Hi Jason and All,
Good topic. As far as I know, simple energy estimation programs like PVWatts don’t take inverter size into account when estimating annual production but more robust simulation programs like PVsyst do. I think it is a matter of really being able to trust your simulator to tell the designer how sizing the inverter will affect annual production. Maybe others can chime in on this. We’ve run different scenarios through PVsyst in house to see how under sizing inverters affects the overall annual production. It is surprising for our area (SF Bay Area) that we can really push an inverter with very little reduction in annual simulated production. I’m talking about putting maybe 5 kW (or even a little more) of PV on a 3.8 kW inverter or thereabouts. Array orientation also factors in. To Issac’s point, if keeping the inverter small reduces the initial installation cost significantly while possibly only reducing annual production by a little bit then I feel it is a good design. All that said, we’re still fairly conservative with pushing our inverters. I’ve found that it is very hard to go back to a client after the salesperson has made the sale and try to reduce the inverter size. In other words, we need to nail the inverter size up front during the sales process or the client will oftentimes feel taken advantage of later. It is hard to explain why we are only giving a client a 3.8 kW inverter when we are calling their system 5 kW DC. To Isaac’s questions: most of our clients wouldn’t notice about mild to moderate clipping during peak periods. However, some would. This is why it is important to setup proper expectations about how we designed the system up front and do our homework to assure that the clipping is accurately estimated and factored in to the production estimates. If we can provide clear logical reasoning for sizing the inverter the way we did then we shouldn’t have any problem. I have no idea how array sizing might affect inverter longevity. I think the general points made by others about the declining costs of the modules themselves and increased cost of BOS components means that these types of discussion are very valid. Best, August *From:* RE-wrenches [mailto:[email protected]] *On Behalf Of *Isaac Opalinsky *Sent:* Tuesday, August 26, 2014 6:11 AM *To:* RE-wrenches *Subject:* Re: [RE-wrenches] Inverters Maximum Input Ratings Jason, This seems to be a regular topic of discussion in our training classes not only for microinverters, but also for string inverters. Especially for 3.8 kW units that are optimal for backfeeding 100A service panels and 7.6 kW units that are optimal for 200A panels. A slightly bigger array can give a higher total yield, maybe some power clipping, without the additional cost of a supply-side connection. As long as you stay below the maximum VOC and ISC, there isn’t a safety issue. So it really just boils down to economics and the overall value proposition for the customer, which makes it hard to provide a blanket recommendation. We’ve been training people for years to model PV system performance to determine an acceptable DC/AC ratio on a project-by-project basis. The inverter manufacturers pretty much all claim that there is no concern about overworking or shortening the useful life of their inverters since limiting operating power limits the operating temperature as well, but that leaves me with two questions: 1. Does anyone have any evidence that high DC/AC ratios does/does not shorten the life of the inverter? 2. If there is a small amount of power clipping (say <1% total annual energy), are many customers likely to notice/care? 3. If they do notice, does the customer service aspect of having to defend a design decision outweigh the potential economic benefits of a smaller inverter? *Isaac Opalinsky *| Technical Trainer | *SunPower Corporation* Desk 443-569-3476 | Cell 443-277-6286 *From:* RE-wrenches [mailto:[email protected] <[email protected]>] *On Behalf Of *Jason Szumlanski *Sent:* Monday, August 25, 2014 4:41 PM *To:* RE-wrenches *Subject:* [RE-wrenches] Inverters Maximum Input Ratings I had a tough customer recently that grilled me on how we can put a 270W solar module on a 215W inverter. Fortunately, Enphase has a wonderful white paper on the subject. However, it got me thinking... Enphase has demonstrated that higher output panels in many climates (hot SW Florida included) can benefit from modules that far exceed the inverter rating, and even exceed the inverter's "recommended input" rating. Enphase has shown that 270W+ modules can show energy harvest on the M215 where it makes sense to "oversize" the module. I also received a similar query from a rather uninformed plan reviewer in an area AHJ along similar lines. Fortunately I was within the "recommended input" rating on the spec sheet of 270W with a 265W module, but I wonder what would happen if I had paired the M215 with a 280W module on my plans, which are becoming readily available now in 60 cell modules with 300W modules on the near horizon. I'm pretty sure my plan would have been kicked back for exceeding the manufacturer's recommendation. My question, which applies to string inverters and microinverters, is how much is too much, what would happen if you paired an array that far exceeded the rating, and how do inverter manufacturers determine the recommended and/or maximum rating of the connected module or array? Also, why do some manufacturers have a simple recommendation while others have a "maximum" rating? Jason Szumlanski Fafco Solar
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