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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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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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seb71 avatar image seb71 logic28 commented ·

The more stings in parallel, the bigger the current.

Bigger current means that the wires between the PV array and the MPPT needs to have a bigger cross section area. These wires are the longest wires in a solar system, so any increase in cross sections quickly adds up to your system cost. This is one reason you want more panels in series (in a string) (instead of more strings in parallel), so higher PV array voltage (but as you increase the PV array voltage, it becomes more dangerous when you have to work on it, so there is a tradeoff).


Power = voltage x current

So you can get the same power at low voltage and high current or at high voltage and low current. You have to find the best compromise, but always keep in mind the MPPT voltage limit.


Bigger current also means that the PV array switch disconnectors get more expensive. For large currents, really expensive.


Whenever you have more than two strings in parallel you must fuse all strings (gPV fuses - in fuse holders - are used for this, on each string wire). The reason for this is that due to a failure in one string, the other strings in parallel with it can supply current through the string which has a problem. With two strings in parallel this is very unlikely to be possible (still you can use string fuses even with only 2 strings in parallel), but as you add more strings in parallel, you increase the available current.


All these add up in cost and raise the possibility of catastrophic failure.

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If not all PV panels are oriented the same, split them into separate PV arrays and use a distinct MPPT for each of those PV arrays. Don't try to connect them all to a single MPPT in such cases. Not sure if it applies to you, but I mentioned this just in case.

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Are those 5 new modules the same as the old PV modules?

Avoid mixing different PV modules in the same PV array (at the same MPPT).

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From the initial posts it sounded as you don't have any component yet. If you already have some PV modules and the MPPT, then your options are limited.

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logic28 avatar image logic28 seb71 commented ·

All components and wires were calculated and acquired months ago, of course, having known about the 450/100, perhaps I would have opted for 2 strings of 7 modules (360V) instead of 3 strings of 5 (246v) but the latter is more convenient for me as I have another 5 identical going onto the solar shed and I like to keep everything symmetrical. So, 5 +5 +5 ground mounted and 5 roof-mounted all being identical.

Also, I'm not combining at the ground mount end but instead at the solar shed so the arrangement can be modified at any future time should it become necessary. All strings are individually fused and protected with an Isolator in a consumer unit at the Ground System location and a second set of isolators at the receiving end before they enter the MPPT in the solar shed for convenience during maintenance.

Ground Mount Delivery Box

img-20230530-203040-320.jpg

Solar Shed Receiving Panel (under construction)

img-20230522-174336-817-copy.jpg


All cables (individual lines per string) from PV to the Solar shed are well above the required section for the strings themself which should never exceed 15A (each) and they would become extremely oversized for higher voltages.

The online calculator for the 250/100 would allow up to six such strings where I’m only planning to use four and perhaps, but not for certain, add a second 250/100, 85 or whatever for the additional string of 5 but, as I said, it does not appear to be necessary according to the calculator as each string is designed for an isc 11.12 A and an im of 10.67 so I can hardly see them reaching more than 15A each hence 60A for the four strings which is below the 70A suggested max input amperage according to Alexandra.

screenshot-2023-05-30-at-204830.jpg

I thought I had done my homework correctly but everyone seems to have a different answer to my question which originally was simply " how many amps would the 250/100 be able to accept since on the box only talks about the 100A charging factor".

I shall monitor closely that the PV temperature doesn't reach much below -1/-2ºc or it might just clip the allowed 250v in which case I will rearrange for a 450/100 since all lines are combined at the receiving end.

The ground-mounted 3 strings are facing south and can be adjusted from 28.5º to 76º from summer to winter, with minor shading on one side only in the winter from some hedges.

The 5 strings on the solar shed are at a 6º angle facing west and will catch the sun almost all day with a minor increment from noon to eve, which is the time of the day when we seem to have more sun in this location.

Ideally, this string should run on a separate MPPT as you quite rightly suggested but, again, I do not consider that imperative but rather safer and more efficient.

Of course, if you think that any of the above is wrongly planned, I am anxious to hear what and why but my question about the exact max amperage input of the MPPT 250/100 remains unanswered a part from what I read in a different thread where Alexandra stated 70A

Thanks for all your input.




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seb71 avatar image seb71 logic28 commented ·

3 strings of 5 (246v)

At first glance, this is the main concern. Quite close to 250V. Have you checked the voltage for the lowest recorded temperature at that location?

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logic28 avatar image logic28 seb71 commented ·

246v is the lowest temperature expected in this region and, as I have said above, yes and I shall monitor during the coldest time of the year should it ever go below.

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seb71 avatar image seb71 logic28 commented ·

246v is the lowest temperature expected in this region

Voltage at the lowest temperature, you mean (I presume).

As the temperature gets lower, the panel voltage increases.


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logic28 avatar image logic28 seb71 commented ·
Also according to the calculator
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seb71 avatar image seb71 logic28 commented ·

Of course, if you think that any of the above is wrongly planned, I am anxious to hear what and why but my question about the exact max amperage input of the MPPT 250/100 remains unanswered a part from what I read in a different thread where Alexandra stated 70A

70A is the value listed in the datasheet of your MPPT.

But as I keep telling you, the vital "NEVER EXCEED" is the PV array voltage.


While a PV array with a higer Isc than 70A might damage the MPPT, a PV voltage higher than 250V will damage the MPPT.

Big difference.

