Mac Lewis was nice enough to run some Helioscope simulations for me (thank you, Mac). The numbers were almost identical to PVWATTS in that per kW, you get about 1% less energy yield with a 420W/IQ7A than a 385W/IQ7+ when operating at 208Vac. Given that the IQ7A is more costly, and presumably the higher powered modules will start off at a higher $/w cost, it probably doesn't make sense to use the 420W/IQ7A on a commercial building, but the energy yield differences were much smaller than I anticipated. On the flip side, higher energy density is an obvious choice if maximum utilization of the roof is more important than a small difference in ROI.
I'm still trying to get a definitive reason that Enphase lists the IQ7+ at a constant 290W for 240/208 (1.21A/1.39A output currents, respectively), but the IQ7A is rated at 349W for 240V and 290W for 208V (1.45A current for both output voltages). Jason On Wed, Mar 3, 2021 at 8:36 PM Dave Tedeyan <dtede...@taitem.com> wrote: > 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. 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