[RE-wrenches] kW/MW v. kWh/MWh

Tue Jan 19 07:43:42 PST 2010

This is one of my pet peeves too...

A few snippets and observations from my frequently given electrical 
terms presentation:

If I asked you "How far is it to Vancouver?" and you answered "45 
miles per hour," everyone listening would know that someone was 
confused. But we tolerate and even perpetuate this kind of confusion 
daily with the terms "watt" and "watt-hour", which are directly 
comparable to miles per hour and miles (NOT the reverse). The terms 
themselves are very confusing, since a watt (which might sound like a 
quantity/unit) is shorthand for "joule per second," the rate of 
energy generation, transfer, or use, while a watt-hour is a unit of 
energy (even though it might remind some people of "miles per hour", 
with "hour" in it).

I'm amazed when some long-time RE folks say things like, "Oh, it 
doesn't matter..." when they say "kilowatt" when they mean 
"kilowatt-hour". This sort of ignorance and brush-off just makes our 
job of energy education harder.

I've begun to teach watts and watt-hours FIRST in my presentations, 
instead of the more common order of volts, amps, amp-hours, watts, 
and watt-hours. Watts and watt-hours are the most important measures 
and the most important distinction, and if I lose students after 
that, it's better than losing them before understanding these crucial 
terms.

For the general public, I completely downplay amps and amp-hours, 
which are really an internal measurement perhaps analogous to 
something like cylinder displacement in an engine, in a world where 
people want to know fuel economy and speed. If I ran the circus, we'd 
rate batteries in watt-hours, have watts and watt-hours on every 
consumer RE system gauge, and leave amps and amp-hours to the geeks 
who need them (for wire sizing, for example).

And going down that road a bit, one of the most helpful moments of 
understanding for me was when years ago my guru Bill Beaty said that 
"two things flow in electrical circuits -- charges and energy". Amps 
and amp-hours describe charge flow and quantity, while watts and 
watt-hours describe energy flow and quantity.

Of course, as others have noted, we can talk about generating or 
storage sources in terms of their power (kW) or energy (kWh) 
capacity, but there is such confusion about the terms, it's better to 
be redundantly clear about what we mean, and make sure we're accurate.

Encouraging electrical terminology literacy, one wrench and newbie at 
a time, ;-)

