Essential system testing every solar project needs for long-term performance

Solar energy systems are built to provide decades of reliable energy production, but hidden issues can silently reduce performance until small failures become large failures, costly repairs, and even safety issues.

For solar owners, developers, and operators, investing in comprehensive and proactive system testing is one of the most effective ways to maximize energy production and extend the operational life of any solar system. From construction and commissioning to ongoing operations and maintenance, proper system testing helps identify electrical faults, equipment degradation, installation issues, and performance concerns before they cause significant damage and revenue loss.

For the long-term success of any solar project, various testing measures should be performed during construction, commissioning, and annually (or semiannual) as a proactive maintenance plan. Utilizing the following essential system testing practices will make the difference between a system that performs long-term and one that faces challenges throughout the project lifespan.

Thermal imaging-handheld and drone

Thermal imaging, either performed by handheld thermal cameras or drone-mounted thermal cameras, detects temperature variations across modules, electrical components, and equipment, pinpointing various production and safety concerns not visible to the plain eye.

Drone thermal inspection

Importance: By pinpointing heat anomalies, thermal imaging can often discover issues early on, allowing for correction before larger failures happen. Thermal Imaging can detect:

• Module hotspots
• Defective modules
• Module soiling
• Array shading
• Failed bypass diodes
• Loose electrical connections
• Offline strings and combiner boxes
• Offline Inverters
• And more!

When to perform: Thermal imaging should be performed during commissioning, once a system has been energized. For maintenance, it should be performed annual as a baseline, with best practice being twice a year, before peak irradiance and before winter hits (spring and fall). If a system is facing ongoing production concerns, performing thermal imaging more often may be needed.

IV curve tracing

IV curve tracing is an essential diagnostic test to determine the performance of modules and strings by recording the relationship between current (I) and voltage (V). That “curve’ can then be compared to module performance expectations.

IV Curve Tracing

Importance: Unlike system monitoring, which can detect when a system is underperforming, IV curve tracing helps determine why underperformance is happening. IV curve testing can detect a wide range of problems, such as:

• Degrading or aging modules
• Damaged modules
• Shading losses
• Soiling impacts
• Damaged connectors
• Open circuits
• String wiring errors
• And more

When to perform: IV curve tracing should be performed during commissioning to verify that newly installed modules and strings meet design specifications. For maintenance, annual testing is suggested to monitor system health and detect degradation over time. However, additional IV curve testing should be performed if production drops significantly or after severe weather events where modules may have been damaged.

Insulation resistance (Megger) testing

Insulation resistance testing, also knowing as Megger testing, is an electrical diagnostic test used to verify the integrity of insulation on cables, equipment, and circuits.

Importance: By measuring the resistance to current leakage, this helps identify weaknesses in insulation before they lead to equipment damage, ground faults, or safety hazards. Over time, insulating material can deteriorate due to moisture, UV exposure, heat, mechanical damage, and more. The goal is to detect deteriorating insulation before larger electrical failures occur.

When to perform: Insulation resistance testing is performed during the mechanical completion stage of commissioning, before a site is energized to ensure a system is safe to operate. During preventative maintenance (annually), insulation resistance testing should be performed to monitor insulation health as a system begins to age. Other instances when IRT (insulation resistance testing) may be performed is during troubleshooting, especially if a ground fault is expected. As well as after repairs, such as cable or electrical repairs.

DC string testing

DC string testing is an essential electrical test that verifies each string of modules is operating correctly and is producing the expected electrical output. By testing individual strings, technicians can identify wiring errors, damaged modules, faulty connectors, or electrical issues.

Importance: On a utility-scale solar site with thousands of strings, identifying even a small percentage of underperforming strings can recover a meaningful amount of lost energy production.

When to perform: DC string testing is usually performed during commissioning before a site is fully energized, but after modules, wiring, combiner boxes, and associated DC equipment have been installed.

• Ground Continuity and Bonding Verification

Ground continuity and bonding verification is an essential test to confirm that all metallic components of the solar electrical system are properly connected to a grounding system, in the case that faults occur.

Importance: Without proper grounding and bonding in place, a fault in the electrical system could energize metal equipment, resulting in electrical shock, equipment damage, or fire hazards. While grounding is essential for the safety of personnel and equipment, if an arc flash or fire occurs, systems may suffer extended downtime as well. This test is essential for the overall safety and reliability of a system.

When to perform: Ground continuity testing is a standard commissioning task that should always be performed. However, it should also be performed during preventative maintenance checks, as grounding systems can degrade over time due to corrosion, loose connections, environmental exposure, etc. Ground continuity testing may also be performed if major repairs or equipment replacement has occurred, to ensure all repaired or new equipment has been properly connected to the grounding system.

