PV String Calculator | Temperature & MPPT Sizing

What This Tool Calculates

This calculator checks whether a proposed PV string — a given module wired in series, a given number of times — is electrically compatible with a chosen inverter. It corrects the module's open-circuit voltage (Voc) and maximum-power voltage (Vmp) to the site's design temperature range, then compares the resulting string voltage against the inverter's maximum DC input voltage and MPPT tracking window, and checks string current against the inverter's per-MPPT current limits.

The output is a clear pass/fail per constraint, plus the recommended modules-per-string window that keeps the string inside every limit at once — not just a single number.

How To Use It

Enter the module's datasheet values: Pmax, Voc, Vmp, Isc, Imp, and its voltage temperature coefficients.
Enter the site's expected minimum and maximum design temperatures.
Enter the inverter's maximum DC voltage, MPPT voltage window, and per-MPPT current limits.
Enter the proposed modules-per-string count and review the pass/fail status for each check.
If a check fails, use the recommended window to find a modules-per-string count that satisfies all of them.

Engineering Principle

A crystalline-silicon module's voltage moves inversely with cell temperature — colder cells produce higher voltage, hotter cells lower voltage. Datasheets state Voc and Vmp at Standard Test Conditions (STC, 25°C cell temperature); actual field temperature is rarely 25°C, so both figures need correcting before they mean anything for design:

V(T) = V(STC) × (1 + (coeff / 100) × (T − 25))

where coeff is the manufacturer's percentage-per-°C voltage temperature coefficient (normally negative). Series-connecting n modules multiplies this corrected voltage by n; current is unaffected by series connection, so it stays at the module's Isc/Imp figure for the whole string.

Calculator

INPUT — ENGINEERING PARAMETERS

DC INPUT — MODULE

MODULE NAME / MODELOPTIONAL

PMAXW

VOCV

VMPV

ISCA

IMPA

VOC TEMP. COEFFICIENT%/°C

VMP TEMP. COEFFICIENT%/°C

STC TEMPERATURE°C

DC INPUT — INVERTER

INVERTER NAME / MODELOPTIONAL

MAX DC VOLTAGEV

MPPT MIN VOLTAGEV

MPPT MAX VOLTAGEV

MAX MPPT / INPUT CURRENTA

MAX SHORT-CIRCUIT CURRENTOPTIONALA

THERMAL & ARRAY CONFIGURATION

MIN DESIGN TEMPERATURE°C

MAX DESIGN TEMPERATURE°C

PROPOSED MODULES / STRING

CALCULATED OUTPUT

SYSTEM STATUSINCOMPLETE

Module power (Pmax) must be greater than 0 W.
Module Voc must be greater than 0 V.
Module Vmp must be greater than 0 V.
Module Isc must be greater than 0 A.
Module Imp must be greater than 0 A.
Voc temperature coefficient is required.
Vmp temperature coefficient is required.
Inverter maximum DC voltage must be greater than 0 V.
Inverter MPPT minimum voltage must be greater than 0 V.
Inverter MPPT maximum voltage must be greater than 0 V.
Inverter maximum MPPT/input current must be greater than 0 A.
Site minimum and maximum design temperatures are required.
Proposed modules per string must be a positive whole number.

Calculations are intended for preliminary engineering assessment only. Final PV string design must be verified against the latest manufacturer datasheets, inverter requirements, applicable standards and actual site design temperatures.

Formulas

V(T) = V(STC) × (1 + (coeff / 100) × (T − 25))Temperature-corrected voltage

Applied separately to Voc (cold-weather check) and Vmp (hot-weather MPPT check).

Vstring = V(T) × modules per stringString voltage

Istring = Imodule (unchanged by series connection)String current

Engineering Notes

This tool performs a preliminary electrical compatibility check only. It does not model shading, soiling, module mismatch, DC cable sizing, or combiner/protection design. Voltage and current limits, along with temperature coefficients, are exactly what you enter — always verify them against the specific module and inverter datasheets for the project, not a generic assumption.

Design minimum and maximum temperatures are project/site-specific (climate data, mounting type, module technology) — this tool does not infer them and does not apply a regional default.

FAQ

Why does cold weather matter for PV string sizing?

A module's open-circuit voltage (Voc) rises as cell temperature falls below the 25°C STC reference. On a cold, clear morning a string can sit well above its nameplate Voc figure — if that corrected voltage exceeds the inverter's maximum DC input voltage, the inverter can trip or, in the worst case, be damaged. String sizing has to check Voc at the site's expected minimum design temperature, not at STC.

Why doesn't string current multiply when modules are wired in series?

In a series string, current is the same through every module — only voltage adds up. Stringing modules in series raises Voc/Vmp proportionally to the module count; the string's operating current stays at the single-module Imp figure. Current only adds when strings are combined in parallel.

What's the difference between MPPT minimum and inverter maximum DC voltage?

Maximum DC voltage is a hard equipment limit — exceeding it is a safety/compliance issue. The MPPT window is the voltage range across which the inverter can actually track the array's maximum power point; a string below the MPPT minimum can still be electrically safe but won't be tracked efficiently, so it's checked separately from the voltage limit.

Is this calculator a substitute for the inverter manufacturer's own string sizing tool?

No — manufacturer tools account for the specific inverter model's full input characteristics and are the reference for final design. This calculator applies the same underlying physics (temperature-corrected Voc/Vmp, MPPT window, current limits) for fast preliminary checks before that detailed design step.