The label on the back of a solar panel contains the values needed to size an array, select a charge controller, and check wiring and protection. Wattage alone does not describe how the panel will behave.
Open-circuit voltage (Voc)
Voc is the panel voltage with no load connected. It is critical when checking the maximum PV input voltage of a charge controller. Panel voltage rises as cell temperature falls, so designers use the panel's temperature coefficient and the expected coldest temperature—not just the printed STC value—to check the array's maximum voltage.
Voltage at maximum power (Vmp or Vmpp)
Vmp is the approximate operating voltage when the panel is producing maximum power under the stated test conditions. It helps establish the normal operating range of the array and controller.
Short-circuit current (Isc)
Isc is the current measured with the panel output shorted under specified conditions. It is used in conductor, connector, and overcurrent-protection calculations with the safety factors required by the applicable design standard.
Current at maximum power (Imp or Impp)
Imp is the current at the maximum-power point. Together, Vmp and Imp approximately produce the panel's rated wattage: volts multiplied by amps equals watts.
Why temperature and test conditions matter
Most nameplate ratings are measured under Standard Test Conditions. An RV roof rarely operates under those laboratory conditions. Higher cell temperature generally reduces voltage and power, while cold conditions increase voltage. The data plate may list separate coefficients for power, voltage, and current.
Common design mistakes
- Looking only at wattage
- Failing to calculate cold-weather series-string Voc
- Ignoring charge-controller voltage or current limits
- Mixing panels with incompatible electrical characteristics
- Using nameplate output as a guaranteed real-world result
Always use the full current data sheet when a nameplate does not show every value required for the calculation.
Need help planning your RV power system? Complete our System Design Form, and the Big Beard Battery team will help you determine the right batteries, inverter, solar, and supporting components for the way you travel.