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Maximum Power Current (Imp)

Definition: Maximum amperage produced by a module or array when exposed to sunlight and connected to a load.

Importance: Maximum power current is one specification used when sizing an array for a given inverter or charge controller.

Open-Circuit Voltage (Voc)

Definition: The maximum voltage generated by a PV module or array when exposed to sunlight with no load (inverter or battery) connected.

Importance: Open-circuit voltage will increase as PV module temperature decreases. To eliminate the possibility of overvoltage conditions that will damage most inverters and charge controllers, a maximum Voc calculation based on the coldest historical temperature for a given site is required during system design.

Short Circuit Current (Isc)

Definition: The amperage generated by a PV module or array when exposed to sunlight with output terminals shorted.

Importance: Modules will not operate at short circuit in the field unless they are incorrectly wired. Using a digital multimeter to check the current of an individual module will briefly short the terminals while the measurement is being taken, allowing you to compare the actual output to the manufacturer's specification during troubleshooting. Additionally, Isc specifications are used for calculating the appropriate amperage rating of overcurrent protection devices.

Maximum Power Temperature Coefficient (% per degree C)_

Definition: The change in module output power in percent-per-degree Celsius at temperatures other than 25°C (STC temperature rating).

Importance: Module voltage decreases as cell temperature increases. A maximum power temperature coefficient is one metric that enables you to predict the real-world power output of an array that's operating at elevated cell temperatures. In hot climates, cell temperatures can reach an excess of 70°C (158°F). For example, consider a module maximum power rating of 200 watts at STC, with a temperature coefficient of minus 0.5% per degree C. At 70°C, the actual output of this module would be approximately 155 watts.

Open-Circuit Voltage Temperature Coefficient (mV per degree C)_

Definition: The change in module open-circuit voltage in millivolts per degree Celsius at temperatures other than 25°C (STC temperature rating).

Importance: Open-circuit voltage will increase as cell temperature decreases, based on the 25°C STC reference temperature. In turn, Voc will decrease as cell temperature increases. Applying the open-circuit voltage temperature coefficient is one way to determine absolute maximum Voc at a site's coldest historical temperature, and allows you to calculate the reduction in module or array voltage at elevated temperatures.

Short-Circuit Current Temperature Coefficient (mA per degree C)_

Definition: The change in module short-circuit current in milliamps per degree C at temperatures other than 25°C (STC temperature rating).

Importance: Short-circuit current will increase in varying degrees as cell temperature increases and Voc decreases. This relationship is interesting in terms of module function, but is not particularly relevant in most system designs.

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Joe Schwartz ([email protected]), Home Power CEO and executive editor, holds a Renewable Energy Technician license in Oregon. His home and home office are powered exclusively by renewable energy.

Special thanks to Home Power Technical Assistant Doug Puffer for module specification research and compilation.

Images on pages 70 & 71 (clockwise from upper left): Courtesy of BP Solar (SX 3195 module); Canadian Solar Inc. (CS5A-180 module); Advent Solar (240 module); Day4Energy (48MC 190 module).

Module Manufacturers:

BP Solar • www.bpsolar.com

Day4Energy • www.day4energy.com

Evergreen • www.evergreensolar.com

Kyocera • www.kyocerasolar.com

Mitsubishi • www.mitsubishielectric.com/solar

Sanyo • www.sanyo.com

Schott • www.us.schott.com

Schuco • www.schuco-usa.com

Sharp • www.solar.sharpusa.com

SolarWorld • www.solarworld-usa.com

SunPower • www.sunpowercorp.com

Suntech Power • www.suntech-power.com

Sunwize • www.sunwize.com

Yingli • www.yinglisolar.com