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logic28 avatar image
logic28 asked

MPPT Oversizing & Calculator

I read this thread which would make sense if I didn’t get a different result from the victron mppt online calculator:

“The max short circuit current accepted in is 70A. The reason for this is that sometimes the pv input is short-circuited to prevent charge.

See the tech spec.

General calculations are batt voltage x charging amps + 30% for watts connected in.

48x 100 =4800 x 30% = 6240W ÷ 400W panel size = 15.6 or 16 panels max”

In my case I was going to use 4 identical strings each of a series of 5 x 415w modules into a 250/100 charger and, if I accept the over 130% oversizing, the calculator agrees with that returning an oversizing of 169% and also suggesting that wanting to, I could use a 250/85 allowing me even up to 6 strings of the same nature with an oversizing of 254%.

Instead, following the calculation of the above thread, I would have 48x100 x30% = 6240 divided by 415w panels = 15.036 so I should have a max of 3 strings of 5 pv into one MPPT.


So which one should I follow and what is the real meaning of “oversizing above 130%” in terms of equipment safety”?

Am I to understand that by accepting the “accept oversizing above 130%” option the 250/100 MPPT would simply restrict the max input amperage to 70A as described above in which case one would simply be wasting the remaining energy when available?

Thanks in advance for any reply



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2 Answers
kevgermany avatar image
kevgermany answered ·

The MPPT will not restrict current above the Isc specification. Wire to ensure that you stay below this.

It will restrict output current based on its current rating.

Oversizing more than 30%is ok. It's a rule of thumb, depending what your local conditions are it may be desirable. Really depends on how much power loss you accept.

Watch input voltage at low temps. Multiple panels in series can easily exceed if temps drop a lot.

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logic28 avatar image logic28 commented ·

Thank you, so does that imply that any excess incoming current would not be a danger to the equipment but just be ignored?

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Alexandra avatar image Alexandra ♦ logic28 commented ·
@Logic28


It does not restrict the incoming.


What is restricted is out going. In the case of the 250\100A it is 100A.

Oversize of 130% means you have the installed pv potential to produce 130A at 25°C.

You can oversize as much as you want as long as you don't break the short circuit rating input rule or the input voltage at low temps rule.

The difference between the online and the downloaded calculator is that the online one does not cater for overpanel setup but rather wants you to be use the full installed potential without cutting the peak.


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kevgermany avatar image kevgermany ♦♦ Alexandra ♦ commented ·
@Logic28

To add, by limiting output current, input current is effectively limited. The difference is the measurement point.

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logic28 avatar image logic28 Alexandra ♦ commented ·

Excellent, so the statement I read earlier “The max short circuit current accepted in is 70A. The reason for this is that sometimes the pv input is short-circuited to prevent charge" is incorrect ?

I will search for the downloadable calculator, I didn't know there was one

Thank you once again

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Alexandra avatar image Alexandra ♦ logic28 commented ·

@Logic28

No there is a max short circuit current input limit. each model is different.

Dont break the two input rules/conditions. Isc and max voltage at low temps.

But if you get a config that allows over sizing without breaking the two input rules, then it will limit to the max output amps of the mppt.

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seb71 avatar image
seb71 answered ·

How much you can "oversize" the PV array also depends on the array orientation (azimuth and angle to horizontal). East or West facing PV arrays can be oversized more than South facing arrays (or North facing in Southern Hemisphere). Same if the elevation is not optimal for your latitude. I don't think the calculator accounts for this.


The MPPT maximum PV voltage is a "never exceed in any condition" limit and if you design a system wrong, you can easily go over it without any PV array "oversizing".


Also avoid more than 3 strings in parallel. In such cases (if 3 strings in parallel are not enough), either get a second MPPT and/or a higher PV voltage MPPT.


Start by finding a good "voltage match" between a PV panel (used in a series string) and a certain MPPT.

For instance a PV panel with Voc= 50V is not a good match for a 150V MPPT.

If you connect 3 such PV panels in a string (so in series), you will destroy the MPPT.

If you connect only 2 such panels in a string, the string Voc will be safe for that MPPT, but using a string with Voc = 100V on a 150V MPPT is a waste.

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logic28 avatar image logic28 commented ·

Thanks for your comment,

I wish I could use higher voltage, unfortunately, this system started off for just 15 ground-mounted modules then, with the addition of a solar shed, I have decided to purchase five more modules and take advantage of this extra roof since 3 strings would hardly produce even half of what the 250/100 can cater for as long as I adhere to the voltage rules.

Even though I had already thought of an extra MPPT, the calculator does not complain about 4 strings as per my project so I would like to find more scientific explanations about the statement “avoid more than three strings” which is a very important point.img-20230302-165122-758-copy.jpg

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