MPPT String Sizing Calculator
Make sure the string voltage stays inside the charge controller’s window.
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Your results
MPPT String Sizing Calculator by TheCalcTools
How the MPPT String Sizing Calculator works
Enter the panel’s Voc, Vmp, Isc and Voc temperature coefficient from the datasheet.
Voc rises in cold weather, so the maximum string length uses the cold Voc.
The minimum length keeps the hot-weather Vmp (assumed 15% below nominal) above the MPPT start voltage.
The formula
Worked example
Using the default values shown in the calculator:
| Panel Voc (V) | 49.5 |
|---|---|
| Panel Vmp (V) | 41.5 |
| Panel Isc (A) | 14 |
| Voc temperature coefficient (%/°C) | -0.27 |
| Lowest site temperature (°C) | 0 |
| MPPT maximum input voltage (V) | 600 |
| MPPT minimum operating voltage (V) | 120 |
| MPPT maximum input current (A) | 26 |
- Cold Voc = 49.5 × (1 + (0 − 25) × −0.27%) = 52.84 V.
- Maximum 11 panels in series; minimum about 4; 1 strings in parallel.
Result:
| Cold Voc per panel (V) | 52.84 |
|---|---|
| Maximum panels in series | 11 |
| Minimum panels in series (rough, at Vmp) | 4 |
| Strings in parallel per MPPT (by current) | 1 |
Frequently asked questions
Why use the coldest temperature?
Panel voltage is highest when cold. Exceeding the controller’s maximum input voltage can damage it.
Is the 15% hot derating exact?
No, it is a rough planning figure. Compute it from the Vmp coefficient and the hottest cell temperature for your site.
Sources and further reading
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This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.