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That's a good presentation/PDF. I suggest anyone who isn't
completely clear about energy vs. power to take a look at it.<br>
<br>
I also suggest that part of the kWh / kW per hour confusion is
based on how we see speed measured. Our speedometers says MPH not
M/H, even though 'per' means divided by. We are familiar with
speed, but less so with invisible electricity flowing in wires.
When people see KWH, perhaps the more familar MPH comes to mind
eventually leading to other confusion as the meaning of P as 'per'
evades them.<br>
<br>
<br>
A few notes related to the presentation:<br>
<br>
1.) an analogy of energy vs power is the water vs water flow (you
did touch on this in the presentation). More specifically, water
used (gallons or cubic feet) vs how quickly it is flowing (gallons
per minute or cubic feet per second). Filling a swimming pool takes
a certain number of gallons of water, this is analogous to the
number of kWh or Joules used. The water company charges for how
much water is used and likewise the electric company charges for how
much electricity (usually measured in kWh) is used. Filling the
swimming pool slowly with a garden hose (fewer gallons per minute)
instead of a fire hose will not change the gallons of water it takes
to fill the pool. Using electricity at a faster (higher watts( or
slower rate (lower watts) will not change the total amount of
electricity used to complete a specific task, all else being equal.
(A 1500 watt electric electric tea kettle will use the same amount
of electrical energy to heat 1/2 gallon of water to 190 degrees as a
750 watt tea kettle will use to heat 1/2 gallon of water to 190
degrees. The 1500 watt kettle will do the job in 1/2 the time, but
the energy used is the same*.)<br>
<br>
<br>
2.) The following caveat mights confuse some people. Be sure to
understand the more basic concepts in the presentation and above
before trying to make sense of the caveats.<br>
<br>
2a.) unlike most residential electric customers, some commercial and
industrial electric customers (mostly those with large capacity 3
phase service) pay both an energy charge and a power demand charge.
The power demand charge is intended to account for the additional
infrastructure (bigger wires and transformers) needed to support
uneven, bursty loads compared to even loads. In the previous
analogy, the water company could charge extra for filling the pool
with a fire hose rather than a garden hose because bigger pipes need
to be installed in the distribution system to support a fire hose
than to support a garden hose. (Water companies do this indirectly
by charging different meter fees based on the size of your water
meter (3/4" vs 1" vs 1.5" etc).)<br>
<br>
2b.) electric company power demand fees are a way to manage customer
power usage. Power demand fees can make big on-demand appliances
(like instant electric water heaters) unattractive to use from a
cost perspective. (An analogy is being charged extra to use fire
hose instead of garden hose to fill a swimming pool, because the
fire hose can deliver the same amount of water in less time.)<br>
<br>
2c.) electric company time-of-use (TOU) rates are a way to manage
customer energy consumption at different times of day. TOU rates
make use of devices during expensive times of day unattractive from
a cost perspective. TOU energy rates are like variable tolls on
toll roads...with extra tolls for the express lanes during rush hour
and deep discounts during the off peak hours.<br>
<br>
2d.) *second order effects can result in differences between how
much energy is used even though mathematically one might expect it
to be the same. One of these are the I^2R (current squared x
resistance) losses in the wires supplying the load. Doubling the
current will quadruple the losses in a wire. These losses are used
chose proper minimum wire sizes for various electric loads. I^2R
losses become dramatically more significant during peak power demand
and peak energy times of day, which is one reason peak shaving, load
shifting, and demand control switches on air conditioners and water
heaters is important. I^2R losses also factor into energy prices.<br>
<br>
---<br>
<br>
Another set of items that are often confused are data storage
capacity (bytes) and communications bandwidth (bits per second) and
latency (delay, lag). Data storage capacity is measured in
bytes/kilobytes(kB)/megabytes(MB)/Gigabytes (GB) etc. Bandwidth is
usually measured in bits per second. 8 bits = 1 byte, but there is
overhead/retransmissions that makes 10 bits = 1 byte a better
approximation for communication bandwidth calculations. Latency is
a measure of delay, lag.<br>
<br>
In an analogy to energy and power, here is how data and power
networks line up:<br>
Data storage capacity (bytes used or moved) (5 gigabytes = 5000
megabytes) ~= energy (kWh used)<br>
Max communications bandwidth (50 Mbps = 5 megabytes/second) ~=
maximum power available (service drop size ... 240 volts x 200 amps
= 48 kW service)<br>
Latency (36 milliseconds to transfer a piece of data from Google's
homepage) ~= seconds to minutes to hours delay in bringing up
standby power plants; years delay in constructing new power plants.<br>
<br>
Low latency in responding to demand allows power networks to more
