Correct PV string design is essential for the safe and reliable operation of any solar inverter system. Selecting the number of modules based only on total power can result in excessive DC voltage, insufficient operating voltage, input-current violations, reduced energy production, or inverter damage.
A complete design must consider the electrical limits of the inverter, the characteristics of the PV modules, the number of MPPT trackers, the arrangement of the strings, and the expected site temperatures.
What Is MPPT?
MPPT stands for Maximum Power Point Tracking.
The voltage and current produced by a PV module change continuously according to:
- Solar irradiance
- Cell temperature
- Shading
- Module orientation
- Operating conditions
The MPPT circuit inside the inverter adjusts the electrical operating point of the PV array to extract the highest available power.
However, the MPPT can only operate correctly when the PV string voltage and current remain within the inverter’s specified limits.
Important Inverter Voltage Ratings
Several voltage values appear in solar-inverter datasheets. These values perform different functions and must not be confused.
Maximum PV Open-Circuit Voltage
This is the highest DC voltage that the inverter input can safely withstand.
The corrected open-circuit voltage of the complete string must remain below this limit under the coldest expected site conditions.
Exceeding the maximum PV voltage can damage the inverter. It should not be treated as a normal operating or protection threshold.
MPPT Operating Voltage Range
This is the voltage range within which the inverter can actively track the maximum power point of the PV array.
The string operating voltage should remain inside this range during normal operation, including high-temperature conditions when module voltage decreases.
Start-Up Voltage
The start-up voltage is the minimum DC voltage required for the inverter to begin operating.
A string may appear to fall within part of the MPPT range but still fail to start reliably if its available voltage does not reach the required start-up value.
Rated MPPT Voltage
The rated or nominal MPPT voltage represents a preferred operating region for the inverter.
It is not normally a fixed operating voltage, but it can help the designer create a balanced string configuration that supports efficient inverter operation.
Important PV Module Data
The following values should be obtained from the exact PV module datasheet:
- Maximum power, Pmax
- Open-circuit voltage, Voc
- Voltage at maximum power, Vmp
- Short-circuit current, Isc
- Current at maximum power, Imp
- Temperature coefficient of Voc
- Temperature coefficient of Vmp or Pmax
- Maximum series-fuse rating
- Maximum system voltage
The selected values must correspond to the exact module model being used.
Series-Connected PV Modules
When PV modules are connected in series, their voltages are added while the string current remains approximately equal to the current of one module.
String Vmp = Number of modules × Module Vmp
String Voc = Number of modules × Module Voc
For example, if one module has:
- Vmp = 42 V
- Voc = 50 V
- Imp = 13 A
A string containing 10 identical modules will have approximately:
- String Vmp = 420 V
- String Voc = 500 V
- String Imp = 13 A
Adding more modules in series increases the string voltage but does not increase the current of the string.
Parallel-Connected PV Strings
When identical strings are connected in parallel, their currents are added while the voltage remains approximately unchanged.
For example, if each string operates at:
- Vmp = 420 V
- Imp = 13 A
Two identical strings connected in parallel will provide approximately:
- Array operating voltage = 420 V
- Combined operating current = 26 A
The designer must therefore check both voltage and current limits.
Important inverter values include:
- Maximum operating current per MPPT
- Maximum short-circuit current per MPPT
- Maximum number of strings per MPPT
- Number of physical PV inputs
- Number of independent MPPT trackers
Maximum Number of Modules in Series
The maximum string length is normally determined using module Voc under the lowest expected temperature.
PV module voltage rises as temperature decreases. Therefore, multiplying the module Voc at standard test conditions by the number of modules is not sufficient for final design.
The module Voc must first be corrected for the minimum expected site temperature.
The corrected string voltage must satisfy the following condition:
Cold-corrected string Voc < Maximum inverter PV voltage
The final design should also include a suitable engineering margin rather than operating directly at the inverter’s absolute voltage limit.
The temperature coefficient must be applied using the method and units provided by the module manufacturer.
Minimum Number of Modules in Series
The minimum string length is usually determined by reviewing Vmp under the highest expected cell temperature.
As module temperature rises, its operating voltage decreases.
The hot-condition string voltage should remain high enough to:
- Enter the inverter’s MPPT operating range
- Support reliable start-up
- Maintain stable operation during high-temperature conditions
The required condition is generally:
Hot-corrected string Vmp > Minimum MPPT operating voltage
The start-up voltage must also be checked separately where required by the inverter documentation.
A string that is too short may start late, stop early, or operate inefficiently during hot weather.
Why Temperature Correction Is Essential
PV module datasheet values are normally stated at standard test conditions, including a cell temperature of 25°C.
Actual module-cell temperature can be significantly higher than the ambient temperature during operation.
In Cold Conditions
- Voc increases
- The risk of exceeding the inverter’s maximum DC voltage increases
In Hot Conditions
- Vmp decreases
- The string may fall below the MPPT operating range
- Inverter start-up and power production may be affected
The design should therefore be verified at both temperature extremes.
Using only the standard datasheet values without temperature correction can produce an unsafe or unreliable string configuration.
Number of MPPTs Versus Number of PV Inputs
The number of physical PV connectors is not always equal to the number of independent MPPT trackers.
For example, an inverter may include:
- Two independent MPPT trackers
- Two PV inputs connected to each MPPT
- Four physical PV inputs in total
The two inputs assigned to the same MPPT may share the same tracking circuit.
