I've never really trusted PVWatts on this. I just did a quick test using my location in the northeast, with all the standard values and then just changing the DC to AC ratio. It's telling me that a ratio of 1.5 is almost exactly the same as a ratio of 1, and that around 1.3 is best. You actually lose about 2% at 0.5 and about 8% production at a ratio of 2.
So I can only conclude that either PVWatts does not have a great calculation, or the efficiency loss due to not running the inverter at rated power is worse than I assumed. Cheers, Dave On Wed, Mar 3, 2021, 6:14 PM Jason Szumlanski < ja...@floridasolardesigngroup.com> wrote: > I've used that document many times when clients question the ratio. I wish > I knew exactly how the PVWATTS ratio calculation worked. I know there's > information on how this is all done, but I don't really have the time or > inclination to study it. I'm wondering if someone knows how accurate It is. > > On Wed, Mar 3, 2021, 4:04 PM Jay <jay.pe...@gmail.com> wrote: > >> Emphase has a white paper showing production vs module size and locations >> around the country. >> >> I don’t know if it’s more accurate than pv watts but you might check it >> out. >> >> Jay >> Peltz power. >> >> On Mar 3, 2021, at 12:13 PM, Jason Szumlanski < >> ja...@floridasolardesigngroup.com> wrote: >> >> >> I'm wondering how accurate the DC to AC ratio is when using PVWATTS. I >> did a quick PVWATTS analysis at my local area using a typical commercial >> roof tilt and found that using a 420W module on an IQ7A versus a 385W >> module on an IQ7+ yields almost identical results. The 420W module has 1% >> less annual production at to the 145% ratio compared to the 132% ratio with >> the 385W module. >> >> Given that, I can't imagine why anyone would use the IQ7A versus the IQ7+ >> on a 208V system. Why incur the additional cost if the AC output current is >> identical for both inverters? >> >> If PVWATTS is right, I guess it's a non-issue in my local area, but it >> may vary in other locations where clipping is more common. The heat here >> results in little clipping at a 132% ratio here, and I guess at 145% the >> morning and afternoon advantages largely outweigh the mid-day clipping >> increase. >> >> Jason Szumlanski >> >> >> >> On Tue, Mar 2, 2021 at 10:00 AM Jason Szumlanski < >> ja...@floridasolardesigngroup.com> wrote: >> >>> I was surprised to learn that the IQ7A is limited to 290 W continuous at >>> 208 volts. That pretty much puts Enphase out of the running for light >>> commercial jobs with new high-powered 72 cell panels. Any thoughts on this? >>> >>> Jason >>> >> _______________________________________________ >> List sponsored by Redwood Alliance >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Change listserver email address & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> There are two list archives for searching. When one doesn't work, try the >> other: >> https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> http://www.re-wrenches.org/etiquette.htm >> >> Check out or update participant bios: >> http://www.members.re-wrenches.org >> >> _______________________________________________ >> List sponsored by Redwood Alliance >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Change listserver email address & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> There are two list archives for searching. When one doesn't work, try the >> other: >> https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> http://www.re-wrenches.org/etiquette.htm >> >> Check out or update participant bios: >> http://www.members.re-wrenches.org >> >> _______________________________________________ > List sponsored by Redwood Alliance > > List Address: RE-wrenches@lists.re-wrenches.org > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > There are two list archives for searching. When one doesn't work, try the > other: > https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > http://www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > http://www.members.re-wrenches.org > >
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