A solar panel label describes two different situations: producing power and having no connected load. Vmp and Imp describe its maximum-power point; Voc and Isc describe its open-circuit and short-circuit ratings. These values help you compare panels and check a proposed camper van array against its charge controller.

What The Solar Panel Ratings Mean

  • Vmp — Voltage At Maximum Power: The operating voltage at the panel’s maximum-power point. It is not the highest voltage the panel can produce.
  • Imp — Current At Maximum Power: The current at that same operating point. It is not the panel’s short-circuit current.
  • Voc — Open-Circuit Voltage: The voltage with no load connected. Use the temperature-adjusted array Voc when checking the controller’s PV voltage limit.
  • Isc — Short-Circuit Current: The rated current under short-circuit test conditions. Read it from the panel label or datasheet.
  • Pmax Or Wp — Rated Peak Power: The panel’s maximum-power rating under its stated test conditions. Calculate it from Vmp × Imp, not Voc × Isc.

Manufacturers may write Vmpp, Impp and Pmpp instead. See the ratings identified in Victron’s Solar Panel Datasheet (PDF).

A 200-Watt Example

Illustrative panel values from the original notes: Vmp 20.4 V, Imp 9.8 A, Voc 24.3 V and Isc 10.2 A.

20.4 V × 9.8 A = 199.92 W, approximately 200 W at the maximum-power point. Multiplying 24.3 V by 10.2 A combines different operating points and does not give the panel’s power rating.

Why The Test Conditions Matter

Standard Test Conditions use irradiance of 1,000 W/m², cell temperature of 25°C and an air-mass value of 1.5. Cell temperature is different from outside air temperature. Victron’s Panel And MPPT Matching Guide explains the maximum-power point and these reference conditions.

Check Cold Voltage Before Choosing A Controller

Voc rises as cells get colder. Use the exact panel’s Voc temperature coefficient and the lowest expected cell temperature to calculate cold-weather array voltage. Keep it within the exact controller’s PV limit, including any required design margin. The panel’s STC Voc is not an absolute voltage ceiling.

Controller PV voltage, PV short-circuit current, permitted array power and battery-side charge current are separate limits. Follow the manual for your actual model. Victron’s SmartSolar Installation Guide gives model-specific examples and explains the cold-temperature check.

Continue Planning Your Van’s Solar Array

Read Solar Panels In Series Vs Parallel to compare array arrangements, then use DC Wire Sizing For A Camper Van for the next design step.