PVUSA rating vs. CEC rating [RE-wrenches]

Bill Brooks billb at endecon.com
Thu Jul 29 13:25:54 PDT 2004


 

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No encouragement here,

It was as a result of the education he received from our PIER website.
People actually read that stuff--I guess it is worth doing.

Bill.

-----Original Message-----
From: Chuck Whitaker [mailto:chuckw at endecon.com]
Sent: Thursday, July 29, 2004 1:14 PM
To: 'Bill Brooks'; RE-wrenches at topica.com
Cc: 'Tim Townsend'; 'Jeff Newmiller'
Subject: RE: PVUSA rating vs. CEC rating [RE-wrenches]


Wow:  Lobbed you a fat one and you crushed it!  Did you tell him to ask you
that question?  If not, how very perceptive of him!

> -----Original Message-----
> From: Bill Brooks [mailto:billb at endecon.com]
> Sent: Thursday, July 29, 2004 10:52 AM
> To: RE-wrenches at topica.com
> Cc: Tim Townsend; Jeff Newmiller; Chuck Whitaker
> Subject: RE: PVUSA rating vs. CEC rating [RE-wrenches]
>
> Jerry,
>
> Now you want to get me in trouble ;-). PVUSA Test Conditions
> are 1000 W/m^2, 20C ambient temp (not cell temperature), and
> 1 m/sec windspeed. Those conditions were developed to test
> the AC power output of a PV system, which takes into account
> all instantaneous losses. This method requires a lot of data
> to perform irradiance, temperature and windspeed correlations
> so it is typically only done on large systems with data loggers.
>
> The CEC process was a compromise between doing a full system
> rating, which is only done after the system has been
> installed (and not so easy), or a computer simulation before
> installation (another option but thought too complicated), or
> just take the STC rating (which is always wrong and misleading).
>
> The upshot is we have a rebate rating number that is based on
> about half to two-thirds of the losses in a PV system--module
> temperature and inverter efficiency (maximum fantasy inverter
> efficiency). So the CEC rating gives you a system number that
> is about 85% of STC module rating and typical systems operate
> in the 70-80% of STC module rating. It also does not account
> for the fact that it is based on manufacturers stated
> performance data that is almost always more favorable than reality.
>
> An instantaneous rating has pros and cons (mostly cons)
> because, unless it is then translated to kWh per year, the
> number is somewhat meaningless. It's like wanting to the gas
> mileage of a car and the dealer keeps telling you how many
> horsepower the car has. A true PVUSA Test Condition AC rating
> can give you a fairly accurate idea of the annual performance
> of a system as long as you properly account for the
> non-instantaneous losses in a system like shading,
> orientation, soiling, and inverter tare losses (if any).
>
> For instance, we are testing system that has a 2500W STC
> array, that has a CEC rating of 2109 Watts and the PTC AC
> rating is 2000 Watts. Using the CEC rating will produce
> estimates that are 5% too high. Other systems have a larger
> separation between rating and reality. To determine how much
> energy the system will produce, you take the annual average
> irradiation (in
> kWh/m^2) for the orientation for the location and reduce that
> by any shading, soiling, and tare losses and multiply that by
> the true PTC AC rating and you will have a very accurate
> annual energy estimate.
>
> Piece of cake-right? Everbody should do this-right? So how
> many people really do an analysis this accurately--not many.
> Many computer programs are on the market today to help do
> this analysis, but garbage in is garbage out--and I see a lot
> of garbage being produced these days.
>
> Bill.
>
> -----Original Message-----
> From: Jerry Caldwell [mailto:solarcowboy at yahoo.com]
> Sent: Wednesday, July 28, 2004 2:04 PM
> To: RE-wrenches at topica.com
> Subject: PVUSA rating vs. CEC rating [RE-wrenches]
>
>
> Bill,
>
> I understand PTC vs. STC ratings for modules but, on the Pier
> Mini Grid website I noticed you distinguish between CEC power
> rating and PVUSA power rating for systems.  Since the CEC
> uses module ratings at PVUSA test conditions, what is the
> difference between the two?  Is it ambient temperature rating
> of the inverter vs. manufacturer's self certified ratings?
>
>
> =====
> Jerry Caldwell
> NABCEP Certified PV Installer
> Light Energy Systems
> 965 Detroit Ave
> Concord, CA
>
> 925-680-4343 office
> 510-205-2798 mobile
>
>
>
>

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