Open-Circuit Voltage (Voc)

Open-circuit voltage (Voc) is the maximum voltage a solar panel produces when no current is flowing — that is, when the circuit is open and nothing is connected to draw power. It is the highest voltage the panel can generate under a given set of light and temperature conditions.

Voc is measured by connecting a voltmeter directly across the panel's positive and negative terminals with no load. For a typical 60-cell residential panel, Voc is around 38-42 volts. For a 72-cell panel, Voc is around 46-50 volts.

This specification is critical for system design because it determines the maximum voltage your charge controller or inverter input will see. If you wire panels in series, the Voc values add up. A string of 10 panels with 42V Voc each presents a maximum voltage of 420V to the inverter or charge controller. Exceeding the input voltage rating of your equipment can cause permanent damage.

Importantly, Voc increases in cold weather. The voltage temperature coefficient (listed on the panel datasheet) tells you how much Voc rises per degree below STC temperature. System designers must calculate the maximum possible Voc at the lowest expected temperature in your location to ensure the string voltage stays within equipment limits.

Voc is one of the four key parameters (alongside Isc, Vmp, and Imp) that define a solar panel's electrical characteristics at STC.