Enter System Details
The grid uses three columns on large screens, two on smaller, and one on mobile.
Example Data Table
| Scenario | Module (W / Voc / Vmp) | Temps (min/max °C) | Inverter (Vmax / MPPT / A / MPPTs) | Typical Output |
|---|---|---|---|---|
| Sample setup | 550W / 49.5V / 41.6V | -10 / 45 | 1000V / 250–850V / 26A / 2 | ~12–18 modules per string, ~1–2 strings per MPPT |
| Colder site | 550W / 49.5V / 41.6V | -25 / 40 | 1000V / 250–850V / 26A / 2 | Lower max modules per string due to higher cold Voc |
| Hotter site | 550W / 49.5V / 41.6V | 0 / 55 | 1000V / 250–850V / 26A / 2 | Higher minimum modules per string to meet MPPT min |
Formula Used
- Temperature-adjusted voltage (STC at 25°C):
V(T) = V(STC) × [1 + (TC%/100) × (T − 25)] - Max modules in series (conservative):
Nmax = floor( min( Vmax_inverter×margin / Voc_cold, Vmax_MPPT×margin / Vmp_cold ) ) - Min modules in series (hot tracking):
Nmin = ceil( Vmin_MPPT / Vmp_hot ) - Max strings in parallel per MPPT (current-limited):
Smax = floor( Imax_MPPT / (Isc × safety_factor) ) - Estimated DC size:
DC(kW) = (modules_total × module_watt) / 1000 - DC/AC ratio:
Ratio = DC(kW) / inverter_AC(kW)
How to Use This Calculator
- Enter your module datasheet values for Voc, Vmp, Isc, and Imp.
- Enter site minimum and maximum temperatures for your project location.
- Enter the inverter’s max DC voltage, MPPT voltage window, max DC current, and MPPT count.
- Adjust the voltage margin and current factor if your design standard differs.
- Click Calculate Sizing to view the recommended string length and parallel strings.
- Use the download buttons to export results for proposals or records.
This tool provides a practical sizing starting point. Always confirm final design with the specific inverter manual and local electrical code.
Why string sizing affects yield
Correct string length keeps the operating voltage inside the inverter’s tracking window. When voltage is too low, the tracker may drop out and energy is lost during mornings, haze, or heat. When voltage is too high, protection trips can stop production and stress components. Designers typically size strings using site temperatures, module coefficients, and the inverter’s published limits.
Cold-weather voltage headroom
Open-circuit voltage rises as cells get colder. For example, a 49.5 V module with a -0.28%/°C Voc coefficient increases about 9.8% at -10°C versus 25°C. That pushes cold Voc near 54.4 V, so 18 modules would reach roughly 979 V. A margin helps account for measurement tolerance, wiring effects, and datasheet rounding.
Hot-weather MPPT tracking
Voltage at maximum power falls in hot conditions. A 41.6 V Vmp module with a -0.35%/°C coefficient drops about 7.0% at 45°C, to roughly 38.7 V. Strings must still stay above the MPPT minimum, such as 250 V, so a practical minimum might be ceil(250/38.7)=7 modules. This check protects stable tracking during peak summer temperatures.
Parallel strings and current compliance
Parallel strings are usually limited by the tracker’s input current. A common design factor is 1.25× Isc to represent high irradiance conditions. With Isc 13.9 A, the design current becomes 17.4 A. If the MPPT current limit is 26 A, only one string is recommended per tracker because floor(26/17.4)=1. If two strings are needed, confirm inverter allowance or choose a higher-current model.
Interpreting DC/AC ratio results
DC/AC ratio compares array DC nameplate to inverter AC rating and guides clipping expectations. Many commercial designs target about 1.15 to 1.35, balancing inverter loading with annual yield. If your calculated ratio is very low, inverter capacity may be underutilized. If it is very high, clipping and thermal loading increase. Use the result as a planning signal, then validate with production modeling. Seasonal temperature spread, cable losses, and future module swaps can shift voltages, so leave documentation and commissioning checks in the design.
FAQs
How do I choose the site temperatures?
Use local design extremes, not daily averages. Minimum temperature drives the cold Voc limit, and maximum temperature drives the hot Vmp tracking limit. If you lack data, use a conservative regional design value and document the source.
Why use Isc instead of Imp for current checks?
Isc represents the highest potential string current and is commonly used for protective and input-limit checks. Applying a safety factor accounts for high irradiance and measurement tolerance. Imp is useful for performance estimates but less conservative for limits.
What does the voltage margin do?
The margin reduces allowable maximum voltages to provide headroom for tolerances, aging, and wiring effects. A value like 0.97 is a practical default. If your inverter manual specifies a different approach, match that guidance.
Can I mix different module types in one string?
Mixing is generally discouraged because Vmp and current mismatch reduces energy and complicates limits. If mixing is unavoidable, use the worst-case Voc and Vmp values for safety checks and consider separate MPPTs for different module groups.
How many modules per string is “normal”?
It varies by module voltage and inverter limits. Many 1000 V systems land between roughly 12 and 22 modules per string, but cold temperatures and MPPT windows can push the result lower or higher. Always verify with datasheets.
Is the recommended design final for construction?
Treat it as a sizing starting point. Confirm with the specific inverter manual, conductor sizing, string fusing rules, rapid shutdown requirements, and local code. Commissioning measurements should verify real string Voc and tracking behavior on-site.
Quick Disclaimer
Outputs are estimates based on your inputs and simplified checks. Real systems may require additional constraints (string fusing, clipping targets, irradiance assumptions, and manufacturer limits).