Panel choice for mppt 100/20

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stewart asked Dec 19, '21

Panel choice for mppt 100/20

Hello. Possible for some advice please. I am trying to match solar panels to a Victron mppt 100/20 controller.

Using the mppt calculator it gives me different panels and configurations but I do not know which is the best set up to buy.

Entering the following details into the calculator .. France (Rennes) 24v system 16mm2 cable length 15m The following panels sizes and configurations all recommend the mppt 100/20.

12v mono panels

90w 2s4p =540w

115w 3s2p=575w

140w 2s2p= 560w

175w 2s2p=700w

24v mono panels;

215w 1s3p =645w

305w 1s2p=605w

360w 1s2p=720w

Out of the above what would be the most efficient panel set up for this controller. South facing and with no problems of shade nor placement of panels(barn roof) minus 10C winter summer 34C

24v 280ah lifepo4 battery

Ignoring cost of panels, fittings, cables Etc

I was thinking more watts the better, but then got confused when the datasheet shows nominal PV power 580w but 4 of the results are above this. So now I am lost as to what is best.

Stewart

Solar Panel

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snoobler answered · Dec 19, 2021 at 08:43 PM

Panels don't matter. MPPT limitations do:

100/20 means the series sum of your panel Voc must not exceed 100V, and the MPPT can't charge more than 20A to the battery.

Additionally, you need to allow for cold weather impact on Voc as cold increases it. At 0°C, your panels will be about 7-10% higher Voc then at 25°C.

It's okay to put more than the 560W of panels on the MPPT. Victron recommends no more than 30% above that, AND the PV input current must not exceed that allowed by the MPPT (on the datasheet). This is called over-paneling. Over-paneling looks like this (example shows a 600W array on a 520W controller):

You lose the red due to the over-paneling, but you gain the green.

Personally, I would choose the 360W panels as they produce the most power.

If shading or partial shading is a concern, then the 90W panels would be the most shade resistant due to the 4P configuration.

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Answer 1 · 2021-12-20T07:05:48Z

stewart answered · Dec 20, 2021 at 07:05 AM

Thank you for answering and the detailed explanation. Over panelling is new to me so something new to learn.

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