Example 1: 10-panel rooftop string
Panel Vmp = 41.2 V, string current = 13.2 A, 10 panels, 45 m run, 4 mm2 cable.
String Vmp = 41.2 x 10 = 412 V
Voltage drop = (2 x 45 x 13.2 x 0.0175) / 4 = 5.20 V
Drop percentage = 5.20 / 412 x 100 = 1.26%
Solar PV Voltage Drop & String Sizer
Estimate total PV string Vmp, long-distance DC voltage drop, and whether the cable run stays within a 1 to 2 percent planning target.
This is a planning tool for solar string sizing and long-distance DC voltage drop. Final PV design should also check Voc, inverter MPPT range, temperature correction, and local PV code rules.
Introduction
Solar installers often need to know whether a long cable run from the roof to the inverter will waste too much energy. Even when the string voltage is high, excessive DC voltage drop can reduce performance and make the PV installation less efficient than expected.
This solar PV voltage drop and string sizer calculator uses the module Vmp, the number of modules in series, the DC string current, the cable size, and the run length to estimate voltage drop in volts and percent. It then compares the result against a typical 1 to 2 percent design target used by many solar designers.
It is useful for long-distance DC voltage drop checks, quick solar string sizing review, and comparing whether a larger cable size may be justified before installation.
Formula
Total string Vmp: Vstring = Vmp x number of panels
DC voltage drop: Vdrop = (2 x L x I x rho) / A
Drop percentage: Drop % = (Vdrop / Vstring) x 100
DC voltage drop: Vdrop = (2 x L x I x rho) / A
Drop percentage: Drop % = (Vdrop / Vstring) x 100
Variable Definitions
| Variable | Meaning | Unit |
|---|---|---|
| Vmp | Module voltage at maximum power | V |
| I | String current at maximum power | A |
| L | One-way cable length | m |
| A | Conductor area | mm2 |
| Vdrop | Estimated total DC voltage drop | V |
Units
Panel Vmp is entered in volts, string current in amperes, length in meters, and conductor size in square millimeters. This calculator uses a copper resistivity planning value of 0.0175 ohm mm2 per meter.
Worked Examples
Example 2: Longer run needing larger cable
Panel Vmp = 38 V, string current = 11 A, 12 panels, 90 m run, 4 mm2 cable.
String Vmp = 456 V
Voltage drop = (2 x 90 x 11 x 0.0175) / 4 = 8.66 V
Drop percentage = 8.66 / 456 x 100 = 1.90%
Practical Notes
This calculator does not replace a full PV design. Real solar string sizing should also review module Voc at low temperature, inverter startup voltage, MPPT operating window, cable temperature rating, grouping factors, and PV installation code rules. Still, this page gives a very useful first answer when deciding if the DC cable run is reasonable or if the string design should be adjusted.