Introduction to Solar Electricity

Voltage

PV modules have three different voltage ratings that it’s handy to understand. The Nominal voltage of a panel could also be called the “conversational voltage.” When we talk about the voltage of the panels and the other components of the system, we’ll most often use the nominal voltage. Nominal voltage actually refers to the voltage of the battery that the module is best suited to charge; this is a leftover term from the days when solar panels were used only to charge batteries. The actual voltage output of the panel changes as lighting and temperature conditions change, so there’s never one specific voltage at which the panel operates. Nominal voltage allows us, at a glance, to make sure the panel is compatible with a given system. The second voltage rating is the maximum power voltage (Vmp). This is the highest voltage the panel can produce while connected to a system and operating at peak efficiency. The third voltage is open circuit voltage (Voc). This is the maximum voltage that the panel can produce when not connected to an electrical circuit or system. Voc can be measured with a meter directly contacting the panel’s terminals or the ends of its built-in cables.

Current

Panels also have two different current ratings: Maximum power current (Imp) and short circuit current (Isc), both listed in Amps. The maximum power current is similar to Vmp: it’s the maximum current available when the panel is operating at peak efficiency in a circuit. Similar to Voc, the short circuit current is the current measurement your meter would show when in contact with the positive and negative terminals of the panel while not connected to a system or load.

All these electrical characteristics are used to help determine the size of the PV system and components. These specifications can be found on the label on each solar module as well as on manufacturers’ specification sheets.

Module connectors

J-Box (top), MC connector (bottom)

There are two main types of connectors on the backs of PV modules. Modules rated at less then 100 Watts, or modules manufactured more then ten years ago, often have openable junction boxes. You can connect wires to the positive and negative terminals of the panel by passing them through knock-out holes in the sides of the junction box. The other type of connector consists of a sealed junction box with wire leads, usually two to three feet in length. In a small number of cases the wires have bare ends, but the majority of modules made today have Multi-Contact (MC) brand connectors at the ends of their wires. MC connectors come in several styles, the two most common of which are often called MC1 and MC2. Because of pending changes in the National Electric Code (NEC), we expect to see several new types of connectors on the market, some of which may be required for code-compliant systems.