MV equipment and cable testing-VLF, etc.

Medium-voltage equipment usually consists of pad-mounted transformers, switchgear, circuit breakers, reclosers, disconnect switches, collection cables, and more. All these components carry a large amount of electrical power and even a minor defect can have major consequences. Performing various testing measures such as VLF cable testing, insulation resistance testing, and more will verify the integrity of MV equipment.

Transformer testing

Importance: A failure anywhere in the MV system can take a large section, or even an entire system offline, resulting in significant production losses, downtime, and costly repairs. Performing testing on MV equipment can reveal moisture intrusion, damaged cable insulation, poor cable splices, loose electrical connections, and protective relay issues. Most of these issues are hidden underground or inside electrical enclosures and require a series of testing measures, not just a visual inspection.

When to perform: Various MV equipment and cable testing measures should be performed during commissioning to ensure equipment was installed correctly and is safe to operate. MV equipment maintenance should be top priority for every site and be included in yearly maintenance. Over time, all electrical systems can age and incur issues due to heat, moisture, electrical stress, mechanical movement, and environmental conditions. Additional MV testing should be performed if electrical faults, lightning incidents, or damage to cables occur.

Ground resistance (fall of potential) testing

Ground resistance testing is commonly performed using a fall of potential method, which measures how effectively a solar site’s grounding system dissipates electrical current into the earth.

Unlike ground continuity testing, which verifies that equipment is connected to the grounding system, Fall of Potential testing measures the effectiveness of the grounding system itself.

The test is performed using a specialized ground resistance tester and two temporary electrodes driven into the soil.

Importance:  The importance of an effective and safe grounding system is essential if ground faults, lightning strikes, short circuits, or electrical surges take place. If ground resistance is too high, equipment may remain energized during faults, resulting in significant damage, or protective devices not operating.

When to perform: Fall of potential testing is most commonly performed during commissioning, to ensure a site’s grounding system meets the project’s design specification and electrical standards. Fall of potential testing should be performed again if a site shows significant soil drying or corrosion, or if significant construction activities or lightning events have happened.

Fiber optic testing

Most modern-day solar sites rely on a robust communication network. Fiber optic cables serve as the backbone of this communication network, connecting inverters, weather stations, SCADA/DAS, protection relays, substations, and control centers.

Unlike electrical testing, fiber optic testing measures the quality of light signals traveling through fiber cables. Two main fiber testing practices that are used is OTDR (Optical Time Domain Reflectometer) testing, as well as Optical Loss Testing (Insertion Loss Testing).

Importance:  If a fiber optic cable is damaged or performing poorly, operators can encounter communication errors with their site, missing underperformance or leaving issues unaddressed that need immediate attention.

When to perform: Fiber testing is performed by a licensed fiber technician during commissioning, after the communication network has been fully installed. Fiber testing is also required if operators face lost inverter communications, SCADA alarms, or network failures. Fiber testing helps isolate whether the problem lies in the cable, connectors, splices, or network equipment. If large repairs that may affect the fiber network occur, such as cable replacements, new equipment, excavation work, storm damage, etc, then further fiber testing may be needed as well. Preventative maintenance testing should also be performed on fiber cables periodically, but not necessarily annually, unless a site encountered any of the listed occurrences that may have disrupted the fiber network. Best practice would be performance testing every 3-5 years, depending on site conditions.

Testing vs. monitoring: Why both are needed

Solar monitoring systems are valuable tools for tracking production and identifying when something changes. However, monitoring typically shows that a problem exists — not always why it exists.

For example:

• Monitoring may show a production drop on a string.
• Testing can determine whether the cause is a failed module, connector issue, wiring problem, or electrical fault.

Comprehensive system testing can also detect small issues before larger issues occur, offering important data to on-site repair teams. The goal is early detection, effective inspection and testing, and quality repairs that increase system production, reliability, and safety.

Protect your solar investment with comprehensive testing

Solar projects are long-term investments that require continual monitoring, inspection, and testing. Regular system testing maximizes production, reduces downtime, and protects valuable equipment.

From commissioning new installations to maintaining aging solar assets, comprehensive system testing provides the information needed to keep systems operating safely and efficiently for the system’s lifespan.

Hannah Smith is the marketing & business development manager at Day Electric, where her goal is to build brand awareness and connect Day Electric with valuable partners within the industry. Her articles reflect the knowledge, experience, and opinions of her field, operations, and executive team.

Day Electric provides comprehensive solar system testing services including thermal inspections, electrical testing, commissioning support, troubleshooting, and performance verification to help solar projects achieve long-term reliability and maximum energy production.

To learn more, visit our website at: https://www.dayelectricusa.com/