Research community reaches consensus on testing indoor photovoltaics

An international team of 73 researchers has established a set of best practices for assessing the performance and reliability of indoor photovoltaic cells, aiming to bring greater clarity and consistency to a field currently characterized by fragmented and differing standards.

“Our study presents the first field-wide consensus on how to evaluate the performance and stability of indoor photovoltaics designed to harvest energy from indoor lighting. Bringing together experts from academia, metrology institutes and industry across 22 countries, the researchers outline a comprehensive framework for standardized measurements,” corresponding author Thomas Brown told pv magazine.

“Key recommendations include how to select appropriate indoor reference light sources, spectra and intensity levels, based on analyses of lighting markets and regulations to ensure that testing conditions accurately reflect those encountered by devices in real-world indoor environments. The study also establishes best practice protocols for assessing long-term stability – an area that has received little attention in previous testing practices,” he added.

The scientists explained that, although the International Electrotechnical Commission (IEC) introduced its first technical specification for measuring indoor photovoltaic efficiency in 2023 – IEC TS 62607-7-2:2023 – variations in indoor lighting still complicate consistent performance assessments. To address this, the group proposed a single reference spectrum and illuminance level as the best-practice benchmark, while also recommending alternative testing conditions that account for the diversity of lighting environments found in buildings. The framework also covers measurement procedures, calibration and masking, while introducing core stability tests that were previously lacking.

The group recommended moving toward standardized lighting conditions to make indoor PV performance measurements more consistent and comparable. With fluorescent lighting being phased out globally, the researchers identified white LEDs as the most appropriate reference technology for indoor PV testing. While the current IEC best-practice reference is the 5,000 K LED B-4 spectrum, they proposed the 4,000 K LED B-3 spectrum as a future standard. The recommendation reflects current market trends, with 4,000 K lighting particularly prevalent in commercial and other non-residential environments.

The researchers also recommended the use of light-tight test chambers with controlled reflections, pseudo-collimated LED illumination and measurement equipment capable of detecting the very low currents produced by indoor PV cells. They said light intensity can be adjusted electrically, optically or by changing the distance between the lamp and device, with electrical power control generally preferred despite potential spectral variations.

They also recommended using LED sources that closely match the reference spectrum, allowing sufficient time for the sources to stabilize and ensuring uniform illumination across the measurement area. For accurate calibration of illuminance and irradiance, they suggested using reference cells, spectroradiometers or luxmeters, with reference cells considered the best-practice approach. They also recommended instruments with calibration traceable to national metrology institutes and measuring absolute spectral irradiance whenever possible. Careful masking of PV cells was also defined as “essential,” particularly for small devices, to minimize errors caused by stray light and edge effects.

For stability testing, the team’s primary recommendation is a light-soaking test at 1,000 lx, where degradation can be observed more readily, while higher illuminance levels can be used for accelerated ageing studies. Additional protocols include dark-storage testing and light-dark cycling to assess degradation, recovery and performance under realistic indoor operating patterns.

As indoor PV degradation can be slow, the researchers also recommended at least 1,000 hours of ageing when a device does not reach 80% of its initial efficiency, with higher illumination levels used to accelerate degradation. Ultimately, they described real-world field testing as “essential” for developing accelerated protocols capable of reliably predicting the lifetime and degradation behavior of different indoor PV technologies.

The proposed guidelines are described in the paper “Best practices for measuring the performance and stability of indoor photovoltaic devices,” published in Nature Energy. “The authors believe the publication marks a milestone for a community that has grown quickly alongside the spread of low-power electronics and indoor PV technologies. By establishing common practices now, the consensus statement lays the foundation for standard testing, paving the way for more complex methods across a wider range of lighting conditions,” the group concluded.

The international team included scientists from Italy’s Tor Vergata University, Spain’s Universitat Rovira i Virgili, Japan’s Toin University of Yokohama, Germany’s Forschungszentrum Jülich GmbH and Australia’s Edith Cowan University, among others.

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