accurately compensate for source variability. Buffering (temporary
short term storage) in the form of batteries, flywheels, and
hydroelectric/pumped storage hydroelectric can bridge the gaps
between sources with high variability and backup sources with high
latency/delays in starting up to replace the high variability
sources.<br>
<br>
bandwidth vs latency (which latency matters a lot in how well
communications systems work)<br>
<a class="moz-txt-link-freetext" href="http://www.stuartcheshire.org/rants/Latency.html">http://www.stuartcheshire.org/rants/Latency.html</a><br>
<br>
<br>
<br>
<br>
On 8/6/2011 3:25 PM, Gordon Howell -- Howell Mayhew Engineering
wrote:
<blockquote
cite="mid:smtpd.4a31.4e3da690.ec0e3.1@mtain-a.tc.umn.edu"
type="cite">
Greenbuilding List:<br>
<br>
Further to the interchange below highlighting people's incorrect
use
units for energy and power, <br>
though it may seem reasonable on the surface to say "almost every
Canadian on this list (including myself) and, I suspect, most
others in
North America and around the world either know or have the
capacity to
figure out what your original message meant":<br>
<br>
it is the same as mixing up speed (km/h) and distance (km), or
volume
(litres) and flow (litres per second) (which we would never do and
would
be disparaged by society if we did)<br>
and <br>
then we get these same people who "apparently" understand how
to "figure it out" then making huge calculating errors when the
look at solar PV economics or sizing (as I've seen many times)
because
they don't fundamentally know the difference between energy and
power.<br>
<br>
<b><u>I would suggest that it is extremely important to not mix up
these
units.<br>
</u></b>A watt is a rate of energy flow -- it is like speed,
which is a
rate of distance "flowing". A watt means a joule per
second. Whenever you use "W" think of "joule per
second"... so a 230 W solar PV module generates 230 joules of
electrical energy per second (at rated solar conditions of 1000
joules
per second per m2).<br>
<br>
A kWh (or preferably joule) is an amount of energy -- it is like
distance.<br>
<br>
kWh means "thousand times watts times hours", which is a
correct energy unit.<br>
<br>
kW/day means "thousand times watts per day", means
"thousands x joules per second per day" -- it is a unit of
energy-production acceleration. It is never used on a small scale
as in kW or days. The electric utilities and regulators use
MW/hour
units, because it speaks of the rate at which generators can ramp
up or
down (which is an acceleration) the rate (MW or millions of joules
per
second) at which they can produce energy. We also use MW/year to
describe the capacity of a PV factory, because it also implies the
acceleration of PV energy generation in the world due to that
factory.<br>
<br>
So I made the attached PowerPoint presentation to help (I hope)
people
understand the difference between the two and the importance for
not
mixing them up.<br>
<br>
I have to say that when I see people mixing up energy and power
units,
then immediately my mind starts to discredit them and their
professional
expertise... and I've even seen engineers, electric regulators and
electric utilities mix them up!<br>
<br>
I am quite fascinated that peoples' mixup in all this
fundamentally
arises because we have non-metric time, and this is due to the
Sumerians
of some 4000 years ago who gave us the sexagecimal system, which
we use
for hours, degrees, minutes and seconds.<br>
<br>
I would value anyone's comments on this.<br>
<br>
+Gordon Howell<br>
Edmonton<br>
<br>
<br>
<br>
<blockquote type="cite" class="cite" cite="">Please see my replies
to your
questions, below. You may not comprehend <br>
all of my answers, but that is all right.<br>
<br>
> We've mounted 13,000 watts DC panels, in agreement with the
Ontario
<br>
> Power Authority to sell, at max., 10 Kw per hour.<br>
<br>
I mean that we have 52 Solar Worldphotovoltaic modules, rated at
250
<br>
watts Direct Current output each, mounted in four strings,
feeding two
<br>
SMA 5,000 watt output inverters, and that the Ontario Power
Authority
<br>
will buy 10 Kilowatts of electricity, maximum, per hour,
produced from
<br>
these modules.<br>
<br>
> These sunny summer days, we vary from 75 to 92 Kw per
day...<br>
<br>
Each day varies, as does the weather, and we typically sell
between 75
<br>
kilowatt hours to 92 kilowatt hours per day, to the Ontario
Power <br>
Authority.<br>
<br>
> They pay 81 cents per Kw produced, up to a maximum of 10 Kw
per
hour...<br>
<br>
Each day, the Ontario Power Authority pays $0.81 Canadian
Dollars for
<br>
each kilowatt that they buy from us. However, they will only pay
for 10
<br>
kilowatts per hour, even if we produce more than that. The limit
of 10
<br>
kilowatts per hour is a part of the definition of the Ontario
microFIT
<br>
distributed generation program.<br>
<br>
> This means we produce more full 10Kw each hour than the
standard
<br>
> 11,000 watts arrays<br>
<br>
I am impressed by your courteous (and slightly humorous)
response to what
I considered a rather inappropriate email.<br>
Almost every Canadian on this list (including myself) and, I
suspect,
most others in North America and around the world either know or
have the
capacity to figure out what your original message
meant.</blockquote>
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