Quick Facts
What Voc is
Open-Circuit Voltage (Voc) is the voltage produced by a solar cell or module when no current flows through the external circuit (the circuit is open). It is the maximum voltage the cell can produce under given conditions of light and temperature, and corresponds to the right end of the cell’s IV curve.
At Voc, the cell is producing maximum voltage but zero current, so the output power is zero. The cell’s useful power output occurs at the maximum power point (MPP), where both voltage and current are positive.
Voc is one of the three key parameters that characterise a solar cell, along with short-circuit current (Isc) and maximum power point. Together, these three parameters describe the cell’s electrical behaviour.
For solar plant design, Voc is the critical parameter that limits string length. The total Voc of all panels in series must stay below the inverter’s maximum input voltage at the coldest expected ambient temperature.
How Voc behaves
Voc depends on cell architecture, temperature, and (weakly) on irradiance.
Temperature: Voc decreases linearly with rising temperature. For Mono PERC, the temperature coefficient of Voc is approximately minus 0.27% per deg C. At 65 deg C cell temperature (typical Indian summer), Voc is about 10% lower than at STC.
Irradiance: Voc has a logarithmic dependence on irradiance. From 200 W per sq m to 1000 W per sq m, Voc rises only about 10%. This means Voc is fairly stable across the operating day; the bigger variation is from temperature.
Cell architecture: Different cell technologies produce slightly different Voc. Mono PERC: about 0.66 V per cell at STC. TOPCon: about 0.69 V per cell. HJT: about 0.74 V per cell.
Cell count in panel: Module Voc is approximately number of cells multiplied by per-cell Voc, with some loss for interconnect resistance.
Cold-temperature Voc and inverter limits
For inverter selection, the critical concern is maximum string Voc at the lowest expected ambient temperature.
Cold ambient temperature pushes Voc above STC nameplate. For Indian sites:
Most locations: lowest annual ambient is around 0 to 15 deg C.
High-altitude sites (Himachal, Uttarakhand mountains): can reach minus 5 to minus 15 deg C.
Cold module temperature (with the modules near ambient on winter nights): close to ambient temperature.
At minus 10 deg C versus STC’s 25 deg C, the temperature difference is 35 deg C. With Voc temperature coefficient of minus 0.27% per deg C, Voc rises by 9.5% above STC.
For a 144 half-cell Mono PERC module with STC Voc of 49 V:
Cold-day Voc: 49 × 1.095 = 53.7 V.
For an inverter with maximum DC input voltage of 1000 V:
Maximum cold-day string Voc must stay below 1000 V (typically with 10% safety margin, so design max is 900 V).
Maximum panels per string: 900 / 53.7 = 16 panels.
If the design uses 18 panels per string (Voc = 967 V), the inverter would trip or be damaged on cold winter mornings.
This is one of the most common design errors in solar plant configuration. Always check cold-day Voc against inverter input limits.
How Voc is measured
In manufacturing:
Flash testers measure Voc as part of standard cell and module characterisation.
Each panel receives an STC flash test before shipment.
Flash test data is recorded in the manufacturer’s database.
Voc is typically measured to plus or minus 1% accuracy in factory conditions.
In the field:
Disconnect the panel from any load.
Measure voltage between the panel’s positive and negative terminals with a high-impedance multimeter.
Record ambient temperature and irradiance for normalisation.
Compare to factory STC Voc after correcting to STC conditions per IEC 60891.
Field Voc measurement is useful for diagnosing degradation, PID, or partial shading.
Voc tolerance and degradation
Manufacturing tolerance for Voc is typically plus or minus 3% to 5%. Premium manufacturers maintain tighter tolerances.
Over the panel’s life, Voc decreases due to several factors:
LID (Light Induced Degradation): Initial 1% to 3% drop in p-type cells.
Annual degradation: 0.5% per year (linear approximation for Mono PERC), of which about half manifests as Voc reduction.
PID: Significant Voc reduction in affected panels.
Hot spot or damaged cells: Localised Voc reduction.
Field Voc tracking is therefore a valuable degradation diagnostic. Comparing today’s field Voc to the original flash test (after STC correction) shows cumulative degradation.
Voc in mismatched strings
In a series string of panels:
The string Voc is the sum of individual panel Voc.
If one panel has reduced Voc (due to damage or degradation), the string Voc drops proportionally.
The string current is limited by the worst-performing panel.
Field IV curves of strings can reveal mismatched modules through irregular Voc patterns.
Common Voc mistakes
Designing strings based on STC Voc without temperature correction. Cold-day Voc exceeds STC.
Using inverter maximum input voltage without safety margin. Aim for cold-day Voc at 85% to 90% of inverter max.
Ignoring location-specific cold temperature. Hill stations and high-altitude sites have much lower winter temperatures.
Treating panels of different brands as interchangeable in voltage. Different products have different per-cell Voc.
Skipping Voc measurement during incoming inspection. Panel-level Voc variations exist; out-of-spec modules should be rejected.
Best practices
For string design, use the panel manufacturer’s published Voc and temperature coefficient.
Calculate cold-day Voc at the lowest expected ambient temperature for the specific site.
Design string length to stay below inverter maximum input voltage with at least 10% safety margin.
For high-altitude sites, use the local extreme low temperature, not generic Indian averages.
For warranty claims, document Voc readings alongside other measurements.
For lender-grade diligence, the string design calculations should be reviewed by an independent engineer.
Standards and references
Voc measurement follows IEC 60891 (correction procedures) and IEC 60904 (PV device measurement). Cell-level Voc is part of IEC 61215 module characterisation. Inverter input limits are specified in IEC 62109 inverter datasheets.
Related glossary terms
- IV Curve
- Short-Circuit Current
- Maximum Power Point
- Fill Factor
- MPPT
- String Inverter
- Temperature Coefficient
- Standard Test Conditions
Key takeaways
Open-Circuit Voltage (Voc) is the maximum voltage a solar cell or module produces when no current flows. Voc is one of three key parameters (with Isc and MPP) that characterise solar cell behaviour. Voc decreases with rising temperature (about minus 0.27% per deg C for Mono PERC) and depends only logarithmically on irradiance. For solar plant design, cold-day Voc determines maximum string length within inverter input voltage limits. Incorrect Voc calculation is one of the most common solar design errors; always use site-specific cold temperature and adequate safety margin.