[RE-wrenches] Inverters Maximum Input Ratings

[Isaac Opalinsky]

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:re-wrenches-bounces at lists.re-wrenches.org] 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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