Mind the Gap: are traditional models underestimating hail damage to PV?

Originally published in PV Tech Power

By Nicole Thompson and Reilly Fagan, kWh Analytics

New data suggests the traditional assumptions behind hail stow modeling may be significantly underestimating the likelihood of damage to a PV system. Nicole Thompson and Reilly Fagan of kWh Analytics dive into the latest hail research and discuss its implications for insurance.

The renewable energy industry has reached a pivotal moment. With nearly 50GW of solar capacity installed in 2024 alone¹ and renewable energy becoming more essential to the US electrical grid, the stakes for the industry have never been higher. Yet beneath this remarkable growth lies a sobering reality: hail damage represents the single most disproportionate threat facing solar installations today. 

The industry’s response has centred on two primary defences: thicker, heat-tempered glass modules and hail-stow protocols that tilt tracking systems to steep angles during storms. These strategies show promise, but a critical question remains: how effective are they really in preventing damage? 

While real-world data are ideal, factors like hailstone density, measurement uncertainty, and varying conditions complicate the answer, making physics-based models essential. However, the latest research shows that the widely used kinetic energy models may be significantly underpredicting the potential for damage by up to 48% for 3in hail, even when panels are in a high-degree hail-stow position. kWh Analytics has developed an empirically corrected hail model to begin to account for these modeling inaccuracies. While stow has been shown to effectively mitigate hail damage in many instances, overestimating its effectiveness can lead to costly miscalculations. Projects that rely heavily on operational protocols while using thinner glass modules may face substantially higher loss rates than anticipated, creating financial strain across the entire value chain from project owners to insurance carriers. To protect solar installations, stow is most effective when combined with thicker, heat-tempered modules, and in some severe hailstorms, the combination of the two is non-negotiable.

Understanding the hail problem

By understanding the frequency and financial impact of hail events, we can better prepare for and mitigate their effects. While hail events account for only 6% of solar loss incidents, they drive a staggering 73% of total financial losses². This knowledge empowers us to take proactive measures to address this imbalance and reduce its impact. The traditional risk maps are changing, too. New research from Dr. John Allen and Central Michigan University in the 2025 Solar Risk Assessment challenges long-held beliefs about hail exposure across the United States.

Using Bayesian modelling, researchers found that 91.18% of utility-scale solar locations in the US have a 10% annual chance (10-year return period) of seeing hail greater than 2 inches (50mm) within approximately 17 miles of their location. Perhaps more concerning, 64% of these locations showed hail over 3 inches for a 25-year return period. This includes sites in traditionally low-risk areas such as California, proving that hail risk is pervasive throughout the United States. 

Hail differs fundamentally from other natural catastrophe perils in both its impact pattern and financial consequences. Major hail events like those that devastated Fighting Jays demonstrate hail’s ability to cause millions in losses across sites within minutes. These events create insurance nightmares, as concentrated losses can exceed hundreds of millions of dollars from single weather events, far surpassing typical fire or wind damage claims. 

Hail creates distinctive damage patterns that pose particular challenges for both operators and insurers. While wind damage typically affects racking and mounting systems (often limited to the perimeter rows), and fire creates localised thermal damage, hail strikes directly at a solar plant’s most vulnerable component: the glass surface of the modules. This creates cascading effects, including glass cracks, hot spot formation, and microcracks, as well as safety and production issues. 

Analysis of loss patterns reveals that 29% of damaged sites have experienced multiple events. However, the data shows an important distinction: sites that implement comprehensive protection measures after initial losses significantly reduce damage in subsequent storms. This suggests that proper risk mitigation can break the cycle of repeated losses that plague some installations. 

Accurately predicting the probability of damage from a natural catastrophe event is imperative to industries like insurance, which base premium pricing on these calculations. The industry leaders use physics-based models to assess the likelihood of damage in different scenarios by comparing the estimated kinetic energy that modules can withstand to the estimated impact energy at different module tilt angles. 

These current modeling approaches assume that hail impacts behave as perfectly elastic collisions, where all kinetic energy (energy associated with movement) is conserved as kinetic energy, as opposed to being converted to other forms of energy (heat, sound, deformation, etc.). However, emerging research from laboratory testing suggests that the inelastic components of real world hail impacts shouldn’t be ignored, and some of these more nuanced details of hailstone impacts should perhaps alter our view of stow effectiveness. 

