Enter module and inverter data
The form uses a single page flow. On large screens inputs appear in three columns, then two, then one on smaller devices.
Example data table
| Example | Voc STC | Vmp STC | Min Temp | Max Temp | Inverter Max DC | MPPT Window | Likely Safe Series Range |
|---|---|---|---|---|---|---|---|
| Commercial rooftop | 49.5 V | 41.3 V | -10 °C | 45 °C | 1000 V | 320-800 V | 9-17 modules |
| Warm climate site | 46.8 V | 38.9 V | 5 °C | 50 °C | 1100 V | 300-850 V | 8-18 modules |
| Cool utility block | 52.1 V | 43.4 V | -20 °C | 40 °C | 1500 V | 500-1300 V | 13-24 modules |
Formula used
String sizing must satisfy both voltage and current limits under realistic site temperatures. This calculator corrects module voltage and current values using the entered temperature coefficients.
| Calculation | Formula |
|---|---|
| Cold corrected open-circuit voltage | Voc,cold = Voc,STC × [1 + (βVoc / 100) × (Tmin − TSTC)] |
| Hot corrected operating voltage | Vmp,hot = Vmp,STC × [1 + (βVmp / 100) × (Tmax − TSTC)] |
| Cold corrected operating voltage | Vmp,cold = Vmp,STC × [1 + (βVmp / 100) × (Tmin − TSTC)] |
| Hot corrected short-circuit current | Isc,hot = Isc,STC × [1 + (βIsc / 100) × (Tmax − TSTC)] |
| Minimum modules in series | max[ ceil(MPPTmin / Vmp,hot), ceil(Start Voltage / Vmp,hot) ] |
| Maximum modules in series | min[ floor(Max DC / Voc,cold), floor(MPPTmax / Vmp,cold) ] |
| Design short-circuit current per string | Isc,design = Isc,hot × Safety Factor |
| Safe strings per MPPT | min[ Allowed Strings, floor(Input Current / Imp), floor(Max SC Current / Isc,design) ] |
Use manufacturer datasheets and project code requirements when finalizing a design. Some jurisdictions require additional margins, conductor adjustments, or code-specific derating rules.
How to use this calculator
- Enter module electrical data from the datasheet, including Voc, Vmp, Isc, Imp, power, and temperature coefficients.
- Enter site minimum and maximum temperatures that represent realistic design conditions.
- Enter inverter voltage limits, MPPT operating window, start voltage, input current, and short-circuit current ratings.
- Set the number of MPPTs, allowed strings per MPPT, safety factor, inverter AC power, and target DC/AC ratio.
- Press Calculate String Sizing to see the result above the form.
- Review the recommended series count, safe series range, maximum string counts, array power range, and the Plotly graph.
- Use the CSV and PDF buttons to save the summary for engineering review or client discussion.
FAQs
1. Why does minimum temperature matter so much?
Module open-circuit voltage rises as cells get colder. A string that looks acceptable at STC can exceed the inverter absolute DC voltage during cold mornings. That is why cold corrected Voc is one of the most important checks in string design.
2. Why is maximum temperature used for minimum series count?
Module operating voltage drops as temperature rises. During hot conditions, the inverter may fall below its startup or MPPT threshold unless enough modules are placed in series. Using hot corrected Vmp protects operating stability.
3. What is the difference between max DC voltage and MPPT max voltage?
The absolute DC voltage is a hard equipment ceiling that should never be exceeded. The MPPT maximum is the upper edge of the preferred tracking range. A string can violate either limit, so both must be checked.
4. Why does the calculator check current in parallel strings?
Parallel strings add current at the inverter input. If the combined operating or short-circuit current exceeds the inverter or MPPT rating, the design can become noncompliant or unsafe. Current limits often reduce allowed strings before voltage does.
5. What does the DC/AC ratio help me decide?
DC/AC ratio helps size the array relative to inverter AC output. A higher ratio can improve energy harvest in some projects, but too much oversizing may increase clipping. The best target depends on climate, economics, and design goals.
6. Are module temperature coefficients always negative?
Voltage temperature coefficients are usually negative because voltage decreases as temperature rises. Short-circuit current coefficients are often slightly positive. Always use the exact values from the module datasheet for reliable sizing.
7. Can I use this for 1500 V utility-scale projects?
Yes, the calculator can support 1500 V style inputs if the inverter and module data are entered correctly. Still, final utility-scale design should also include code compliance, conductor checks, fuse coordination, and project-specific engineering review.
8. Does this replace a stamped engineering design?
No. This tool is a planning and validation aid. Final construction documents should still be reviewed against local electrical code, equipment manuals, temperature assumptions, and professional engineering requirements where applicable.