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How photovoltaics are tested & rated

Richard Perez

Have you ever wondered how PV modules are rated for power output? How do those magic wattage numbers appear on the back of every module? Well, virtually every module is tested by their manufacturers. This article discusses how

PV makers test and rate their modules. And how these power ratings may be different from actual module performance out in the sunshine.

A long and winding road...

This series of articles grew from our PV testing over the last three years. We found differences between the performance ratings printed on modules and their actual performance in the sun. We set out to find out why. This turned out to be a very long journey indeed. We got information from the modules' makers, we talked to the Solar Energy Research Institute (SERI), and we set up module "test jigs" for evaluating modules ourselves.

During the next few issues of Home Power, we will be printing the actual performance data of virtually every module, new and used, now available. This article defines the terms, standards and procedures used by PV makers and by us during our "in the sun" PV testing.

The Standards

All measurement depends on standards. Without using clearly defined standards, measurement is meaningless. Rating the power output of a photovoltaic module is done in a highly structured and standardized fashion. Here are the various measurement parameters & a schematic of our test jig.


Modules are rated at two voltage levels. The first is called "Open Circuit Voltage (Voc)" and is just that. The voltage output of the module is measured with the module disconnected from any load. The second voltage rating point is called "Voltage at maximum power point (Vmp)" and is the voltage at which the module puts out the most power. All voltage measurements are made at the module's electrical terminals on the module's back. These measurements are made with a highly accurate voltmeter. We use the Fluke 87s with 0.1% accuracy.


Current is also rated at two important levels. The first is called "Short Circuit Current (Isc)" and is the amount of current that the module supplies into a dead short. The second current rating is called "Current at maximum power point (Imp)" and is the number of Amperes

Home Power's PV Test Jig


DMM measuring voltage

Shunt 0.1% 10 A. @ 100 mV.

3Ω rheostat 250 W.

1.6Ω 225 W. as needed

41.5 DMM measuring

20 Home Power #23 • June / July 1991

temperature probe

PV Module under test

delivered by the module at its maximum power point. Current is measured with a shunt in series with one of the PVs' lead. The voltage loss across the shunt provides accurate current measurements. We use 10 Amp., 100 mV. Deltech shunts with an accuracy of 0.1%. We use a Fluke 87 in 4 1/2 digit mode to take these measurements.

Maximum Power and Maximum Power Point

Power is equal to Amperes times Volts (P=IE, or Watts=Amperes X Volts). Every module has a specific point on its power curve where the product of Amps times Volts yields the greatest Wattage. This is the Maximum Power Point, and the module's wattage output is rated at this point's voltage and current.

So to find the module's maximum power point we take data over the entire range of voltage and current. Because we have taken the modules voltage and current


module temperature

DMM measuring current



DMM measuring sunshine

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