This modelling gap has profound implications. Because inelastic components are at play, current models that assume all energy remains as kinetic energy (and thus decreases predictably with increasing stow angle) may overestimate the effectiveness of protective strategies by as much as 48%, creating blind spots in risk assessment that affect everything from insurance pricing to technology investment decisions. 

The goal of all hail resilience strategies remains straightforward: reduce the probability of glass damage. When glass breaks, modules cannot effectively produce electricity and hot spots form, causing cascading failures. But achieving this goal requires confronting uncomfortable truths about modelling limitations, reassessing protection strategies and embracing new technologies that can withstand increasingly severe weather.

How hail actually damages solar panels

To understand adequate hail protection, we must first examine the physics governing these destructive impacts and how models can be used to assess the probability of glass breakage when hail events occur. Further, we can scrutinise these models’ assumptions to understand how simplifications are introduced in modeling, which may lead to inaccuracies in loss estimates.

Explaining kinetic energy

Hail damage begins with kinetic energy— the energy of motion carried by falling hailstones. This energy can be represented by the classic physics equation KE = ½mv², where mass (m) and velocity (v) determine the total kinetic energy of a falling hailstone. This kinetic energy can then be compared to the kinetic energy required to break a module, often obtained via lab tests, to ultimately determine the probability of a module breaking given a hailstone of a certain mass and velocity. 

Panel glass thickness plays a crucial role in determining the likelihood of breakage. RETC’s research in the 2023 Solar Risk Assessment shows that 3.2mm glass/ polymer backsheet modules substantially outperform 2mm glass/glass alternatives across all impact energies, with the protection benefit increasing at higher energy levels, as seen in Figure 2. 

This lab testing data provides a guide for how probability of damage scales with hailstone kinetic energy. We can take this a step further and assume this relationship holds, regardless of the stow angle, so long as we can calculate the effective kinetic energy imparted onto the module. This can be accomplished by assuming the collision is perfectly elastic, so that only the portion of kinetic energy that is perpendicular (normal) to the module contributes to breakage (i.e. angled impacts result in predictably less kinetic energy being transfered to the module and thus lower breakage risk). So, for angled impacts (e.g. when modules are placed in a high-degree hail stow), the kinetic energy which contributes to damage can be represented follows, where KE is total kinetic energy of the hailstone and KE_normal is the effective kinetic energy the module “sees”:

KE_normal=KE cos²(θ)

Therefore, a panel tilted at 60 degrees would be expected to receive only one-quarter of the impact energy of a flat installation, assuming no wind and that hail is falling straight down. When wind is present, the calculation remains the same; however, the velocity and fall angle of the hail may be affected by the wind (Φ below, Figure 3), ultimately affecting the impact angle with the module. While this model provides a valuable baseline, it is built on simplifications that don’t fully capture real-world hail behaviour.

Elastic vs. inelastic conditions

While the simple kinetic energy model provides a useful starting point, it overlooks key complexities in how impacts cause breakage. The kinetic energy of a falling hailstone represents the total energy available to be transferred, absorbed, or dissipated during the collision. But fully understanding how, why and when modules break requires knowing how that total energy is distributed during a collision. Factors such as how concentrated and abrupt the energy transfer is (i.e., the force applied to the module), how kinetic energy converts into deformation or vibrational energy and how the tangential component of kinetic energy generates additional shear forces all play a role in breakage—factors that a simple kinetic energy model fails to capture. 

In short, using kinetic energy as the sole predictor for breakage probability— and assuming it scales with cos²(θ) as in a perfectly elastic collision — may be fundamentally flawed. 