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logic28 avatar image logic28 seb71 commented ·

Sure, never brake the voltage rule, am fully aware of that.

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seb71 avatar image seb71 logic28 commented ·

All cables (individual lines per string) from PV to the Solar shed are well above the required section for the strings itself which should never exceed 15A (each) and they would become extremely oversized for higher voltages.

If you are talking about string wires, increasing the string voltage (while keeping the same current, so adding more of the same panels in series) does not make the string wires oversized.

The voltage only matters for the wire insulation, not for the wire cross section area.

Wire cross section area is dictated by the current it must carry (without exceeding a certain temperature).

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logic28 avatar image logic28 seb71 commented ·
If I were to go from strings of 5 to strings of 7 no current changes would occur, just higher voltage which is what I was trying to say. More pv in series in a string would only add voltage, I'm sure we agree on that.
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seb71 avatar image seb71 logic28 commented ·
I was talking about "the wires become oversized" part. Which is not true.
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seb71 avatar image seb71 logic28 commented ·

The ground-mounted 3 strings are facing south and can be adjusted from 28.5º to 76º from summer to winter, with minor shading on one side only in the winter from some hedges.


The 5 strings on the solar shed are at a 6º angle facing west and will catch the sun almost all day with a minor increment from noon to eve, which is the time of the day when we seem to have more sun in this location.

Those need to be put on separate MPPTs.

No point in connecting South facing and West facing panels and with different inclinations to the same MPPT.

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logic28 avatar image logic28 seb71 commented ·
I'm not sure what would happen internally if different amperages were entering the MPPT that is why I said I would consider a second MPPT.

Would it reduce the overall current to the lowest value connected perhaps?

I would like to see a scientific explanation for this and I shall look for it, I should know from school really but that was 55 years ago, lol.

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seb71 avatar image seb71 logic28 commented ·
MPPT is the acronym for Maximum Power Point Tracking.


PV panel arrays oriented and angled in different ways will not have the maximum power at the same point on the current-voltage curve at a certain moment, so the MPPT can't properly adjust.

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logic28 avatar image logic28 seb71 commented ·
In this case, I'll have to understand how the MPPT works internally because, from standard rules, two currents of different values add up to the sum of them, at least in normal circumstances.


If the MPPT ( and excuse me but I am new to this) deals with currents in its own fashion, then yes, my initial idea of adding another 250/100 would come back into play.
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kevgermany avatar image kevgermany ♦♦ logic28 commented ·
There's a steep correlation between PV output and voltage. One set will need the MPPT to maintain a different panel voltage to another due to illumination difference. It compromises, resulting in too low a voltage on some panels, reducing their power, and too high a voltage on the other reducing their power.

It's lose lose.

Forget about the internal workings. Just look at the power/voltage graphs on your existing kit. You'll see small voltage changes with high power changes.

MPPTs have very sophisticated algorithms to manage this.


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klim8skeptic avatar image klim8skeptic ♦ logic28 commented ·

@Logic28 Fronius used to approve of east/west arrays on one mppt.

I dont think they still approve of this now though.

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seb71 avatar image seb71 logic28 commented ·
Can you fit 16 of those panels (instead of 15) on the ground mount?


If not, then can you fit 8 of those panels (instead of 5) on that solar shed roof?


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logic28 avatar image logic28 seb71 commented ·

Unfortunately, 16 pv will not fit as I have already 1 standing vertical in order to fit next to the 14 horizontal.

Cannot fit 2 rows vertically for local regulation.

No 8 modules for the calculator, 7 PVs appear to be the max that the online calculator would allow with the 450/100

The solar shed roof could only accommodate 6 max.

When the new outbuilding will be completed I will have 80m2 west and 80m2 east at my disposal but I doubt that I shall ever need more PV for my consumption, we'll see.

In that case, the 450/200 (or more if there is one) will be the best answer with a second inverter.

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seb71 avatar image seb71 logic28 commented ·
I was not thinking about strings of 8 panels. But about an array of 8 panels.


By "vertical" you mean "portrait" (and not "landscape") panel orientation (terms borrowed from photography)? Or actually vertical?

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logic28 avatar image logic28 seb71 commented ·
I meant portrait of course, in that position when raised to 78º for the winter sun angle, the array would exceed the 2m height permitted.

I could put an application through the local authorities but there is no point when in the future I could use my new building roof and have as many PV modules as money can buy.


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logic28 avatar image logic28 seb71 commented ·

An array or more of 8 panels would require a higher voltage Mppt where in my case I am trying to maximise power using what I have.


My philosophy is that increasing voltage to overcome potential risks is a no-brainer exercise and can be adopted if nothing else works.

I know someone who is using the same Mppt as mine with a 3 strings array of almost identical modules producing higher Voc than mine and even the calculator negates the usage of it.

Here is his situation at a very optimistic min temperature of 0ºcscreenshot-2023-05-31-at-082920.jpg



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seb71 avatar image seb71 logic28 commented ·

An array or more of 8 panels would require a higher voltage

Not necessarily.

An PV array means one string or more strings in parallel.

A string means one or more PV panels (PV modules) in series.

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logic28 avatar image logic28 seb71 commented ·

Sorry I meant a string of more than 8 modules but I see what you mean however given the number 15/14 ground mount I would have to think of a different combination to keep the voltage further away from the max limit. as an instance groups of 2 in parallel in series by 4 (=8) but again I cannot fit 16 modules or that would be ideal with a max voltage of just over 180v. thank you.

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