String Sizing Calculator
Size PV string configurations per NEC 690 with dual voltage correction methods, inverter MPPT compliance checks, and 10 panel and inverter presets — free, no signup.
PV String Sizing Calculator
Enter panel specs, site temperatures, and inverter MPPT window. Get minimum and maximum panels per string, parallel strings, total array power, and NEC compliance status.
| Parameter | Method 1 | Method 2 | Status |
|---|---|---|---|
| Voc at T_min | - | - | - |
| Max String V (at - panels) | - | - | - |
| Vmp at T_cell_max | - | N/A | - |
| Min String V (at - panels) | - | N/A | - |
| String Isc × 1.25 | - | - | |
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What This Tool Covers
The String Sizing Calculator determines how many solar panels to connect in series (per string) and how many strings to connect in parallel for a given inverter. It applies NEC 690 temperature-corrected voltage calculations to ensure the array stays within the inverter's MPPT window and never exceeds its maximum input voltage.
Inputs You Provide
- • Panel Voc, Vmp, Isc, Imp
- • Panel Voc temperature coefficient (%/°C)
- • Site minimum and maximum design temperatures (°C)
- • Inverter MPPT voltage window (Vmin, Vmax)
- • Inverter maximum DC input voltage
- • Inverter maximum input current per MPPT
- • Number of MPPT inputs available
Outputs You Get
- • Minimum panels per string (MPPT lower limit)
- • Maximum panels per string (voltage ceiling)
- • Maximum parallel strings per MPPT input
- • Total array power (kW DC)
- • Temperature-corrected Voc at minimum temp
- • NEC 690 compliance status (pass/fail)
Features
Handles both voltage ceiling (cold temperature Voc) and floor (hot temperature Vmp) constraints simultaneously - the two calculations that most sizing errors miss.
Temperature-Corrected Voc
Applies the NEC 690.7 method: multiplies cold-temperature Voc by the panel's temperature coefficient to find the worst-case open-circuit voltage that the inverter must withstand.
MPPT Window Compliance
Ensures the string's hot-temperature Vmp stays above the inverter's MPPT minimum and cold-temperature Voc stays below the inverter's MPPT maximum.
Multi-MPPT Array Planning
Enter the number of available MPPT inputs and max current per input to determine how many parallel strings each input can accept - useful for multi-orientation roof designs.
How It Works
Four checks run in sequence: voltage ceiling, voltage floor, current limit, and NEC compliance status.
Enter Panel Electrical Specs
Input Voc, Vmp, Isc, Imp from the panel datasheet. Include the Voc temperature coefficient - this is the critical value for NEC 690 cold-weather calculations. It's typically expressed as %/°C or mV/°C.
Enter Site Temperature Range
Input the coldest recorded temperature for the site (NEC design minimum) and the highest expected module temperature for Vmp calculations. ASHRAE 99% design temperatures are the standard source.
Enter Inverter Specs
Input the inverter's absolute maximum DC input voltage, MPPT voltage range (min and max), and maximum input current per MPPT channel from the inverter datasheet.
Calculate String Limits
The tool calculates maximum panels per string (cold Voc × N must not exceed inverter max DC voltage) and minimum panels per string (hot Vmp × N must stay above MPPT minimum).
Review Results & Compliance
The tool returns the valid string length range, maximum parallel strings, total array power, and a pass/fail NEC 690.7 compliance status for the configuration.
Use Cases
Permit Set Documentation
Produce temperature-corrected string voltage calculations required by AHJs for residential and commercial permit submissions. Many inspectors require NEC 690.7 documentation explicitly.
New Panel or Inverter Selection
Quickly check whether a specific panel model is compatible with an inverter before purchasing. Run multiple combinations in minutes to find the configuration that maximizes string length and minimizes combiner boxes.
Cold-Climate String Verification
In northern states where temperatures reach −20°C to −30°C, standard string lengths often push Voc close to inverter limits. Verify compliance before design sign-off.
