[Greenbuilding] Energy and Power...

Sat Aug 6 15:25:28 CDT 2011

Greenbuilding List:

Further to the interchange below highlighting people's incorrect use 
units for energy and power,
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":

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)
and
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.

I would suggest that it is extremely important to not mix up these units.
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).

A kWh (or preferably joule) is an amount of energy -- it is like distance.

kWh means "thousand times watts times hours", which is a correct energy unit.

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.

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.

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!

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.

I would value anyone's comments on this.

+Gordon Howell
Edmonton

>Please see my replies to your questions, below. You may not comprehend
>all of my answers, but that is all right.
>
> > We've mounted 13,000 watts DC panels, in agreement with the Ontario
> > Power Authority to sell, at max., 10 Kw per hour.
>
>I mean that we have 52 Solar Worldphotovoltaic modules, rated at 250
>watts Direct Current output each, mounted in four strings, feeding two
>SMA 5,000 watt output inverters, and that the Ontario Power Authority
>will buy 10 Kilowatts of electricity, maximum, per hour, produced from
>these modules.
>
> > These sunny summer days, we vary from 75 to 92 Kw per day...
>
>Each day varies, as does the weather, and we typically sell between 75
>kilowatt hours to 92 kilowatt hours per day, to the Ontario Power
>Authority.
>
> > They pay 81 cents per Kw produced, up to a maximum of 10 Kw per hour...
>
>Each day, the Ontario Power Authority pays $0.81 Canadian Dollars for
>each kilowatt that they buy from us. However, they will only pay for 10
>kilowatts per hour, even if we produce more than that. The limit of 10
>kilowatts per hour is a part of the definition of the Ontario microFIT
>distributed generation program.
>
> > This means we produce more full 10Kw each hour than the standard
> > 11,000 watts arrays
>
>I am impressed by your courteous (and slightly humorous) response to 
>what I considered a rather inappropriate email.
>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.
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