yamezz avatar image
yamezz asked

MPPT Sizing Calculator

I am getting confused using both the online Sizing Calculator and Excel. I have four (4) panels with the following specifications.

325 Watts

Voc = 36

Vmpp = 30

Isc = 11.88

Impp = 10.82

Can anyone help me determine which controller(s) I need and in which configuration I need to wire the panels? I have 430Ah of 12V AGM batteries to connect. Thanks.

MPPT Controllers
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2 Answers
wkirby avatar image
wkirby answered ·

So you have 375W x 4 = 1.5KW worth of max. PV power.
You have a 12V battery bank. 1500W / 12V = 125A of charge current at full PV power.
A battery system Voltage of 12V brings some limitations which we'll go into below.

The largest charge controller available is a 100A one. The 150/100 would work, but it will not harvest the full potential of your array because its maximum charge power is 100A (1200W)
As your battery Voltage increases with charge, you'll get 1480W of charge power, which is very close to your maximum PV output, but this will only happen close to the top of the charge cycle and when the battery is at the Absorb stage,
This is not normally a problem, just a loss of potential PV power.
Wire your panels with 2 Series strings in parallel.

If you are really concerned about getting every last drop of PV energy then you'll need to consider two charger controllers. 2x 150/60 will get you there with full power available after 12.5V which is realistic.
Wire these two controllers up with one series string each.

If you are even remotely thinking about expanding your PV power in the future, plan for this now and get bigger charge controllers if you can afford it.

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nigelfxs avatar image nigelfxs commented ·

It is not clear how many 12V batteries make up the total 430Ah capacity. If it is an even number of batteries (e.g. 2 x 215Ah), then it may be better to connect these in series, assuming that is compatible with the rest of the system. This will halve the current so will be well within the range of a single MPPT.

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yamezz avatar image yamezz nigelfxs commented ·

Nigel, Thanks for your advice. I see now, the higher battery voltage would have been better. Unfortunately I have wired everything in my shed for 12V already. Live and learn, hey.

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yamezz avatar image yamezz commented ·

My current setup has 400W of panels, so this is my 'future upgrade'.

When looking at the sizing calculator, I had thought I'd wire up 2 x 2 panels, which, according to the spec label, would get me two strings of 650W, with max voltage of 72V and max current of 11.88A. With these numbers I thought two 75/15 controllers would be sufficient, but I now understand the current rating on the controller refers to the 12V battery side.

The panels will rarely receive full power all at once and the increase from 400W to 1,300W was planned to generously exceed any requirement I currently have , so I'm not too concerned with a little clipping, but as you say, bigger is better.


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nigelfxs avatar image nigelfxs yamezz commented ·

Take care not to exceed the maximum voltage of the MPPT 75/15. The MPPT will safely limit the maximum current but will be damaged if the input voltage is exceeded.

The open-circuit voltage (Voc) of the panels is usually specified at 25C and increases as the temperature drops - typically by 0.33% per degree Celsius. Two panels in series will therefore have a Voc of around 78V at 0 Celsius, which will almost certainly damage your MPPT.

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yamezz avatar image yamezz nigelfxs commented ·

Thanks Nigel. I'll look at 100V models. It could have been an expensive lesson if not for your advice.

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Pat Davitt avatar image
Pat Davitt answered ·

When the PV Array size gets over about 1200W I always recommend that one take a look at increasing battery voltage to 24V, or preferably 48V. Especially when their are no DC loads in the system. Many advantages and very few disadvantages.

If you do have 12VDC loads, it is not a big deal to install a DC/DC converter to power those loads. There is some efficiency loss to take into consideration. Victron makes some. Below is the spec sheet for a industrial grade 60A converter that accepts 19-72VDC and outputs a pot adjustable 11-15VDC. (I have mine set at 13.2VDC.)


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yamezz avatar image yamezz commented ·

Thanks Pat. Unfortunately all the loads in my of-grid shed are already wired for 12V, including the inverter that runs power tools and a microwave. If I was to start again, I think you're right - the higher voltage system would be better.

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