What is the difference between nominal voltage, Voc, Vmp, Isc, and Imp in the case of a solar panel? Which parameters are important to check before the installation of solar panels?
Different solar panels or photovoltaic (PV) modules have different specifications. There are several terms associated with solar panels and their ratings such as nominal voltage, Voc, Vmp, Isc and Imp. All these parameters are checked before installation. Brief information about each is given below.
Used just for classification, it is not a real voltage you are going to measure. It is not a fixed voltage either and, normally, it is not mentioned in the specification sheet of a PV module. Some of the common parameters mentioned in the specification sheet are listed in the table.
Voltage at open circuit (Voc)
This voltage is checked with a voltmeter across the output terminals of the solar panel module, without connecting any load. This parameter is used to check/test the module during installation and later for system design. It is an important parameter under standard test conditions. Voc is used while determining the number of solar panels required for a particular load.
Voltage at maximum power (Vmp)
This is the voltage available when the panel is connected to a load and is operating at its maximum capacity under standard test conditions. Most solar panel manufacturers specify Vmp to be around 70 to 80 per cent of the Voc.
Short circuit current (Isc)
This is the value of current obtained when the positive and negative terminals of the panel are connected to each other through an ammeter in series. This is the highest current the solar panel will produce under standard test conditions. Isc is used to determine how many amps a panel can handle when connected to a device like a solar charge controller or an inverter circuit.
Current at maximum power (Imp)
This current is obtained when solar panel is producing its maximum power. It is the amperage you would want to see when connected to a solar equipment.