Lab testing from Groundwork Renewables (2025 Solar Risk Assessment) reveals that the inelastic complexities of hailstone collisions may be significant when accounting for angled impacts. The simple elastic kinetic energy model KEcos²(θ) underestimates the energy delivered to a sensor under angled impacts by up to 69% at 75°. This observation may be explained by the simple model assuming that all transferred energy follows the cos²(θ) relationship, a relationship that only applies to perfectly elastic collisions. It overlooks energy converted to deformation or vibration and ignores tangential kinetic energy that can generate shear forces—mechanisms at play in an inelastic collision. Furthermore, previous studies on rockfalls³ have noted that the inelasticity of the collision increases with increasing impact angle, meaning that this divergence from the perfectly elastic model would be expected to increase with higher stow angles. In plain terms, the benefits of higher stow angles would be especially overstated if the simplified model were used when compared to the lower tilt angles. 

Taking these corrections for inelasticity into account, kWh Analytics and GroundWork Renewables derived an increase in the probability of module breakage of up to 48% for 7.5cm (~3”) hail when compared to the simple elastic model (2025 Solar Risk Assessment). 

The real-world implications are striking. Using a traditional physics-based model that does not account for inelasticity, a 2mm glass module has approximately a 36% chance of breakage at ~3in (7.5cm) hail under 40mph winds when stowing at 75°. When we include inelasticity into modelling assumptions, the probability of breakage jumps to approximately 84%. 

While impact dynamics with a sensor differ from those with a PV module, this analysis provides a directionally accurate approach to adjusting for stow angle. We urge PV testers to conduct hail test-to failure experiments at various stow angles to better capture real-world impact behaviour, including inelastic effects. This direct approach would reveal the true influence of stow angle, providing far more reliable insights than simply assuming breakage probability scales with the normal (perpendicular) component of kinetic energy.

An aside about wind

Studies show that the probability of module breakage from hail decreases significantly when panels are faced away from the wind, but this scenario is not always possible. Large utility-scale solar installations can span many acres, and the wind direction at one corner of the plant may differ from that at the opposite end. In these non-ideal scenarios where modules are tilted into to the wind, high-degree tilt angles are more likely to prevent breakage than low angles, especially for thinner glass modules.

Insurance loss modelling

Because insurers rely on physics-based models to price hail risk, flaws in those models can lead to inaccurate assessments of project vulnerability and mispriced premiums. For the few insurers offering premium differentiation for stow, utilising the simple kinetic energy model may overestimate the effectiveness of stow by nearly 50%. When these models predict lower damage probabilities for installations with stow capabilities, insurance companies may price policies based on protection levels that differ from field reality. Understanding the true physics of hail impacts is helping the industry develop more realistic expectations about the effectiveness of protection. While stow strategies remain valuable components of comprehensive protection, recognising their actual performance levels allows for better planning, risk management and cost-benefit analyses. This understanding encourages continued innovation in material improvements and multi-layered protection approaches that can deliver the reliability both project owners and insurers require. Insurance companies are actively addressing this challenge. Progressive renewable energy insurers now request detailed documentation of protection measures and are developing more sophisticated models that better account for the actual performance of stow strategies. Some carriers are beginning to offer premium differentiation for projects that combine multiple protection approaches rather than relying solely on positioning systems.

The path forward

The research reveals a fundamental challenge facing the solar industry: current hail modelling may be underestimating damage risk by up to 48% for large hailstones, even when panels are positioned at high-degree stow angles. This modelling gap could potentially create cascading effects across technology investment, insurance pricing, and operational strategies that the industry must address through comprehensive protection approaches as well as further quantification of the effects of stow. While our corrected modelling shows that stow provides less protection than traditional calculations suggest, effective hail protection still works when implemented as part of a multi-layered strategy. This shift has created new requirements for project development. Asset hardening measures now influence project economics from initial design through ongoing operations. VDE Americas, in collaboration with Wells Fargo, has developed a best practice guide for solar resilience⁴, identifying several critical protection strategies:

Module selection: This represents the most fundamental choice in hail protection. The popular 2mm glass/glass construction performs poorly when subjected to hail impacts, due to the thinner, untempered front glass. Upgrading to a 3.2mm glass/ polymer backsheet module provides measurably better resilience, especially if the front glass is tempered. Even better, using a thicker front glass, such as 4mm glass, is thought to increase resiliency, and the latest 3.2mm/2mm glass/glass modules also offer increased protection compared to 3.2mm glass/polymer backsheet. These configurations use 3.2mm tempered glass for the front surface where hail impacts occur, with 2mm glass on the rear for structural integrity. Initial studies are showing a marked improvement over standard 3.2mm/polymer backsheet construction, with panels sustaining up to 1.7x higher impact energies to the front glass without glass breakage (Groundworks and kWh Analytics, 2025 Solar Risk Assessment). While the front glass thickness is the same as the 3.2mm/polymer backsheet, industry speculation suggests this increased resilience is due to the increased rigidity of the module as a whole from using the 2mm glass backsheet, but research is still ongoing.