The inverter wiring diagram and datasheet should be reviewed to identify which inputs belong to each MPPT.
Strings connected to the same MPPT should generally have:
- The same number of modules
- The same module type
- The same orientation
- The same tilt angle
- Similar shading conditions
- Similar electrical characteristics
When Should Separate MPPTs Be Used?
Separate MPPT trackers should be considered when PV groups have different operating conditions.
Examples include:
- East-facing and west-facing arrays
- Different roof slopes
- Different tilt angles
- Different numbers of modules per string
- Different module models
- Unequal shading conditions
- Separate building sections
Connecting significantly different strings to the same MPPT can reduce energy production because the tracker attempts to establish one operating point for all connected strings.
String Current Verification
Correct voltage does not automatically mean that the design is acceptable.
The combined current of all strings connected to one MPPT must remain within the inverter limits.
The designer should verify:
- Combined Imp against the maximum operating current
- Combined Isc against the maximum permitted short-circuit current
- Cable ampacity
- Connector ratings
- Isolator ratings
- Fuse requirements
- Surge-protection-device ratings
The inverter may limit excess operating current, but exceeding the specified current or short-circuit-current limits may violate the manufacturer’s requirements and compromise system safety.
Common PV String Design Mistakes
Exceeding the Maximum DC Voltage
This commonly occurs when Voc is calculated only at standard test conditions without considering the voltage increase during cold weather.
Using Too Few Modules in a String
A short string may fall below the MPPT operating range when the modules become hot.
Exceeding the MPPT Input Current
The string voltage may be acceptable while too many parallel strings cause excessive input current.
Confusing Imp and Isc
Imp represents the normal current at the maximum power point.
Isc is used when reviewing short-circuit-current limits, protection devices, cables, and safety margins.
Connecting Unequal Strings to the Same MPPT
For example, connecting a string of eight modules in parallel with a string of ten modules on the same MPPT is generally unsuitable.
Assuming Every Input Has a Separate MPPT
Several physical inputs may be connected internally to the same MPPT tracker.
Ignoring Maximum Permitted PV Power
A configuration may comply with the voltage and current limits while exceeding the maximum PV-array power permitted by the inverter manufacturer.
PV Array Oversizing
It is common for the installed PV-module capacity to exceed the inverter’s rated AC output power.
For example:
- Inverter rated AC output: 20 kW
- Permitted PV-array capacity: 30 kWp
This does not mean that the inverter will produce 30 kW on the AC side.
Oversizing can improve energy production during periods of low irradiance, but it may also produce power clipping when the available PV power exceeds the inverter’s output capability.
Any oversizing must remain within the manufacturer’s limits for:
- Maximum PV-array power
- Maximum PV voltage
- MPPT voltage range
- Maximum operating current
- Maximum short-circuit current
- Number of permitted strings
Practical Design Procedure
A reliable PV string design should follow a clear process.
Step 1: Confirm the Exact Equipment
Obtain the datasheets for the exact inverter and PV module models.
Step 2: Identify Site Temperatures
Determine the minimum expected ambient temperature and the maximum expected module-cell temperature.
Step 3: Calculate Maximum String Length
Correct the module Voc for the minimum temperature and confirm that the complete string remains below the inverter’s maximum PV voltage.
Step 4: Calculate Minimum String Length
Correct the module Vmp for the maximum cell temperature and confirm that the string remains within the MPPT range.
Step 5: Verify Start-Up Performance
Confirm that the available string voltage can satisfy the inverter’s start-up requirements.
Step 6: Verify Current Limits
Check Imp and Isc for every MPPT after applying the required design factors.
Step 7: Review the MPPT Distribution
Group together only strings with compatible orientation, tilt, module count, and shading conditions.
Step 8: Verify Total PV Power
Confirm that the total installed module power is within the inverter manufacturer’s permitted limits.
Step 9: Check the Balance of the Design
Where practical, distribute the PV power evenly across the available MPPT trackers.
Step 10: Document the Final Configuration
Record the calculations, string quantities, module counts, temperature assumptions, current values, and MPPT allocation.
Final Engineering Checklist
Before procurement or installation, confirm that:
- The exact PV module and inverter models are known
- The maximum corrected string Voc is below the inverter limit
- The minimum corrected string Vmp remains inside the MPPT range
- The inverter start-up voltage has been checked
- The current per MPPT is within the permitted operating-current limit
- The short-circuit current is within the inverter limit
- The number of strings per MPPT is acceptable
- Strings connected to the same MPPT are compatible
- Different orientations or shading conditions use separate MPPTs where possible
- The total PV-array power is within the manufacturer’s permitted range
- Cable, connector, isolator, fuse, and SPD ratings are suitable
- Manufacturer-required safety and design factors have been applied
- All assumptions and calculations have been documented
Technical Conclusion
Correct PV string design is not based only on dividing the inverter’s maximum voltage by the module voltage.
The complete design must confirm that:
- Voc remains safe during the coldest conditions
- Vmp remains within the MPPT range during the hottest conditions
- Start-up voltage is available
- Input current and short-circuit current remain within the inverter limits
- Strings sharing one MPPT have compatible characteristics
- The total PV-array capacity complies with the manufacturer’s specifications
A well-designed PV string improves safety, energy production, inverter reliability, and long-term system performance.