Kirk, I would recommend you consider using a tool like PVsyst or Helioscope to model DC:AC ratios. A quick modeling of roughly 200 kW on three Solectria 50 kW inverters shows about 1.3% clipping here.
It then comes down to analysis of your costs for additional modules, dc wiring, and inverters. Admittedly I sit on the opposite side of the fence. I try to provide at least a 1.2 ratio to minimize cost per kWh. I am not aware of any large scale studies comparing PVsyst or Helioscope clipping projections to real world performance. I can tell you that our arrays with monitoring and a ~1.2 DC:AC ratio perform as well as our older 1:1 systems. Not that I use Enphase often but they have some research/whitepapers on the topic since their micros tend to undershoot 60 cell module power ratings. Good luck! Matt Matthew Partymiller Solar Energy Solutions LLC (877) 312-7456 m...@solar-energy-solutions.com On Wed, July 13, 2016 3:02 pm, Kirk Herander wrote: > Hello folks, > > > I am composing a response to a question a potential customer asked me. It > seems a competitor is trying to talk him into a 20% larger KW array than > the inverter AC max output rating is. The idea of course is to generate > more power on either side of peak output, but at a cost I don't feel is > justified. My opinion of this particular big-box installer I will keep to > myself. My response to the customer, trying to keep it simple: > > > > "On the DC array input side, most inverters do allow an overload factor. > For > instance, a 10kw AC inverter may allow for 12 kw of DC array as an input. > Whether or not this is a good idea boils down to economics and technical > reasons. > > On a sunny day, the inverter generates power as a typical bell curve. > Power > output rises in the morning, peaks at noon, declines in the afternoon. In > my example, the inverter can't output more than 10 kw AC. What overloading > the input will do will widen the bell curve, i.e. generating more power in > the morning and afternoon, BUT clipping the peak at 10kw on either side of > noon. So there is power to be gained in morning and afternoon, but peak > power is lost(if conditions allow the peak output to be reached), since > the 12 kw array can never be converted to more than 10 kw of AC power. > Depending upon > time of year(ambient / cell temperature) and weather conditions, the peak > may be clipped at 10 kw for several hours a day. So you are both gaining > and losing power using this method. And typically the inverters are only > overloaded in this manner on large-scale farms where the economics are > favorable. > > In your case, if you could actually put 200kw of DC array into 150 Kw of > inverter, the economics would never justify it. That extra 50 kw of array > would cost you $100k of more, and the dollar payback for the power that > extra 50Kw would generate will take 2 - 3x the time that the array size > does that stays inside the output limit of the inverters. This is why I'm > not a big fan of dramatically overloading the inverters, if at all, in > your case. Any KW portion of the array which is above the nameplate kw > rating of the inverter is going to have a longer payback for these > reasons. " > > I feel my reasoning is sound, But I don't want to be too loose with the > facts. Comments are appreciated. Thanks. > > > > Kirk Herander > > > Owner|Principal, VT Solar, LLC > > > Celebrating our 25th Anniversary 1991-2016 > > > www.vermontsolarnow.com > > dba Vermont Solar Engineering > > NABCEPTM 2003 Inaugural Certificant > > > VT RE Incentive Program Partner > > > 802.863.1202 > > > > > > > > > _______________________________________________ > 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 > > > List-Archive: > http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.ht > ml > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > 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 List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