Calculation Methodology
Follows NEC 690.7 temperature-correction method for Voc and standard MPPT window analysis for Vmp.
Cold-Temp Voc (NEC 690.7)
Voc(cold) = Voc(STC) × [1 + (Tmin − 25) × TempCoeff]
At cold temperatures, Voc rises above STC rating. This is the voltage that must remain below the inverter's absolute maximum DC input voltage.
Max Panels Per String
Max Panels = floor(Inverter Max DC Voltage ÷ Voc(cold))
This is the hard ceiling. String Voc at minimum temperature must never exceed the inverter's absolute maximum input voltage - violation can destroy the inverter.
Min Panels Per String
Min Panels = ceiling(MPPT Vmin ÷ Vmp(hot))
At high temperatures, Vmp drops below STC. If string Vmp falls below MPPT minimum, the inverter loses tracking and production drops to zero.
Max Parallel Strings
Max Strings = floor(Inverter Max Input Current ÷ (Isc × 1.25))
NEC 690.8 requires a 125% safety factor on Isc. The number of parallel strings cannot cause total current to exceed the inverter's rated input current.
Pro Tips
Use ASHRAE 99% Dry Bulb Temperature for Tmin
The ASHRAE 99% heating design temperature is the NEC standard for minimum design temperature. It represents the temperature exceeded 99% of the time - not the absolute record low. Look it up for the project zip code before running the calculation.
Add 25°C to Ambient for Rooftop Module Temperature
Roof-mounted panels run 25–35°C above ambient. Use ambient high + 25°C as your maximum module temperature for Vmp calculations. This is the standard approximation for close-roof-mount installations.
Stay 5–10% Below Max DC Voltage for Safety Margin
Targeting 95% of the inverter's maximum input voltage gives a safety buffer for record cold years or mismatch in temperature data. Many experienced designers won't go above 600V on a 600V inverter even if the math technically allows it.
Verify Combiner Box Requirements for Parallel Strings
When more than two strings connect in parallel to one MPPT input, you typically need a DC combiner box with fusing. Single-string and two-string inputs may not require fusing per NEC 690.9, but check your specific configuration.
Frequently Asked Questions
What happens if string voltage exceeds inverter maximum?
The inverter's input protection may trip, permanently damaging internal components. In the worst case, it voids the inverter warranty. NEC 690.7 exists specifically to prevent this - cold-weather Voc can be 10–25% higher than the STC datasheet value, which is why temperature correction is mandatory.
What is an MPPT input?
Maximum Power Point Tracking (MPPT) is the algorithm inverters use to continuously extract peak power from a PV string as voltage and current change with sunlight and temperature. Each MPPT input can accept one or more strings within its voltage and current limits. Multi-MPPT inverters allow different roof orientations to run independently.
Can I mix different panel models on the same string?
Technically possible but strongly discouraged. Mismatched Vmp values cause the MPPT algorithm to operate at a compromise point, reducing production from both modules. Different Isc values in parallel strings can cause reverse current issues. Use the same panel model throughout a string.
What is the difference between Voc and Vmp?
Voc (open-circuit voltage) is the panel voltage with no load connected - the maximum voltage it can produce. Vmp (voltage at maximum power) is the voltage at which the panel produces peak power under MPPT control. Voc is typically 20–25% higher than Vmp. Both are listed on panel datasheets at STC (25°C, 1000 W/m²).
Does string sizing apply to microinverter systems?
No. Microinverters attach to individual panels - there are no DC strings to size. String sizing is relevant for string inverters and power optimizers paired with a string inverter. For microinverter systems, the relevant calculation is AC circuit sizing, not DC string voltage.
What NEC article governs string sizing?
NEC Article 690 covers solar photovoltaic systems. Specifically, Section 690.7 covers maximum voltage calculations (including the temperature-correction requirement), Section 690.8 covers circuit sizing and current calculations, and Section 690.9 covers overcurrent protection for PV source circuits.
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