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--></style><title>Re: [RE-wrenches] kW/MW v.
kWh/MWh</title></head><body>
<div><br></div>
<div>This is one of my pet peeves too...</div>
<div><br></div>
<div>A few snippets and observations from my frequently given
electrical terms presentation:</div>
<div><br></div>
<div>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).</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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).</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>Encouraging electrical terminology literacy, one wrench and
newbie at a time, ;-)</div>
<div><br></div>
<div>Ian</div>
<div><br></div>
<div>At 9:42 AM -0800 1/18/10, Peter Parrish wrote:</div>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">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.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">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).</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">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.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">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.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">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.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">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).</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">Some
examples:</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">(1)</font></tt><font face="Times New Roman" size="-2"
color="#000080"> </font><tt><font size="-1"
color="#000080"> 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).</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">(2)</font></tt><font face="Times New Roman" size="-2"
color="#000080"> </font><tt><font size="-1"
color="#000080"> 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.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">(3)</font></tt><font face="Times New Roman" size="-2"
color="#000080"> </font><tt><font size="-1"
color="#000080"> A peck is one fourth the volume, maybe more
convenient for nuts and berries.</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">(4)</font></tt><font face="Times New Roman" size="-2"
color="#000080"> </font><tt><font size="-1"
color="#000080"> 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. </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080">(5)</font></tt><font face="Times New Roman" size="-2"
color="#000080"> </font><tt><font size="-1"
color="#000080"> 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!</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">Happy
MLK Jr. Day to everyone! NOW what do I do to keep
busy?</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">-
Peter</font></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1" color="#000080">Peter
T. Parrish, Ph.D., President<br>
California Solar Engineering, Inc.<br>
820 Cynthia Ave., Los Angeles, CA 90065<br>
CA Lic. 854779, NABCEP Cert. 031806-26<br>
</font></tt><a href="mailto:peter.parrish@calsolareng.com"><tt><font
size="-2"
color="#000080">peter.parrish@calsolareng.com</font></tt></a><tt><font
size="-1" color="#000080"> <br>
Ph 323-258-8883, Mobile 323-839-6108, Fax
323-258-8885 <span
></span
> <span
></span
> <span
></span
> <span
></span
> <span
></span
> <span
></span
> <span
></span
> <span
></span
> </font
></tt></blockquote>
<blockquote type="cite" cite><tt><font size="-1"
color="#000080"> </font></tt></blockquote>
<blockquote type="cite" cite>
<hr size="2"></blockquote>
<blockquote type="cite" cite><font face="Tahoma"
size="-1"><b>From:</b> re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org]<b> On Behalf Of</b>
William Dorsett<br>
<b>Sent:</b> Sunday, January 17, 2010 4:38 PM<br>
<b>To:</b> RE-wrenches; Dan Fink<br>
<b>Subject:</b> Re: [RE-wrenches] kW/MW v. kWh/MWh</font></blockquote>
<blockquote type="cite" cite><font
face="Times New Roman"> </font></blockquote>
<blockquote type="cite" cite><font face="Times New Roman">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.<br>
<br>
Bill Dorsett<br>
SunwrightS<br>
1715 Leavenworth<br>
Manhattan, KS 66502<br>
Home/Office 785/539=1956<br>
Cell 785/564-2583<br>
wmdorsett@sbcglobal.net<br>
<br>
See Amory Lovins July 08 on Charlie Rose<br>
http://www.charlierose.com/guests/amory-lovins<br>
<br>
--- On<b> Sat, 1/16/10, Dan Fink<i>
<danbob@hughes.net></i></b></font><font face="Times New Roman">
wrote:</font></blockquote>
<blockquote type="cite" cite><font face="Times New Roman"><br>
From: Dan Fink <danbob@hughes.net><br>
Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh<br>
To: "RE-wrenches"
<re-wrenches@lists.re-wrenches.org><br>
Date: Saturday, January 16, 2010, 8:51 PM</font></blockquote>
<blockquote type="cite" cite><font face="Times New Roman">Hello
Marco.<br>
No d'oh!<br>
Your assessment is correct; you can't store kilowatts of power, you
*can* store kilowatt hours of energy.<br>
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.<br>
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.<br>
DAN FINK<br>
<br>
<br>
<br>
Marco Mangelsdorf wrote:<br>
> I'm reading a piece on energy storage in the latest issue of
Distributed Energy mag.</font></blockquote>
<blockquote type="cite" cite><font face="Times New Roman">><br>
> <br>
> The writer, Lyn Corum, repeatedly refers to the capacity of
storage mediums (thermal, compressed air, flywheel, battery) in kW and
MW terms.<br>
><br>
> <br>
> Heck, even my friends at Beacon Power refer to their flywheel
systems in MW terms.<br>
><br>
> <br>
> Am I missing something here? Shouldn't all references to
energy storage be in kWh and MWh terms?<br>
><br>
> <br>
> Or am I experiencing a d'oh! moment?<br>
><br>
> <br>
> Marco<br>
><br>
> ProVision Solar<br>
><br>
> <br>
><br>
>
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