty watt. We might not have completely understood the nuances of the watt, but it didn't take us long to figure out that light bulbs with more watts, are brighter and hotter. The bill payers in our house were noticeably irritated when we left the lights with big watt numbers turned on.
Returning to our elementary physics class, we of course remember that in electrical terms a watt is equal to potential (measured in volts) x current (measured in amperes). And of course, current is really the amount of charge that moves each second.
Hence, one ampere equals the movement of one coulomb (unit of charge) per second. So, a watt is really equal to 1 volt-coulomb per second. Most of us familiar with units of energy will no doubt remember that 1 volt-coulomb is the same as 1 joule - the metric unit of energy. Still with me?
Well, for those of you that I lost, let's return to the notion of power - not power in the purely electrical sense, but power or the sake of getting work done. Power is the application of energy per unit of time - most notably the second. Note that you really can't get any work done with power itself unless you apply power over a length of time. The old imperial system of measurement that most of us are familiar with is the horsepower.
Ironically enough, the horsepower as a unit of power was invented by Scottish inventor James Watt when he was looking for a way to describe the power in his steam engines, the name sake of the metric equivalent: 1 hp = 745.56 watts. So, when you apply power for a period of time, you use energy. It doesn't actually matter whether your power comes from steam, gasoline or electricity. In order to get any work done (energy), you need to apply power for some time.
So, if I use power over a period of time, I consume energy. Remember that energy is measured in joules - which is the use of one watt for one second. If I turn a 60 watt light-bulb on for 60 seconds, I've used 60 joules of energy.
So what's up with the Kilowatt-hour?
Well, it turns out that those who originally put in our electric metrology forgot all about their physics class or thought the notion of joules was too complicated. So they invented a brand new energy measure instead. One kilowatt-hour is the use of 1000 watts for 1 hour, or rather 1000 joules/second for 3600 seconds or rather 3.6 mega joules. So we should be paying for our electricity by the mega joule instead of the kilowatt-hour.
Of course, if we really wanted to return to the old imperial system of units (as created by James Watt), then we should be measuring electricity by the horsepower-minute. Since 1 hp = 745.56 watts, 1 kilowatt-hour would be 60 x 1000 / 745.56 or 80.5 horsepower-minutes. Wouldn't that be fun?
I fully expect that most readers will by puzzled by my nerdy rant. To that I offer the following thought. What would you say, if after completing a long car road trip, the driver said, "Well it looks like we travelled 60 miles-per-hour-days"? You were expecting 1,440 miles?
Previous Weird & Wireless:
Joel Young, VP of Research and Development and CTO at Digi International,
has more than 22 years of experience in developing and managing data
and voice communications. He joined Digi International in June 2000 and
in his current role he is responsible for research and development of
all of Digi's core products.
Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.
Hence, one ampere equals the movement of one coulomb (unit of charge) per second. So, a watt is really equal to 1 volt-coulomb per second. Most of us familiar with units of energy will no doubt remember that 1 volt-coulomb is the same as 1 joule - the metric unit of energy. Still with me?
Well, for those of you that I lost, let's return to the notion of power - not power in the purely electrical sense, but power or the sake of getting work done. Power is the application of energy per unit of time - most notably the second. Note that you really can't get any work done with power itself unless you apply power over a length of time. The old imperial system of measurement that most of us are familiar with is the horsepower.
Ironically enough, the horsepower as a unit of power was invented by Scottish inventor James Watt when he was looking for a way to describe the power in his steam engines, the name sake of the metric equivalent: 1 hp = 745.56 watts. So, when you apply power for a period of time, you use energy. It doesn't actually matter whether your power comes from steam, gasoline or electricity. In order to get any work done (energy), you need to apply power for some time.
So, if I use power over a period of time, I consume energy. Remember that energy is measured in joules - which is the use of one watt for one second. If I turn a 60 watt light-bulb on for 60 seconds, I've used 60 joules of energy.
So what's up with the Kilowatt-hour?
Well, it turns out that those who originally put in our electric metrology forgot all about their physics class or thought the notion of joules was too complicated. So they invented a brand new energy measure instead. One kilowatt-hour is the use of 1000 watts for 1 hour, or rather 1000 joules/second for 3600 seconds or rather 3.6 mega joules. So we should be paying for our electricity by the mega joule instead of the kilowatt-hour.
