Solar String Calculator

A solar string calculator determines the allowable number of PV modules connected in series to one inverter input. The controlling result is the maximum permitted module count: 11 modules per string for the values shown.

PV modules wired in series add voltage. The string must remain below the inverter’s absolute DC voltage limit during the coldest expected condition, while its operating voltage must also fall within the inverter’s MPPT range. These limits are used during preliminary array layout, inverter selection, DC homerun planning, combiner design, and subsequent conductor and voltage-drop review.

The calculator uses:

Input	Value	Electrical purpose
Module Voc	49.5 V	Open-circuit module voltage at STC
Module Vmp	41.2 V	Module voltage at maximum power at STC
Voc temperature coefficient	-0.28%/C	Change in Voc as module temperature changes
Minimum temperature	-10 C	Cold-condition basis for maximum string Voc
STC temperature	25 C	Reference temperature for module voltage data
Inverter max voltage	600 V	Absolute maximum inverter DC input voltage
MPPT minimum voltage	150 V	Lowest inverter voltage for the MPPT operating range
MPPT maximum voltage	480 V	Highest inverter voltage for the MPPT operating range

Cold Module Voc

Module Voc rises as temperature falls when the module’s Voc temperature coefficient is negative. The cold-voltage calculation therefore uses the difference between Minimum temperature and STC temperature.

\(\text{Temperature delta} = \text{Minimum temperature} - \text{STC temperature}\)

\(\text{Cold module Voc} = \text{Module Voc} \times \left(1 + \left( \frac{\text{Voc temperature coefficient}}{100} \times \text{Temperature delta} \right) \right)\)

Using the entered values:

\(\text{Temperature delta} = -10\ C - 25\ C = -35\ C\)

\(\text{Cold module Voc} = 49.5\ V \times \left(1 + \left( \frac{-0.28}{100} \times -35 \right) \right)\)

\(\text{Cold module Voc} = 54.351\ V\)

The result is Cold module Voc: 54.351 V. This is the voltage used to screen the maximum series module count against Inverter max voltage.

Maximum Modules by Voltage

The inverter’s Inverter max voltage is an absolute upper limit. The series string’s cold open-circuit voltage cannot exceed it.

\(\text{Max modules by voltage} = \left\lfloor \frac{\text{Inverter max voltage}} {\text{Cold module Voc}} \right\rfloor\)

\(\text{Max modules by voltage} = \left\lfloor \frac{600\ V}{54.351\ V} \right\rfloor = 11\ \text{modules}\)

Twelve modules would produce a calculated cold open-circuit string voltage of:

\(12 \times 54.351\ V = 652.212\ V\)

That exceeds the 600 V inverter limit. The voltage ceiling therefore permits 11 modules.

MPPT Operating Range

Module Vmp is used to determine whether the series string has enough operating voltage to enter and remain within the inverter’s stated MPPT range.

The minimum module count is:

\(\text{Min modules by MPPT} = \left\lceil \frac{\text{MPPT minimum voltage}} {\text{Module Vmp}} \right\rceil\)

\(\text{Min modules by MPPT} = \left\lceil \frac{150\ V}{41.2\ V} \right\rceil = 4\ \text{modules}\)

Three modules would provide only:

\(3 \times 41.2\ V = 123.6\ V\)

That is below the 150 V MPPT minimum voltage. Four modules provide 164.8 V at the entered Module Vmp value.

The maximum module count allowed by the MPPT upper limit is:

\(\text{Max modules by MPPT} = \left\lfloor \frac{\text{MPPT maximum voltage}} {\text{Module Vmp}} \right\rfloor\)

\(\text{Max modules by MPPT} = \left\lfloor \frac{480\ V}{41.2\ V} \right\rfloor = 11\ \text{modules}\)

The resulting MPPT-compatible range is 4 through 11 modules per string.

Calculation Result

Result	Value
Cold module Voc	54.351 V
Temperature delta	-35 C
Max modules by voltage	11 modules
Min modules by MPPT	4 modules
Max modules by MPPT	11 modules
Recommended max modules	11 modules

Recommended max modules is the lower of the two upper limits: Max modules by voltage and Max modules by MPPT. In this example, both limits are 11 modules.

A design using 11 modules has a calculated STC maximum-power string voltage of:

\(11 \times 41.2\ V = 453.2\ V\)

That is within the entered 150 V to 480 V MPPT range, while its calculated cold open-circuit voltage remains below the 600 V inverter maximum.

Field Verification

This calculation establishes a preliminary series-string voltage range. Final PV design requires verification against the installed equipment and the actual design conditions.

• Use the exact module nameplate and manufacturer electrical data for Module Voc, Module Vmp, and Voc temperature coefficient.
• Use the project’s applicable low-temperature design basis for Minimum temperature rather than a general regional average.
• Confirm the inverter’s published Inverter max voltage, MPPT range, input-current limits, and permitted DC input configuration.
• Review final string quantity and parallel connections separately for inverter input current, combiner or source-circuit configuration, DC disconnect ratings, and available equipment listings.
• Size PV source-circuit and output-circuit conductors separately. The series module count determines voltage; conductor ampacity depends on the applicable PV current calculation, terminal rating, insulation temperature rating, adjustment factor, correction factor, and installation conditions.
• Complete voltage-drop review after the array layout establishes string homerun length, conductor material, AWG or kcmil size, raceway routing, and source-to-inverter path.
• Verify the final installation with the applicable code requirements, equipment instructions, utility requirements where applicable, and the AHJ.