Choosing a panel

As a technology enthusiast, digging into the underlying system specifications was of particular interest to me, both for the panel and inverters. I’m not going to get into detail around the inverter debate, but based on my research I’m solidly in the “micro” side of things, given the type of system I wanted installed here.

Qualifying it that way, is important. Because if we were going to start over from scratch, without a generator, I suspect that a pure DC installation would be more ideal, at least from conceptual level. No one I spoke with tried to sell me a DC system, so I’m not sure it’s even possible to get one here in Illinois. Full DC would only make sense if using local battery storage, with the benefit coming from increased system efficiency as the DC/AC conversion happens once as it leaves the panel+battery ecosystem. As we won’t be installing a battery here, just one conversion will occur in the system as well. In our case at the panel, or more specifically, in the micro-inverter attached to each panel.

Each of the vendors pitched a different panel, so it wasn’t possible for me to directly compare cost-per-panel across the proposals. Interestingly enough, some of the vendors weren’t even that forthcoming around which panels and inverters they were trying to sell me. Everyone did disclose them, after being asked, but it was odd that it wasn’t in the original bid. With the panel models in hand, I found the specifications from the OEM’s website and compared on the efficiency, output, warranty, inverter type (string, micro, or something else), and projected efficiency loss over the duration of the panel’s life.

Superiority-by-the-numbers was not the only factor, with aesthetics playing a small role in the selection process. This is of course a highly subjective metric, and even personally, if we lived in a different area I’m not confident we would have given the black-stealth install the priority that it received.


  • Sunpower X21-350-BLK (selected panel): The X21 isn’t Sunpower’s best panel, that distinction belongs to the X22-360, but it’s still very efficient (21.5%), carries a great, 25 year, physical warranty, and 92% efficiency warranty after 25 years. Unfortunately, Certasun doesn’t sell the X22, so I couldn’t get an apples-to-apples cost comparison, but I think we still would have opted for the X21 for aesthetic reasons (once you go black…) even though the X22 is 10W/0.6% more efficient. That might not seem significant, but 26 panels * 10W = 260W, so ~75% of another panel in the system. I did ask Certasun why they don’t sell the X22, and it basically came down to a lack of demand. Too many of their other clients prefer a uniform black look over the better performing panel for them to bother with it. $1.47/W
  • Sunpower E20-327: The E20 is 20% efficient, doesn’t have the same longevity as the X21/X22 models, but does carry the same 25 year physical warranty as Sunpower’s other panels. To get the same output as the Certasun installation, this vendor was pitching more panels, not all of which were in great locations. It would have been interesting to see where the cost/benefit intersection would have been between the cheaper-per-panel E20 and X21, but with the total system cost higher and suspect placement, I didn’t pursue it. $1.67/W
  • Panasonic N325K: The specs for this panel are pretty good: all black looks, 19.4% efficiency, micro inverters, and a 25 year product and efficiency (90.76% @25 years) warranty. This was a contender, but given the numbers from Sunpower, both in efficiency and cost it didn’t quite make the grade. I also had concerns that the massive panel count proposed by the vendor wasn’t going to play out in reality. $2.26/W
  • AXITEC AC-330 MH/120S: With 18.96% efficiency, 15 year physical / 85% longevity warranty, and string inverter install, this panel was a tough sell. It was also the most expensive proposal because they wanted to install a mind-blowing 44 panels, many in places I know are in the shade. Had we pursued it further, they did note that these were just preliminary counts, and at some point they would stop by and fly a drone around the house to get a realistic count. $1.44/W
  • Silfab SIL-320 BL: A 25 year physical, coupled with a 30 year performance warranty seem like winning points for this panel. But, digging into the details, Silfab only guarantees 82% efficiency after 25 years, but seeing as they started at 18.84% efficiency in the first place, and use string inverters, it would have to be massively cheaper to even be considered. Which wasn’t the case, most likely due again to the need to bump up panel count to hit an output number. I wasn’t able to see placement details because this vendor “doesn’t provide a detailed design” in their proposals; which took them off the list immediately. On the plus side, they do have that “all black” aesthetic. $1.69/W

If the $/W numbers seem low compared to what you’re expecting, it’s because I included the projected Illinois incentive in them. Different states have different incentive programs, so it will vary significantly state-to-state. I was planning to include the number without the Illinois incentive, but some of the proposals didn’t break the federal and state incentives into discrete buckets. It should be possible to figure them out roughly, but getting a pure number wasn’t really the point of this post :).

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4 years ago

We have LG panels for the past 5 years now – also in black – 24 270 W panels. They cover about 85% to 90% of our annual electrical usage. Looks like things have gained 80 – 90 watts in the past five years. Not bad – not jealous at all.

Reply to  Andrew Van Til
4 years ago

That is pretty much what I had been doing until your post – thanks for ruining my PV system for me :).