Module Performance

The total electrical power output (wattage) of a photovoltaic module is equal to its operating voltage multiplied by its operating current. Photovoltaic modules may produce current over a wide range of voltages. This is unlike voltage sources such as batteries, which produce current at a relatively constant voltage.

The output characteristics of any given module are characterized by a performance curve, called an I-V curve, that shows the relationship between current and voltage output. The chart shows a typical I-V curve. Voltage (V) is plotted along the horizontal axis. The current is plotted along the vertical axis. Most I-V curves are given for the standard test conditions (STC) of 1,000 watts per square meter irradiance (often referred to as one peak sun) and 25°C (07°F) cell temperature. It should be noted that STC represent the optimal conditions as a consistent means for measuring — rarely are these conditions recreated in outside environments. The IV curve contains three significant points:

• Maximum Power Point (both Vmp and Imp)
• Open Circuit Voltage (Voc)
• Short Circuit Current (Isc)

1. Maximum Power Point (Vmp & Imp)

This point, labeled Vmp and Imp, is the operating point at which the maximum output will be produced by the module at operating conditions indicated for that curve. In other words, the Vmp and Imp of the module can be measured when the system is under load at 25°C cell temperature and 1,000 watts per square meter. The voltage at the maximum power point can be determined by extending a vertical line from the curve downward to read a value on the horizontal voltage scale. The example in the chart above displays a voltage of approximately 17 volts at the maximum power (Vmp). The current at the maximum power point can be determined by extending a horizontal line from the curve to the left to read a value on the vertical current scale (Imp). The example above displays a current of approximately 2.5 amps at the maximum power.

The wattage at the maximum power point is determined by multiplying the voltage at maximum power by the current at maximum power. In the chart, the maximum wattage at STC would be approximately 43 watts. This power is represented by the rectangle under the curve.

The power output decreases as the voltage drops. Current and power output of most modules drops off as the voltage increases beyond the maximum power point.

2. Open Circuit Voltage (Voc)

This point, labeled Voc, is the maximum potential voltage achieved when no current is being drawn from the module. Since no current is flowing, the module experiences maximum electrical pressure. The example at left displays an open circuit voltage of approximately 21 volts. The power output at Voc is zero watts. Open Circuit Voltage can be measured in the field in several common circumstances. When buying a module, it is recommended to test the voltage to see if it matches the manufacturers specifications. When testing voltage with a digital multi-meter from the positive to the negative terminal, an open circuit is created by the meter which allows Voc to be measured. It is also common to see a module operating at Voc early in the morning and late in the evening.

3. Short Circuit Current (Isc)

This point, labeled Isc, is the maximum current output that can be reached by the module under the conditions of a circuit with no resistance or a short circuit. The example to the left displays a current of approximately 2.65 amps. The power output at Isc is zero watts. When first purchasing a module, it is recommended to test the short circuit current to see if it matches the specification sheet. The short circuit current can be measured only when making a direct short across the positive and negative terminals of a module. Creating a direct short across more than one module at a time (or a module with voltage greater than 24V nominal) is not recommended and can be extremely dangerous. All Isc measurements should be taken when the module is not connected to other components in the system.

Note: When testing modules with ‘quickconnects’ it is recommended to use test leads to avoid leaving carbon deposits (which cause high resistance) on the module’s leads. Before testing amperage with a digital multi-meter, check to ensure the module’s Isc does not exceed the meter’s DC amperage rating and always use the appropriate personal protective equipment.

4. Specification Label

All of the values found on the I-V curve above are used to create a specification label for each module. All modules are rated under standard test conditions, thereby allowing their values to be compared. The specification label can be found on the back side of the module or through the manufacturer.

Source: “PHOTOVOLTAICS - Design and Installation Manual” by Solar Energy International.

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