The limits of open circuit voltage and short circuit current testing

By Will White, Fluke Solar Product Application Specialist

In the field of photovoltaic (PV) module testing, two common methods are used to assess the performance and health of solar panels: I-V curve tracing and open circuit voltage (Voc)/short circuit current (Isc) testing. While both methods can provide valuable insights, they differ significantly in their diagnostic capabilities and the depth of information they offer.

In this article, we’ll compare these two main types of PV testing so you know which one suits your needs best.

Open circuit voltage and short circuit current testing

Open circuit voltage (Voc) and short circuit current testing (Isc) are straightforward methods to measure the voltage and current of a solar module exclusively at the endpoints of the I-V curve. Voc is measured when the module is not connected to any load; Isc is measured when the module's terminals are shorted.

These tests provide basic information about the module's maximum voltage and current capabilities. However, neither open circuit voltage nor short circuit current offers insights into performance under actual operating conditions.

I-V curve tracing

I-V curve tracing involves measuring a solar module's current (I) and voltage (V) characteristics under varying loads. This generates a detailed I-V curve, which is a graphical representation of the module's performance across its entire operating range. Compared with open circuit voltage and short circuit current testing methods, I-V curve tracing provides a fuller picture of a PV module’s performance.

By analyzing the I-V curve, technicians can identify various deviations that indicate specific classes of issues within the module or the PV circuit. Each of the deviations reduces the maximum power that the string can produce; detection and repair are important to overall PV system health.

Detecting deviations: I-V curve tracing vs. open circuit voltage/short circuit current testing

I-V curve deviations serve as crucial information for PV system maintenance. While open circuit voltage (Voc) and short circuit current (Isc) testing have a purpose, they’re unable to identify most performance deviations. I-V curve testing, however, provides a more complete picture.

For example, common I-V curve deviations such as steps or notches, soft knee, reduced slope in the vertical leg, and increased (steeper) slope are all detectable with an I-V curve tracer. They point to a number of potential causes that require attention: shading, soiling, shorted bypass diodes, increased series resistance, or other possible issues. Voc and Isc fall short here — neither test can find these deviations.

Isc and Voc testing do play a role in maintaining PV systems, but their respective scopes are simply too narrow. Out of the six common I-V curve deviations, these testing methods can only detect two. In contrast, I-V curve testing can detect all six deviations — including low current and low voltage — making it the superior choice for detection.

Read our article “6 deviations to the I-V curve” for a detailed breakdown of each deviation, potential causes, and interpretations.

I-V curve tracing: The clear choice

While open circuit voltage and short circuit current testing provide basic information about a solar module's maximum voltage and current, I-V curve tracing offers a comprehensive analysis of the module's performance under actual operating conditions. It also provides valuable diagnostic information for troubleshooting performance issues. Voc and Isc are significantly more limited in scope and do not analyze PV modules under real operating conditions.

I-V curve tracing verifies that PV modules are operating correctly and producing the expected power output. It also aids in detection — comparing the measured I-V curve with the expected curve, technicians can identify issues such as damage, shading, or defective components. Additionally, regular I-V curve tracing helps with maintaining PV system efficiency.

Compared with open circuit voltage and short circuit current testing, I-V curve tracing is the superior choice for assessing your PV system’s health and performance. Don’t miss critical signs of system degradation due to inferior testing methods. Voc and Isc only tell a fraction of the story; for the whole thing, turn to I-V curve tracing.

About the Author

Will White has been a Solar Product Application Specialist at Fluke since 2022, supporting renewable energy testing equipment like I-V curve tracers, electrical meters, and thermal imaging cameras. With nearly two decades of experience in the solar industry, Will’s expertise spans residential and small commercial PV systems, energy storage, and wind power. He is a NABCEP Certified PV Installation Professional and a seasoned educator in solar energy, having developed and taught courses for Solar Energy International (SEI). Connect with Will on LinkedIn.