Of course, if we really wanted to return to the old imperial system of units (as created by James Watt), then we should be measuring electricity by the horsepower-minute. Since 1 hp = 745.56 watts, 1 kilowatt-hour would be 60 x 1000 / 745.56 or 80.5 horsepower-minutes. Wouldn't that be fun?
I fully expect that most readers will by puzzled by my nerdy rant. To that I offer the following thought. What would you say, if after completing a long car road trip, the driver said, "Well it looks like we travelled 60 miles-per-hour-days"? You were expecting 1,440 miles?
Previous Weird & Wireless:
- Weird & Wireless: Why is the use of cell phones discouraged around petrol pumps?
- Weird & Wireless: What is the difference between a human eye and an antenna?
- Weird & Wireless: What's the deal with electronics and radios on airplanes?
- Weird & Wireless: Can batteries be left out in the cold?
- Weird & Wireless: GPS, and how do those satellites know where I am?
- Weird & Wireless: Do microwave ovens cause cancer?
- Weird & Wireless: Why can I use a 2.4-GHz phone and 802.11 network at the same time?
Joel Young, VP of Research and Development and CTO at Digi International,
has more than 22 years of experience in developing and managing data
and voice communications. He joined Digi International in June 2000 and
in his current role he is responsible for research and development of
all of Digi's core products.Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.
Comments (6)
It's a good unit for everyday use - in the days of electric fires with each bar being rated at 1kW it was easy to calculate running costs. And for many years it was priced conveniently at about 5p (or a shilling, I suppose...) although obviously less so now.
Posted by Paul Harding | August 27, 2009 11:21 AM
Posted on August 27, 2009 11:21
The statement above "If I turn a 60 watt light-bulb on for 60 seconds, I've used 60 joules of energy." will confuse people as it is wrong.
60 Watts for 1 second is 60 joules. 60 watts for 60 seconds would be 3600 joules.
Posted by EW | August 27, 2009 11:29 AM
Posted on August 27, 2009 11:29
60W for 60s is 3600J (8th paragraph). Maybe 60J is a good enough approximation for those engineers that have read this, after all it is within a couple of orders of magnitude, but physicists are more pedantic?
Posted by MJ.Wells | August 27, 2009 3:17 PM
Posted on August 27, 2009 15:17
Don't forget that the horsepower is defined as 550 foot-pounds per second.
Multiply by seconds to turn power unit to an energy unit, which means that in the imperial system electricity should be supplied by the FOOT-POUND.
2,655,223 foot-pounds make a kilowatt-hour.
So, obviously we need a bit of a larger unit, so how about the mile-ton being equal to 4.45kWh?
Posted by Ian Benton | September 2, 2009 11:26 AM
Posted on September 2, 2009 11:26
Don't forget also, that electrical metering is further confused by the Watt and Volt-Ampere. Watt/Hours are the accumulated sum of the instantaneous product of Volts and Amps which may be negative (for part of the cycle, in an AC circuit). This negative, or reactive, power represents power flowing from the load back to the utility. The Volt-Ampere or "Apparent power" is what one would deduce from a pair of analogue meters measuring average (RMS) current and voltage with no respect to phase.
Domestic users are usually billed in kWh i.e. "real power" whilst an industrial user may well be billed in kVAh and be penalised for the reactive power drawn or kVAr. kVAh is always the same or greater than kWh. I believe the fundamental unit of energy is the erg, which is one ten-millionth of a Joule (or Watt/Second) and is defined in terms of electron-Volts.
Posted by Mike Watson | October 14, 2009 1:15 PM
Posted on October 14, 2009 13:15
I like the unit kWh/year, which is how you often quote household power consumption.
It has the same dimensions as W and I think 1W is 8.76kWh/year (taking a year as exactly 365x24 hours)
So if your power consumption is 4000kWh/year you could quote it as 457W (i.e. your average power consumption over a long period is 457W). You'd spend a lot more time explaining what you meant than if you said 4000kWh/year though...
Posted by Tim Morley | January 21, 2010 2:24 PM
Posted on January 21, 2010 14:24