Module Measurements
Photovoltaic (PV) module measurements at NLR include current versus voltage (I-V) under simulated and outdoor conditions to increase measurement accuracy and reduce uncertainty.
Interested in Module Measurements?
Simulated Module Current Versus Voltage
We use two I-V measurement systems to assess the performance parameters for PV modules under simulated conditions:
- Spire 5600 SLP pulsed solar simulator
- Large-area continuous solar simulator.
The following table is a condensed list of characteristics for module I-V measurements under simulated light.
| System | Special Features | Light Source | Dimensions | Voltage | Bias |
|---|---|---|---|---|---|
| 0.2- to 1-sun module I-V | 30- to 100-ms flash; sweep both directions | 2 xenon flash lamps | 200 cm × 137 cm | 1 mV /250 V | 0.1 mA / 25 A |
| 1-sun modules | 100% duty-cycle continuous light allows the sample to be in any state prior to measurements; spectral irradiance measured; user-controlled bias | 25-kW xenon lamp | 170 cm × 130 cm | 5 mV / ±300 V | ± 1 µA to ±60 A |
How NLR Reduces Uncertainty in Module Calibrations
NLR developed a procedure for measuring the power of commercial crystalline-silicon PV modules that reduces the uncertainty in module maximum power (Pmax) to ±1.1% (k=2 coverage factor). This value is the lowest Pmax uncertainty reported by an accredited test laboratory.
NRL coined the term "module self-reference" for this procedure. The two biggest factors contributing to uncertainty in module calibration are temperature uncertainty and spatial nonuniformity of the light source. The module self-reference procedure minimizes both by using the module itself as a thermometer and a reference device to set the light level of the simulator in the final measurement of module power.
Spire flash simulator
SOMS outdoor test bed
LACSS continuous simulator
Thermal equilibrium T = 25°C Minimal heating during flash
Non-uniformity ~ 0.2% Measurement at 25°C
Use module lsc to set simulator intensity, eliminates effects of non-uniformity and spectral correction
The procedure is:
- Calibrate module open-circuit voltage (Voc) versus irradiance using a pulsed simulator with the module in thermal equilibrium at 25°C. This calibration provides a sensitive internal "thermometer" for subsequent measurements.
- Cool the module to ~15°C, then mount it on an outdoor test bed and monitor Voc as the module heats. The current-voltage (IV) curve is measured when the module reaches 25°C, which provides a measure of short-circuit current (Isc) with minimal uncertainty because of the highly uniform irradiance of natural sunlight.
- Again cool the module to ~15°C, and mount it in a continuous simulator. Voc is monitored until the temperature reaches 25°C. Use the module Isc from the outdoor measurement to set the light level on the simulator—thus eliminating errors due to spatial nonuniformity of the light source and spectral mismatch with the reference cell.
- Finally, again cool the module to ~15°C, remount it on the continuous simulator, and monitor Voc until the temperature reaches 25°C. Then measure the IV curve, which provides an accurate measure of Pmax at 25°C and 1,000 W/m2.
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Last Updated Sept. 2, 2026