Hail stow: The act of tilting panels into steep angles to reduce the probability of glass breakage during wind or hail events demands reliability across multiple interconnected components: weather monitoring alerts must be live and in real-time, the trackers must have reliable power to enter into stow, operators must know and employ the appropriate procedures and communication networks must be fully functional to deploy a stow command uniformly across the entire solar array. Regular testing can reveal potential failures ahead of a storm, and the most effective installations ensure redundancies across critical components (weather alerts, communication nodes, etc.).

Operational protocols: These extend protection beyond equipment specifications. Night stow procedures ensure protection during overnight storms when manual intervention is more difficult. Documentation protocols that satisfy insurance requirements are becoming essential for favourable coverage terms.

For operational sites that do not have 3.2mm or thicker glass installed, all is not lost. VDE Americas shared a case study in the 2025 Solar Risk Assessment that demonstrates how proper operational protocols can deliver exceptional results, even without thicker modules. Three projects in Fort Bend County, Texas, using standard 2mm dual glass panels successfully weathered ~4in (100mm) hailstones that devastated the nearby Fighting Jays site. Their success came from flawless execution: reliable 52° stow positioning, robust communication systems and comprehensive operational protocols that ensured every tracker responded properly. Two sites sustained zero damage, while the third saw minimal impact only due to a pre-existing tracker motor issue and flying debris. This validation demonstrates that while thicker glass provides superior protection, operational excellence with proven materials can still deliver remarkable resilience. Getting the hail modelling right matters for everyone in the solar value chain. Accurate risk assessment enables appropriate insurance pricing, proper economic incentives for effective protection strategies, and continued innovation in technologies that deliver real-world resilience. The combination of improved materials, reliable stow systems and comprehensive operational procedures works when implemented together. To close the gap between perceived and actual risk, the industry must adopt empirically validated models, optimised around the physics of what actually happens when hailstones hit solar panels to ensure that our renewable energy infrastructure can withstand the increasingly severe weather it will face.

References

[1] https://www.woodmac.com/news/opinion/solar-surge-the-us-solar-industry-shattersrecords- in-2024/

[2] 2025 Solar Risk Assessment: https://kwhanalytics.com/solar-risk-assessment

[3] Wang, Yanhai, et al. “Effects of the Impact Angle on the Coefficient of Restitution in Rockfall Analysis Based on a Medium-Scale Laboratory Test.” Natural Hazards and Earth System Sciences, Copernicus GmbH, 19 Nov. 2018, doi.org/10.5194/nhess-18-3045-2018.

[4] VDE Americas and Wells Fargo: Best practices for hail stow of single-axis tracker-mounted solar projects, https://www.vde.com/en/vde-americas/newsroom/240221-hail-stow-tech-memo

Authors

Nicole Thompson is a senior manager of data science at kWh Analytics. Prior to joining the team, Nicole worked as a data scientist at an AI company where she developed explainable industrial AI solutions, with a focus on reasoning algorithms. In her graduate studies, her research was centred around nanocrystals for bioimaging as well as applying data science to differential capacity analysis of batteries. Nicole earned her M.S. in chemical engineering with a data science option from the University of Washington and her B.S.E in chemical engineering from Case Western Reserve University.

Reilly Fagan is a senior data analyst at kWh Analytics. Prior to joining kWh Analytics, Reilly worked at a as a lead research analyst in the research department of a venture capital firm. There, she created research reports for corporate clients on solar technology, battery recycling, sustainable aviation fuels, and more. Reilly received a B.S. in chemical engineering from the University of Colorado at Boulder.