Ian

At 9:42 AM -0800 1/18/10, Peter Parrish wrote:
>Just a pedagogical note. Since I have been teaching PV Installation 
>now fairly continuously since July of this year. The kW/kWh problem 
>is pretty pervasive: I even slip once and a while. Here is the 
>problem as I see it.
>
>Most basic units like mass, distance, energy are simply defined: 
>kilogram, meter and joule. The rates of change are derived: 
>kilometers per hour, meter per second, joule per second (watt).
>
>The basic units related to energy (at least within the electrical 
>utility and related fields) are defined first with the rate of 
>change: watts (or joule per second). Then the other quantities are 
>derived from the watt: watt-hour (energy), watts/meter-squared 
>(insolation) and the like.
>
>Well that's just a fact, and in my classes I drive that point home 
>early and often - so as to minimize confusion down the road. To 
>enliven things a bit, I also add a little history.
>
>In the 18th and 19th centuries, there was a lot of work done trying 
>to understand the nature of and interrelationship between mechanical 
>work, heat and then later electricity. The early work focused on the 
>rate at which work could be done, and concepts like horsepower and 
>later watts were used and compared. An exception to this trend is 
>the unit of heat the British Thermal Unit (BTU) and the rate BTU/hr.
>
>Another interesting fact: since the beginning of time (wo)man has 
>invented various units of measure to assist them in carrying out 
>business. The rule of thumb: invent a unit of measure that can be 
>used for the puposes at hand, that is convenient (gives you the 
>level of accuracy you need) and doesn't use zeros (invented in India 
>in 800 AD) or the decimal point (an even later invention).
>
>Some examples:
>(1)     A cord of wood (128 cu-ft) is rougly the amount that can fit 
>in the back of a wagon (or large pick up truck). In New England, you 
>can sustainably harvest about 1 cord per acre (if I remember 
>correctly). Whether you cut your own wood or order it from someone 
>else, you would probably think for example in terms of 3 or 4 cords 
>of wood (not 3.5 cords).
>(2)     A bushel is a convenient measure of fruit and vegetables, 
>about a cubic foot. If you needed bake a few of pies and have some 
>left over for eating, you might buy a bushel of apples.
>(3)     A peck is one fourth the volume, maybe more convenient for 
>nuts and berries.
>(4)     A hand (4 inches) is used to measure how tall a horse is. An 
>average horse is about 16 hands; a pony 14 hands or less and a big 
>draft horse could be 19-20 hands.  
>(5)     When talking to a customer about their usage per day, the 
>unit of energy kWh is similarly convenient. A small family might use 
>10 kWh/day and a larger family might use 35 kWh/day. In either case 
>1 kWh/day one way or another won't make much difference in designing 
>a system that will eliminate their bill. I must admit that with 
>larger commercial accounts we would be looking at numbers like 150 
>kWh/day and we start using zeros to reflect the fact that we aren't 
>particularly interested in measuring energy usage to a precison of 
>less than 1%. In any event, the kWh is much more convenient than the 
>Joule: there are 3,600,000 Joules in 1 kWh. Of course we could 
>"shorten" the notation to MJ and now we would have a unit of energy 
>measure that fits the criteria: typical usage and the level of 
>precision doesn't rquire zeros or decimal points. So the small 
>household might use 36 MJ/day and the large household 126 MJ/day. 
>Perhaps one day, we could switch from kWh to MJ!
>
>Happy MLK Jr. Day to everyone! NOW what do I do to keep busy?
>
>- Peter
>Peter T. Parrish, Ph.D., President
>California Solar Engineering, Inc.
>820 Cynthia Ave., Los Angeles, CA 90065
>CA Lic. 854779, NABCEP Cert. 031806-26
><mailto:peter.parrish at calsolareng.com>peter.parrish at calsolareng.com 
>Ph 323-258-8883, Mobile 323-839-6108, Fax 
>323-258-8885                                                                                                  
>
>
>From: re-wrenches-bounces at lists.re-wrenches.org 
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of 
>William Dorsett
>Sent: Sunday, January 17, 2010 4:38 PM
>To: RE-wrenches; Dan Fink
>Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh
>
>But you know they keep talking about the potential contribution that 
>plug-in hybrids might have for storing energy for utility support. 
>Most of our systems have hugely larger capacity than current Prius'. 
>And I suspect that many RE system owners will want some backup, 
>especially if they are compensated for installing it.
>
>Bill Dorsett
>SunwrightS
>1715 Leavenworth
>Manhattan, KS 66502
>Home/Office 785/539=1956
>Cell 785/564-2583
>wmdorsett at sbcglobal.net
>
>See Amory Lovins July 08 on Charlie Rose
>http://www.charlierose.com/guests/amory-lovins
>
>--- On Sat, 1/16/10, Dan Fink <danbob at hughes.net> wrote:
>
>From: Dan Fink <danbob at hughes.net>
>Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh
>To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
>Date: Saturday, January 16, 2010, 8:51 PM
>Hello Marco.
>No d'oh!
>Your assessment is correct; you can't store kilowatts of power, you 
>*can* store kilowatt hours of energy.
>Author Lyn Corum might have slipped up a couple times in there and 
>dropped an 'h' from the kw, but maybe not -- because when storing 
>energy on a utility scale, the engineers are greatly concerned with 
>how fast the energy can come in, and how fast they can send it out. 
>So when they refer to a '50 kw flywheel storage unit,' that often 
>means the max power coming in or out at any given instant.
>In our (comparatively) puny end of the energy storage business, 
>batteries, we rarely have to deal with the issue. Assuming our 
>battery bank is big enough to keep the charge or discharge rate 
>below C10, or even C1, we just stack more inverters.
>DAN FINK
>
>
>
>Marco Mangelsdorf wrote:
>>  I'm reading a piece on energy storage in the latest issue of 
>>Distributed Energy mag.
>  >
>> 
>>  The writer, Lyn Corum, repeatedly refers to the capacity of 
>>storage mediums (thermal, compressed air, flywheel, battery) in kW 
>>and MW terms.
>>
>> 
>>  Heck, even my friends at Beacon Power refer to their flywheel 
>>systems in MW terms.
>>
>> 
>>  Am I missing something here?  Shouldn't all references to energy 
>>storage be in kWh and MWh terms?
>>
>> 
>>  Or am I experiencing a d'oh! moment?
>>
>> 
>>  Marco
>>
>>  ProVision Solar
>>
>> 
>>
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