MPPT sizing calculator - results Online vs Excel sheet

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ulfertg asked Aug 1, '20

MPPT sizing calculator - results Online vs Excel sheet

Hi all,

I'm quite new to PV and still have no clear idea of what I don't know yet.

So the answer to my question might be very obvious ;)

However I'd appreciate any insight you are willing to offer :)

I installed three 120W modules in series on the roof of my RV and the online calculator recommended the MPPT 100/20 for them.

So I went ahead and bought it, not understanding at that time that "20" is the max A to the battery (I belive at 14,4V? - so max output to the battery would be about 290W?).

The MPPT 100/20 charges a LiFePo4 battery, most of the bulk charge happens at 13 to 14V, so 260-280W..

The highest peak power I got out of the panels during the last couple of days was about 270W according to the Victron App. Battery SOC between 20% and 90%.

Conditions: Clear Sky, Germany, about 51° latitude and 7° longitude (I've calculated an airmass of about 1.2, but I could be wrong), module temp about 50-55°C, panels flat. Cables about 6m 4mm² (single length) from the modules to the controller and 1m 4mm² to the LiFePo4.

I'm wondering if the panels are just not delivering their rated power (within this conditions I'd expect to see above 300W) or if the MPPT 100/20 is a bit undersized. Or if the values in the app are simply not accurate ;)

Today I played with the Excel calculator "VE-MPPT-Calc-3_7" and after slightly altering it to show the calculated max current instead of limiting it to the max output current of the charger the diagrams look like this:

To me it looks as if the available power of the panels is limited by the MPPT 100/20 even at high temps.

So now (finally) my question: why is the online calculator recommending the MPPT 100/20?

Is it (for example) normal that panels are far from their rated power so it makes sense to undersize the controller in relation to the specs of the panels?

What am I missing?

Thanks for helping me understand :)

MPPT Controllers mppt charging

screenshot-2020-08-01-at-210925.png (125.1 KiB)

screenshot-2020-08-01-at-214328.png (212.0 KiB)

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snoobler answered · Aug 01, 2020 at 08:06 PM

20A is the limit at any voltage. There are also input limits on panel voltage and panel current.

Panels rarely put put their MAX power. While I can't find the reference, I could swear I've seen Victron commonly recommends over-paneling the charge controller by up to 30% provided you stay within the input limits.

You're over-paneled by about 24%, which is in line with the above.

Given the conditions you describe, 270W sounds really good, but it might be controller limited to some degree. I wouldn't expect the 360W without optimal positioning. My 2970W array rarely puts out over 2700, and I have them positioned at optimal year-round tilt for my latitude - not flat on the ground. If you pointed them at solar South and tilted them to be perpendicular to the sun, then you would likely get very near the actual 360W.

You have slightly more panels than the charge controller can use, IF the panels are performing nearly perfectly. In most real-world situations, the controller is well matched to the panels.

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Answer 1 · 2020-08-06T08:06:26Z

ulfertg answered · Aug 06, 2020 at 08:06 AM

Thank you!

At first it seemed to me that the little difference in price to the MPPT 100/30 wouldn't be worth to sacrifice performance (even though it might only be a little bit), but then again the much smaller form factor of the MPPT 100/20 and its load output is, for my use case, an advantage over the bigger model - so I'll stick with it.

Thanks again for explaining!

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Answer 2 · 2020-08-06T11:55:42Z

launacorp answered · Aug 06, 2020 at 11:55 AM

Think abaout the temparature of the MPPT. I think the maximum output is only below 45°C of the MPPT.

My 100/20 is in my basement/cellar under my garage with a fan blowing air to the fins. On high current the temparature gets to about 38°C to 40°C.

Hot pv panels loose efficeny too.

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