What would be a save limit for oversizing a mppt?

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Jan asked Dec 10, '23

What would be a save limit for oversizing a mppt?

At this point it's getting more and more difficult to find solar panels that actually accommodate smaller mppt's as most trustworthy suppliers nowadays mainly supply panels for industrial size installations. I have several 75/15 smart solar models that are used to charge (standby) deep cycle gel batteries (mostly for lights, cctv cameras and internet equipment ea wifi) but can't find any panels that actually come close to the recommended specs as per manual and victron mppt calculator- at this point I'm contemplating just buying a bigger panel and hope for the best.... at what point does oversizing a solar panel become unacceptable?

https://mppt.victronenergy.com#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

I attached a typical calculator example of a widely available solar panel*

MPPT SmartSolar Solar Panel

screenshot-20231210-142046-chrome.jpg (170.3 KiB)

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Answer 1 · 2023-12-10T12:47:18Z

ejrossouw answered · Dec 10, 2023 at 12:47 PM

As long as you don't exceed the e.g. 75V input voltage, you will be fine as the MPPT 75/15 will limit output to it's specified maximum 15A. Oversizing will provide benefits at lower irradiance levels. However, on a 12V system it means you will only get max 220W benefit from the above panel which makes for less than an optimal solution.

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Jan commented · Dec 10, 2023 at 12:59 PM

Considering it's the smallest (trustworthy) panel I can come by- I'm willing to settle for some losses (on power allocation from the pv side).

Thinks for the quick response

Do you think additional cooling would be required as the unit probably would turn the excess power into heat? Any thoughts?

According to the manual, if installed upright- it should be capable of sufficient cooling under normal conditions

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ejrossouw Jan commented · Dec 10, 2023 at 01:19 PM

Graag gedaan. The MPPT is designed for 15A and as long as it is installed as per recommendations and ventilated adequately, it should be fine. The MPPT will only draw the power it requires so there should not be excess e.g. if you have a load requiring only 100W, that is all both panel and MPPT will produce.

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Answer 2 · 2023-12-11T11:40:02Z

Michelle Konzack answered · Dec 11, 2023 at 11:40 AM

Are you using a 12V or 24V battery?

With a 12V battery, the panel you have choosen is perfect with the 42,6Vmp but you will loose the half of the power, because yoz can not charge with more then 220W. Better use 100/30 if you want to use the 455Wp.

With a 24V battery, the 42,6Vmp are a little bit to low, in the winter or if you have somw dark rainy days. But in the summer you can use the full power of the solarpanel.

I use a 24V battery and use 3 panels in series, to have even in estonian winter a little bit energy. 2kWp give me 0,25kWh a day. And with my provisory installed Fronius Primo 8.2-1 with its 8,2kWp i get now around 2kWh a day (the 21 solarpanels are in series and produce nicely energy, but i had to set the output to 4kW because otherwise I would overload the Multiplus-II 24/5000.

Hence in your case, the Vmpp should be 4-8 times higer then the Battery voltage to get in the winter enough energy for your equipement. Hence a 100/50 with two of the solarmodules would work until -30°C and you get up to 730W in the summer and in the winter maybe 40-80W an hour...

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