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Open Circuit Voltage (Voc) In Solar Panels: Definition And Typical Values

Open circuit voltage (Voc) is the maximum voltage a solar panel produces when no current is flowing and the output terminals are disconnected from any load. It is the single most important parameter for string sizing and the first thing you measure when troubleshooting a panel with a multimeter.

What Voc means

When a solar panel sits in sunlight with nothing connected to its output terminals, a voltage builds up across the positive and negative leads. This voltage is Voc, the open circuit voltage. No current flows because the circuit is open, and all the energy the cells generate appears as voltage.

Every solar panel datasheet lists a Voc value measured under Standard Test Conditions (STC): 1,000 W/m2 irradiance, 25 degrees C cell temperature, and AM1.5 spectrum. The Voc you measure in the field will differ from this rated value depending on temperature and light conditions.

At the cell level, Voc depends on the semiconductor material, doping concentration, and cell quality. A single crystalline silicon cell produces about 0.60-0.68V open circuit. Panels achieve their rated Voc by wiring cells in series.

Typical Voc values by panel type

Panel TypeCell CountTypical Voc (STC)
Standard 60-cell residential60 cells37-40V
Half-cut 120-cell residential120 half-cut cells37-41V
Standard 72-cell commercial72 cells44-49V
Half-cut 144-cell commercial144 half-cut cells45-50V
12V off-grid panel (36 cells)36 cells21-22V

A 120-cell half-cut panel has the same Voc as a 60-cell panel because the half-cut cells are wired as two parallel groups of 60 series cells. The series count stays the same, so the voltage is the same. The current doubles instead.

How temperature affects Voc

Voc has a negative temperature coefficient, typically between -0.25% and -0.35% per degree Celsius for crystalline silicon panels. This means Voc decreases as temperature rises and increases as temperature falls.

Example calculation: A panel rated at 49.5V Voc (STC at 25 degrees C) with a temperature coefficient of -0.30%/degree C:

  • At 45 degrees C (hot rooftop): Voc = 49.5 x (1 + (-0.003 x 20)) = 49.5 x 0.94 = 46.5V
  • At -10 degrees C (winter morning): Voc = 49.5 x (1 + (-0.003 x (-35))) = 49.5 x 1.105 = 54.7V

That cold-weather Voc of 54.7V is 10.5% higher than the STC rating. When you wire 10 panels in series, the string Voc reaches 547V on a cold morning versus 495V at STC. This is why NEC 690.7 requires you to calculate maximum Voc at the lowest expected ambient temperature before finalizing your string size.

Why Voc matters for system design

String sizing. The total Voc of panels in series must not exceed the inverter's maximum input voltage. Most residential inverters have a 500V or 600V DC input limit. Exceeding this limit trips the inverter or causes permanent damage. Always calculate maximum Voc at the coldest expected temperature, not at STC.

Troubleshooting. Voc is the first measurement a technician takes when diagnosing a panel. A reading of 0V means the panel is dead, a cracked cell has severed the circuit, or a wire is disconnected. A reading that is 60-80% of the rated value suggests one of the three bypass diodes has failed, shorting out one-third of the cells. A reading within 90-100% of rated Voc confirms the panel is electrically functional.

Charge controller compatibility. For off-grid systems, the MPPT charge controller's maximum input voltage must exceed the string Voc at the coldest expected temperature. A PWM controller requires the panel Voc to be reasonably close to the battery voltage (within about 5V above nominal), making Voc the key compatibility parameter.

How to measure Voc with a multimeter

  1. Disconnect the panel from the inverter, charge controller, or any other equipment.
  2. Set your digital multimeter to DC voltage mode. Use a range above the expected Voc (200V range works for most panels).
  3. Touch the red probe to the panel's positive MC4 connector and the black probe to the negative connector.
  4. Read the voltage. In full sun (close to 1,000 W/m2), the reading should be 90-100% of the rated Voc on the panel label.
  5. Compare to the datasheet value. If the reading is within 5% of the rated Voc (adjusted for temperature), the panel is healthy.

No current flows during this test, so there is no arc risk and the measurement is safe with standard DC voltage precautions. See our full guide on how to test a solar panel with a multimeter for detailed troubleshooting procedures.

Voc vs Vmp

ParameterVocVmp
Full nameOpen circuit voltageVoltage at maximum power
Current flowingNone (0A)Imp (maximum power current)
Typical value (400W panel)41-50V34-42V
RatioReference (100%)80-85% of Voc
Used forString sizing, troubleshootingMPPT tracking, operating point

Voc is always higher than Vmp because drawing current from a solar cell causes a voltage drop due to internal resistance and recombination losses. The operating point where the product of voltage and current is maximized (Vmp x Imp = Pmax) occurs at a voltage roughly 80-85% of Voc.

Related terms

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Frequently Asked Questions

What is a normal Voc reading for a solar panel?
A normal Voc depends on the cell count. A standard 60-cell (or 120 half-cut cell) residential panel reads 37-41V. A 72-cell (or 144 half-cut cell) commercial panel reads 44-49V. In full sun, your reading should be within 90-100% of the rated Voc on the datasheet. Lower readings indicate a problem.
Why is my solar panel Voc reading low?
Common causes include high cell temperature (Voc drops about 0.3% per degree Celsius above 25 degrees C), partial shading, a failed bypass diode (which drops Voc by roughly one-third), cell degradation, or a dirty panel surface. If your reading is 60-80% of rated Voc, suspect a bypass diode failure. If it is 0V, the panel or wiring is dead.
What is the difference between Voc and Vmp?
Voc is the maximum voltage when no current flows (panel disconnected). Vmp is the voltage at maximum power output when the panel is connected to a load. Vmp is always lower than Voc, typically 80-85% of Voc. For a panel with Voc of 40V, Vmp is usually 32-34V.
Why does Voc matter for inverter selection?
Solar panels wired in series add their voltages. On a cold morning, Voc can rise well above the STC rating. If the total string Voc exceeds the inverter's maximum input voltage, the inverter shuts down or is permanently damaged. NEC 690.7 requires calculating maximum Voc at the lowest expected temperature.
Can I measure Voc safely with a multimeter?
Yes. Set your digital multimeter to DC voltage mode, disconnect the panel from all equipment, and touch the red probe to the positive MC4 connector and the black probe to the negative connector. The panel produces voltage but no current flows through the meter, so there is no shock or damage risk beyond normal DC voltage safety precautions.
Does Voc change with irradiance?
Voc is relatively stable across a wide range of irradiance levels. It only drops noticeably at very low light (under 200 W/m2). At 50% irradiance (500 W/m2), Voc is typically only 2-5% lower than at full sun. This is unlike current, which scales almost linearly with irradiance.
What happens to Voc in cold weather?
Voc increases in cold weather. The temperature coefficient of Voc is typically -0.25% to -0.35% per degree Celsius, meaning Voc rises as temperature drops below 25 degrees C. At -10 degrees C, Voc can be 10-12% higher than the STC rating. This is why cold-weather string sizing is critical to avoid exceeding inverter voltage limits.

Sources

Marko Visic
Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.