An important step in kicking off a successful PV installation is verifying module operation. This includes checking the open-circuit voltage (Voc) and short-circuit current (Isc) of each module on the ground—before it gets mounted.
Exactly how to measure a module’s voltage and current depends on the type of meter you are using. You can take measurements with a digital multimeter (DMM), which uses test leads for measuring voltage and current, or a clamp meter, which has a openable jaw that goes around the wire to measure current. (Some clamp meters also have test lead jacks for plugging in leads for measuring voltage, and some DMMs have clamp accessories that plug into jacks.)
Use a meter capable of reading DC measurements up to the expected module Voc and Isc. If you are testing a Schott Solar ASE 310-watt module, for example, the specification sheet shows that the Voc equals 63.8 V and Isc equals 6.5 A.
Before turning on the meter: Plug the red lead into the “V” jack and the black lead into the “COM” jack. Set the dial for DC volts and the appropriate value range for the given Voc. Connect the red lead to the positive connector or terminal on the module. Connect the black lead to the module’s negative connector or terminal.
For every 1°C higher than 25°C (77°F) that the PV cell experiences, the module will show a 0.5% drop in voltage. On a clear, sunny day, cell temperature will be about 25 to 30°C higher than ambient air temperature. For example, if the ambient air temp is 20°C, cell temperature may be 50°C (depending on how long the module has been sitting in the sun). This translates into a 12.5% voltage loss:
50°C - 25°C = 25°C rise
25°C rise x 0.5% voltage drop per °C = 12.5% voltage drop
Under these conditions, the Schott Solar module Voc should measure about 56 V DC (63.8 V x 0.875).
While it isn’t recommended to measure the short-circuit current of multiple modules wired together, we can measure the Isc of a single module. For dependable readings, the current measurement should be done with the module receiving good solar exposure (unshaded and directly facing the sun on a sunny day). When working with modules that have pre-attached quick-connect cables, it is easiest to measure Isc with a clamp meter.
To avoid sparking, which can damage the connectors, plug the module leads together with the module in the shade or turned over. Set the meter to DC current and clamp the jaws of the meter around the connected wires—then expose the module to the sun.
Current is directly proportional to the irradiance on the module, and even a little bit of haze can affect it. So don’t be worried if the Isc value is slightly lower than stated on the spec sheet, which is based on full light of 1,000 W/m2. Conversely, reflected light from snow, high altitudes, or edge-of-cloud effects may cause slightly higher readings. Having an irradiance meter handy can give you an idea of the available solar resource. With its reading, you can create a multiplier to factor into your measurements. For example, if your irradiance meter measures 800 W/m2, multiply the expected amps by 0.8 to compare to your measured value.
You can measure a module’s Isc with a DMM and test leads by plugging the leads into the correct jacks for measuring current, and setting the dial to the appropriate DC current range. However, sparking will likely occur when you’re trying to connect the DMM leads to the module connectors or terminals, and fingers can get burnt and connectors damaged. This is why a clamp meter is preferable.
Most modules carry an initial power warranty that guarantees operation within 10% of its rated output, minus the tolerance variance. If measurements are less than that, and irradiance and temperature impact have been accounted for, you might need to replace the module. Your meter’s accuracy may influence the measurements, making it tough to call if you are not too far off from the expected limits. If all module measurements deviate from the expected values, then your meter’s accuracy is more suspect.
To single out below-spec modules, an easier indicator is large variances between the modules you are measuring. If modules are all the same model, and field-tested under the same conditions, yet one module yields significantly lower measurements than the others, it may need to be replaced. To make sure module temperatures are consistent while you’re measuring, keep all the modules in the shade. To test, pull out one module at a time. This will make it easier to spot modules that are truly reading low voltage.