[RE-wrenches] insolation v. actual output

[Peter Parrish]

Hi Marco,

First off, the P-ptc/P-stc ratio is 90% if you have a high-quality module.
>From their things get progressively worse.

IMHO it is like peeling an onion. From the outside it can be reduced to one
simple statement. Then you can investigate some of the simplifying
assumptions and make more sophisticated calculations based “actual data”. I
won’t start with an “equation” but a “rule of thumb”.

In Southern California 20-60 miles east from the ocean, with a roof facing
South and pitched at 5:12, with 3-6” of air space between the roof and the
modules, no shading, a good inverter (weighted 95% efficiency), and modules
with a PTC/STC rating of 90%, and all dc and ac runs using #10 wire...

One (1) kW-stc should produce 4.25 kWh per day, averaged over a year.

Less than optimum orientation, closer to the ocean and say a 3:12 roof
pitch, less than or equal to 3” off the roof, maybe an inverter with a 93%
efficiency, 50’ plus of #12 wire runs


The number drops to 3.9 kWh per kW-dc averaged over a year.

I am surprised how well the two numbers bracket the actual numbers.

If you want an “equation” with a dozen independent parameters that you can
set
ask yourself how many of those parameters you actually have independent
verification and can “plug and chug” with any confidence? 

You can try PVWatts (which takes into account everything you have asked for)
or the EPBB calculator and compare them. 

PVWatts breaks everything down into two steps: (1) the microclimate,
orientation, array type (fixed, or tracking) and P-stc, and (2) inverter
efficiency, ac and dc losses, module mismatch, soiling, shading, etc. I
think that the algorithms behind PVWatts are pretty straightforward, and in
the case of step #2, it is pretty much a series of multiplicative factors.

Let me drop in one question, "If PV mfgr XYZ says that their P-stc numbers
are +0 / -3%, how do you translate that into a "module mismatch" percentage?

But ultimately you have to take into account that a given year’s weather
differs from the 30 year mean by +/- 6% (experts correct me here, and if you
do please introduce the concept of standard deviation), so don’t let your
customer pin you down based on 1 year’s data.

Bottom line: take a close look at PV Watts and consult with the folks at
NREL.

- Peter

 
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

 
________________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Marco
Mangelsdorf
Sent: Tuesday, October 12, 2010 6:17 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] insolation v. actual output

Could someone please provide me with that generally accepted equation when
it comes to estimating AC output from a PV array versus the STC rating?

That is, I’m looking for that equation which estimates the losses due to mod
mismatch, soiling, wire losses, etc., etc.  I’ve got someone who mistakenly
expects their PV array to put out 90 or more percent of the STC rating.

Thanks,
marco