Solar String Sizing Calculator
Panel Specifications
Inverter Specifications
Site Conditions
String Voltage Summary
Wiring solar panels in series is a balancing act: too many panels and the cold-weather open-circuit voltage can exceed the inverter's maximum input; too few and the array drops below the MPPT tracking window on hot days. This calculator applies temperature coefficients to find the worst-case Voc and Vmp, then returns the maximum, minimum, and recommended panels per string plus the parallel-string limit.
Formula
Voc(T) = Voc_stc × (1 + (coeff / 100) × (T − 25)); Nmax = floor(Vdc_max / Voc_cold)
- Voc_stc
- Open-circuit voltage at Standard Test Conditions (25 °C)
- coeff
- Voc temperature coefficient in %/°C (typically negative, e.g. −0.29)
- T
- Cell temperature; the record low is used for the worst-case high Voc
- Vdc_max
- Inverter absolute maximum DC input voltage
- Nmax
- Maximum panels per series string before exceeding Vdc_max
How it works
- Enter the module nameplate values (Voc, Vmp, Isc, Imp, wattage) and its Voc temperature coefficient, then the inverter limits (MPPT minimum and maximum voltage, absolute maximum Vdc, and maximum input current) and your record low and high site temperatures.
- Because the coefficient is negative, the coldest temperature raises Voc to its peak; the engine corrects Voc at the record low and Vmp at the record high to capture both extremes.
- Maximum string length is the inverter's max Vdc divided by the cold Voc (rounded down); minimum length is the MPPT minimum divided by the hot Vmp (rounded up); the recommended length keeps the cold string voltage within the MPPT window, and parallel strings are limited by inverter current over panel Isc.
Worked example
A 370 W module (Voc 40 V, Vmp 31 V, Isc 10 A, coeff −0.29 %/°C) on an inverter with a 200–800 V MPPT window, 600 V max Vdc, and 15 A max input, at a record low of −10 °C and high of 45 °C.
- Cold Voc: 40 × (1 + (−0.29/100) × (−10 − 25)) = 40 × 1.1015 ≈ 44.06 V.
- Hot Vmp: 31 × (1 + (−0.29/100) × (45 − 25)) = 31 × 0.942 ≈ 29.20 V.
- Max string: floor(600 ÷ 44.06) = 13 panels; min string: ceil(200 ÷ 29.20) = 7 panels.
- Parallel strings: floor(15 ÷ 10 Isc) = 1. Cold string voltage at 13 panels: 44.06 × 13 ≈ 572.78 V (under 600 V).
Recommended 13 panels per string (7 minimum), 1 parallel string, array power ≈ 4810 W, cold string Voc ≈ 572.78 V.
Frequently asked questions
- Why is the coldest temperature the critical case for string length?
- A solar cell's open-circuit voltage rises as it gets colder. The highest Voc the array will ever produce happens on the coldest sunny morning, and if that voltage exceeds the inverter's maximum DC rating it can damage the inverter, so string length is limited by the record low temperature.
- Why does the minimum string length depend on the hottest temperature?
- Operating voltage (Vmp) falls as panels heat up. On the hottest day the string must still stay above the inverter's MPPT minimum, or the inverter loses the ability to track the array's maximum power point, so a minimum number of panels is required.
- What limits the number of parallel strings?
- Parallel strings add current, not voltage. The inverter's maximum input current divided by the module short-circuit current (Isc) sets how many strings can be paralleled before the inverter's current rating is exceeded.
- Should I use NEC 690.7 or my own record-low temperature?
- NEC 690.7 allows using either the ASHRAE extreme minimum design temperature or a temperature-coefficient method like this one. Use your site's actual record low (or the ASHRAE value) to be conservative; using a milder figure risks an overvoltage condition the inverter cannot tolerate.