If the PTC is calculated from the STC and not the results of testing at "PVUSA Test Conditions", then it's of somewhat dubious utility as a predictor of performance, since it's a function of an already questionable quantity - the STC.
What the PTC and STC _should_ measure is well-defined: "PTC refers to PVUSA Test Conditions, which were developed to test and compare PV systems as part of the PVUSA (Photovoltaics for Utility Scale Applications) project. PTC are 1,000 Watts per square meter solar irradiance, 20 degrees C air temperature, and wind speed of 1 meter per second at 10 meters above ground level. PV manufacturers use Standard Test Conditions, or STC, to rate their PV products. STC are 1,000 Watts per square meter solar irradiance, 25 degrees C cell temperature, air mass equal to 1.5, and ASTM G173-03 standard spectrum." - http://www.gosolarcalifornia.org/equipment/pvmodule.php Most explanations imply that the STC represents what each module rated that way will test, but any group of same model modules will have a distribution of results. So what does the number reported by a manufacturer represent? The median of their measurements? The average? The target? The very best they ever measured? What are the tolerances? When you get modules rated 200 watts STC, are you getting modules that range between 180 and 220 watts or modules that range between 180 and 200 or ? Are the variations in power because of differences in voltage or current? I suspect current. If modules are in strings, the string is going to be limited by the worst performing module. The California CSI program is very concerned about maximizing PV production, and so requires installers to very accurately measure slope, azimuth, shading, etc. requiring accuracy to 2% for some measurements, yet there is this huge loophole of accountability: The manufacturers are testing every module, but they don't share results, and don't give distribution of testing as compared to STC. The warranties differ greatly between module brands. The company that most "pushes the limits" will appear to be delivering more power, while the company that has the best process control, and delivers the most real watts per dollar may seem more expensive. Example: Module A, STC 200 watts, PTC 175 watts, Power warranted to be at Pmin which is -7% of Pmax, which is -5% of STC. So module could test at 176.7w. So PTC could really be 154.6w. Module B, STC 200 watts, PTC 175 watts, Power warranted to be -0% STC at delivery. So PTC might really be 175 watts. So if A is priced at 5% less than B, it seems like a better deal. But power delivered is 12% less. > From: David Katz <[EMAIL PROTECTED]> > Subject: Re: [RE-wrenches] CEC module rating testing > Jerry, > The CEC formula for PTC uses the NOCT of the module, the STC rating, the > power temperature coefficient and the aperature area of the module. > The data is supplied by the manufacturer but must make sense. Starting > July 1, 2009, this will have to be certified by an acredited testing lab. > Let me know if you want the exact formula and i will send it to you. i > can't attach anything to this email > David > > David Katz > > Jerry Caldwell wrote: >> Wrenches, >> >> Can anyone tell how PTC ratings are determined for modules on the CEC >> list of eligible modules? I seem to recall having heard that the >> manufacturers provide this data on the honor system. Can anyone >> verify this? >> >> The test is obviously more involved than flash testing at STC. Even >> though PTC is supposed to simulate more real-world conditions, it must >> still require a controlled environment. How exactly is this done, and >> how long does it take? >> >> Best, >> Jerry Caldwell >> Recurrent Energy _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
