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valvetdk avatar image
valvetdk asked

is the calculator wrong, or something i dont know?

Am I mistaken, or is the calculator wrong?

I used the calculator to determine the size of my MPPT. My solar panels have the following specifications, and I used two units of this model in the calculation, resulting in an indicated MPPT of 100/30 for my 24V system:

  • Pmax = 405
  • Open circuit voltage (Voc/V) = 37.15
  • Short Circuit current (Isc/A) = 13.78
  • Vmp = 31.18
  • Imp/A = 12.99

I connected both panels in series, so the formula for my voltage is 37.15 + 37.15 = 74.3, and the current remains the same at 13.78 A. However, when I use the calculator again and input that I want to connect 2 strings in parallel, the calculator suggests upgrading to an MPPT 100/50 model.

But why? Doesn't the voltage remain the same, 74.3 Volt , and the current becomes 13.78 + 13.78 = 27.56 A? Shouldn't I still be able to use my MPPT 100/30 with 2 series + 2 strings, or why does the calculator recommend upgrading to 100/50?"




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klim8skeptic avatar image
klim8skeptic answered ·

@ValvetDK Shouldn't I still be able to use my MPPT 100/30 with 2 series + 2 strings, or why does the calculator recommend upgrading to 100/50?"

It comes down to the array power and mppt output current at battery voltage.

The 100-30 can handle an array of about 880w, with its output current of 30a into a 24v battery. 2 of your panels is 810w, a good fit for the 100-30.

The 100-50 can handle an array of about 1400w, with its output current of 50a into a 24v battery. 4 of your panels is 1620w, a good fit for the 100-50.


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valvetdk avatar image valvetdk commented ·

Oh, so it's not enough to just look at the maximum voltage and current an MPPT can handle; one also needs to take wattage into account, right?

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prein avatar image
prein answered ·

Yes, the 30 amps is not only on the mppt side, but also on the battery side.


That means that with 24V battery you can only charge with 30A= (*24=720 watts)

Nothing breaks when you do connect them, because you stay within spec. But there is much solar energy unharvested

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