Adding dissimilar panels – do I need a separate MPPT?

[taxesarewaytoohigh]

I was able to find a lot of help on my topic on this forum, especially this really helpful article by Lacho Pop, that was linked from his signature block in this forum. (Unfortunately, I couldn’t find his forum name to tag him and give credit.) But the article rasied more questions for me than it answered, so I’m looking for help.

We’re getting closer to building our 600 sqft off-grid home (finally got the building permit) but I realized recently that when I sized the system, I was looking at the STC rating on the panels, which is the advertised rating. Now that I understand the NOCT rating, I’m thinking I don’t have enough panels. I bought 10 Sirius panels, 410 W Bifacial (Max 533 w).

My plan was to use 9 of them, 3 strings of 3 in series, connected in parallel, and keep one for a spare. Our MPPT is a Schneider Conext 60/150, with a max short circuit current of 60 amps and max PV array rating of 5220 watts. Our inverter is a Schneider XW Pro. (Specs attached.)

Now my thought is to set up the array with 12 panels instead of 9, but the Sirius 410’s are getting hard to find and I don’t want to buy 10 of them. We have an option to buy 4 new (never installed) Renogy 450W bifacial panels for $600 from a local friend. (Specs attached). They are 6” longer, which I don’t think is a huge issue. My initial thought was that because they are a higher wattage, they wouldn’t downgrade the rest of the panels. However, after reading the article I mentioned above, I did the math.

If I add one 450 W Renogy panel to each of the 3 strings (wired in series), using STC figures, I get 1659 watts per string, for a total array wattage of 4977. My original array wattage was 1230 w per string, totaling 3692 watts. We gain 1285 watts out of 3 – 450 watt panels (assuming I did the math right).

This seemed just fine until I read the rest of this article. It says “Different solar panels reduce the effectiveness of the controller to track this optimal power point.”

So – how bad would this be? Would I be making a big mistake? Is it better to buy a separate MPPT and put 4 new Renogy panels on a separate MPPT? If so, does it need to be the same as the one I already have? To buy another Conext 60/150 MPPT is about $600. I'd be spending $1200.

And then to throw out another option – Sirius 415 w panels are nearly identical, from same manufacturer and series, just slightly more efficient with slightly more output in both amps and volts…. and I AM able to get those in a quantity of just 3 from Solex for $915 including shipping. More money – but maybe then I wouldn’t need a different MPPT??

I’m attaching specs for these options and I’m hoping someone with more experience can guide me!
THANK YOU!

 

[SplitUniverse]

assuming the Vmp and Voc are the same then the "power curve" is probably similar and you will not lose a lot from placing a string of them in parallel.
otherwise you will bump into exactly what the MPPT manual warns about.

 

[taxesarewaytoohigh]

Well, the Vmp and the Voc are not at all the same on the renogy panels. Its becoming obvious to me that to add these renogy panels, I'd have to buy another MPPT.

What about getting the 415W panels by the same manufacturer in the same series? The difference in the Voc ratings and Vmp ratings is 1/10th of a volt.

Does anyone know if this is close enough that I can call them "the same" and wire them together without confusing the MPPT?

 

[SplitUniverse]

Grab the manual or spec sheet for both panels and compare the voltage curves. If they are close enough and pretty much the same slope then you shouldn't lose much when paralleling.
The issue with different panels in parallel is that the compounded power curve is not the shape that the MPPT expects so it could easily lock into the wrong "peak" and just sit there while a more productive peak is available.

 

[Solee]

I would definitely use a separate MPPT if you decide to go with the Renogy modules. The reason is that the difference in Voc and Vmp between the Sirius 410W and Renogy 450W modules is around 10%. It’s safer to use the 415W Sirius modules if you plan to run the array on a single MPPT. The difference in both current and voltage between the 410W and 415W Sirius models is negligible, so assuming your entire array faces the same direction, mismatch losses will be minimal.

I’ve learned that the different models listed in a datasheet, for example 405W, 410W, and 415W in the datasheet you've shared, are typically produced on the same manufacturing line. They’re then sorted based on the current and voltage readings measured at the end of production. So, in practical terms, the 410W and 415W Sirius modules are essentially identical. However, this isn’t the case with the Renogy modules, which have more cells than the Sirius modules—hence the higher operating voltage.

 

[AH Trimble]

I recently referb'd and redid 2 of my arrays. Both had mismatched PVs (v & a & w). They were also slightly different angles and compass points. But they were paralleled into the same Victron 150/70 MPPT. Got a burr under my saddle and completely redid them...broke them into totally different arrays with the same PV, same model, ect., although both were different in total watts.
Bought a Victron 100/20 for the smaller array and kept the larger array on the 150/70.
Whoa!!!!!!! Worth the time and minimal cost for the 100/20. I am seeing far better charging from both arrays. Kinda makes sense when you think of it. For me...it was worth it.

 

[ksmithaz1]

It doesn't matter how many panels you have, or think you need. You don't have enough.