PVUSA rating vs. CEC rating [RE-wrenches]

[Bill Brooks]

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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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