Solar PV String Sizing Calculator
Sizes a solar PV string: temperature-corrects the module Voc and Vmp, then finds how many modules fit in series within the inverter's MPPT window and maximum system voltage — with a window-fit diagram and design checks.
Module (datasheet, STC)
Temperatures
Inverter MPPT input
Temperature coefficients are usually negative (voltage falls as it gets hotter). Use the coldest and hottest cell temperatures you expect on site. Everything runs in your browser.
String sizing Valid range found
MPPT window fit
The string is sized so the cold open-circuit voltage (orange) stays left of the max system voltage (red) and the operating range (blue) sits inside the MPPT window (green). A design aid only — verify against the inverter and module datasheets, the wiring rules and a licensed installer.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the module's Voc, Vmp, Imp, Pmax and the temperature coefficients of Voc and Vmp (usually negative %/°C).
- Enter the coldest and hottest cell temperatures you expect on site.
- Enter the inverter's MPPT minimum and maximum voltage, its maximum system voltage and its maximum input current, then read the valid string range and the window-fit diagram.
How it works
Module voltage rises as it gets colder and falls as it gets hotter. The tool corrects Voc to the coldest temperature (the highest voltage the array will ever reach) and Vmp to the hottest temperature (the lowest operating voltage), using Voc·(1 + β·(T − 25)) with the temperature coefficient β in %/°C.
The maximum modules per string is the largest number whose cold open-circuit voltage stays under both the inverter's maximum system voltage and the top of its MPPT window. The minimum is the fewest whose hot maximum-power voltage stays above the bottom of the MPPT window, so the inverter can still track the array on the hottest day.
The maximum number of parallel strings on one MPPT input is the inverter's maximum input current divided by the module's Imp. Multiplying the recommended module count by Pmax gives the string's rated power.
Worked example
Worked example. A 540 W module (Voc 49.5 V, Vmp 41.5 V, βVoc −0.25 %/°C, βVmp −0.29 %/°C) on a 1000 V inverter with a 200–800 V MPPT window, at −5 °C to 70 °C cell temperature: cold Voc rises to 53.2 V and hot Vmp falls to 36.1 V, so a string may run from 6 to 17 modules. At 17 modules the cold open-circuit voltage is ~905 V — safely under the 1000 V limit — for a 9.18 kWp string.
Common mistakes
- Entering the temperature coefficients as positive numbers — for voltage they are almost always negative (voltage drops as temperature rises).
- Using ambient air temperature instead of cell temperature — cells run hotter than the air, so the hottest cell temperature is well above the peak ambient.
- Sizing to the MPPT maximum only and forgetting the absolute maximum system voltage, which the cold Voc must never exceed.
Frequently asked questions
Why does the cold temperature set the maximum string length?
A module's open-circuit voltage rises as it gets colder, so the highest voltage the string will ever reach is at the coldest expected cell temperature. That cold Voc must stay below the inverter's maximum system voltage, which is why cold — not hot — governs the longest permissible string.
What temperature coefficients should I use?
Use the values on the module datasheet: the temperature coefficient of Voc (often around −0.25 to −0.30 %/°C) for the cold-voltage check, and the coefficient of Vmp (or of Pmax) for the hot-voltage check. Both are entered as %/°C.
How many strings can I put on one MPPT?
As many parallel strings as the inverter's maximum input current allows: maximum input current divided by the module's Imp, rounded down. The tool shows this figure alongside the string length.
Related tools
- Solar Panel Count Calculator
- Solar Output Calculator
- Solar Irradiance to kWh Calculator
- Solar Payback Period Calculator
- Solar Charge Controller Size Calculator
- Inverter Sizing Calculator
Explore more in Electrical, Electronics, Solar & Energy.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.