PV String Voltage Calculator

A PV string voltage calculator determines the maximum expected open-circuit voltage of a series-connected photovoltaic string at the entered low-temperature condition. Its primary result, String cold Voc, is used to compare the expected DC string voltage against the inverter’s entered maximum-voltage limit before finalizing a string configuration.

PV modules connected in series add voltage. Cold weather increases module open-circuit voltage, so a string that appears acceptable using nameplate Voc at standard test conditions may approach or exceed an inverter’s DC voltage limit at the minimum design temperature. The calculator applies the entered Voc temperature coefficient to determine the cold-corrected voltage of one module, then multiplies that value by Modules in series.

The calculation also produces String Vmp, the series string’s maximum-power voltage at the entered STC module value. String Vmp is useful for preliminary review of the operating voltage available from the array, while String cold Voc is the value used for the entered maximum-voltage comparison on this screen.

Voltage Inputs

Input	Electrical use
Modules in series	Number of PV modules electrically connected in one series string. Series connections increase voltage while string current remains approximately the module current.
Module Voc	Open-circuit voltage of one module at STC, entered in volts. This is the base voltage used for the cold-voltage calculation.
Module Vmp	Maximum-power voltage of one module at STC, entered in volts. This value is multiplied by the number of modules to produce String Vmp.
Voc temperature coefficient	Percentage change in module Voc for each degree C of temperature change. A negative coefficient means Voc rises as module temperature falls below the STC reference temperature.
Minimum design temperature	Lowest temperature used by this voltage screen. The lower the entered temperature, the higher the calculated cold Voc when the coefficient is negative.
STC temperature reference	Reference temperature used with the entered coefficient, commonly 25 C.
Entered inverter maximum voltage	Inverter voltage limit used for the calculator’s comparison and margin result.

The inputs must represent one module model and one intended series-string arrangement. Module Voc, Module Vmp, and Voc temperature coefficient should be taken from the applicable module data supplied for the design condition. A mixed-module string cannot be evaluated reliably by entering values from only one module.

Cold Voc Calculation

The voltage screen first determines the difference between the Minimum design temperature and the STC temperature reference:

\(\displaystyle \Delta T = T_{\text{minimum}} - T_{\text{STC reference}}\)

It then applies the entered percentage-based Voc temperature coefficient to Module Voc:

\(\displaystyle V_{\text{oc,cold module}} = V_{\text{oc,STC}} \times \left[ 1 + \left( \frac{\text{Voc temperature coefficient}}{100} \times \Delta T \right) \right]\)

For a typical negative coefficient and a temperature below the STC reference, both the coefficient and temperature difference are negative. Their product is positive, increasing the calculated cold Voc.

The calculator then calculates string voltage:

\(\displaystyle V_{\text{string cold Voc}} = V_{\text{oc,cold module}} \times \text{Modules in series}\)

String Vmp uses the entered STC maximum-power voltage without a temperature adjustment on this screen:

\(\displaystyle V_{\text{string Vmp}} = V_{\text{mp}} \times \text{Modules in series}\)

The inverter comparison is:

\(\displaystyle \text{Entered inverter voltage margin} = \text{Entered inverter maximum voltage} - \text{String cold Voc}\)

A positive margin indicates that the entered string cold Voc is below the entered inverter maximum voltage. A negative margin indicates that the entered cold-voltage value exceeds it.

Calculation Example

Use the following entered values:

Input	Value
Modules in series	10 modules
Module Voc	49.8 V
Module Vmp	41.5 V
Voc temperature coefficient	-0.29%/C
Minimum design temperature	-10 C
STC temperature reference	25 C
Entered inverter maximum voltage	600 V

The temperature difference is:

\(\displaystyle -10 C - 25 C = -35 C\)

The cold-voltage adjustment is:

\(\displaystyle \frac{-0.29}{100} \times -35 = 0.1015\)

Cold Voc per module is therefore:

\(\displaystyle 49.8 V \times (1 + 0.1015) = 54.8547 V\)

For 10 modules in series:

\(\displaystyle 54.8547 V \times 10 = \text{548.547 V}\)

The screen reports these results:

Result	Value	Use
Cold Voc per module	54.8547 V	Expected open-circuit voltage for one module at the entered minimum design temperature
String cold Voc	548.547 V	Cold-condition series-string voltage compared with the entered inverter maximum voltage
String Vmp	415 V	STC maximum-power voltage for the 10-module series string
Entered inverter voltage margin	51.453 V	Difference between 600 V entered inverter maximum voltage and 548.547 V String cold Voc

The inverter-voltage comparison shows that the entered string voltage is below the entered inverter maximum because 548.547 V is less than 600 V.

Design Use and Review Boundary

String cold Voc is used during preliminary PV string configuration to determine whether the selected number of series modules remains below the inverter’s DC voltage limit at the entered low-temperature condition. It can also help compare alternate module counts before equipment, string layout, DC conductors, raceway routing, and combiner or inverter locations are finalized.

String Vmp supports preliminary operating-voltage review. It does not establish inverter MPPT acceptance, startup behavior, operating temperature performance, production performance, conductor ampacity, conductor AWG or kcmil selection, voltage-drop performance, raceway fill, disconnect ratings, overcurrent protection, or equipment listing compatibility.

This voltage screen uses only the entered module voltage values, Modules in series, Voc temperature coefficient, temperature inputs, and Entered inverter maximum voltage. Final PV design requires verification of the actual module and inverter documentation, the applicable installation conditions, equipment ratings, conductor insulation and terminal ratings, required disconnecting means, and AHJ requirements.