# PV System Maintenance: Hail, Soiling and System Performance Under Control

Maintaining a PV system from 100 kWp comprises continuous monitoring, visual inspections after storms, early cleaning and specialist tests with thermography and electroluminescence. The effort pays: soiling costs 3 to 4 percent of yield per year in central Europe (BSW/Fraunhofer ISE), and hail damage reached CHF 292 million in 2021 according to AXA. Modules of hail resistance class HW3 or HW4, a fixed maintenance regime and trained staff secure system performance over the operating life.

Maintaining a PV system from 100 kWp: hail damage from AXA statistics, hail resistance classes HW3 and HW4, yield loss from soiling and a maintenance regime with thermography and electroluminescence.

Kanonische Seite: https://www.ampere-dynamic.ch/en/knowledge/pv-system-maintenance-hail-cleaning/
Veröffentlicht: 2026-09-02

**Key Takeaways**

- **Hail is the largest single risk:** Storm damage at AXA Switzerland fluctuated between CHF 61 million (2020) and CHF 292 million (2021); the 30-year average is around CHF 115 million (AXA, June 2025).
- **HW3 is the baseline, HW4 in hail regions:** SIA 261/1 and the VKF hail register set hail resistance class HW3 as the standard; Swissolar fact sheet 17 recommends HW4 in exposed regions.
- **Soiling costs 3 to 4 percent per year:** Without cleaning, soiling losses in central Europe add up to more than 10 percent (BSW/Fraunhofer ISE). Vegetation can cost around 25 percent in our experience.
- **Cell damage is invisible:** Microcracks after hail go undetected in a visual inspection. Thermography and electroluminescence reveal them before the yield measurably drops.

A PV system from 100 kWp is an asset whose yield depends on factors that keep changing: falling feed-in tariffs, dynamic tariffs, shifts between production and consumption, storms and soiling. Maintenance decides whether the system holds the performance from the planning stage over its operating life. This article shows what damage hail and soiling cause, which [hail resistance class](https://www.ampere-dynamic.ch/en/knowledge/glossary/hagelschutzklasse/) (Hagelschutzklasse) is required today and what a maintenance regime that secures the yield looks like.

## How large is the hail risk for solar systems in Switzerland?

Hail is the most expensive weather hazard for roofs in Switzerland, and solar modules sit exposed on the roof.

The storm statistics of AXA Switzerland show the range ([AXA, press release of 4 June 2025](https://www.axa.ch/de/ueber-axa/medien/medienmitteilungen/aktuelle-medienmitteilungen/2025/20250604-unwetterstatistik.html)):

- **61 million CHF** — Storm damage AXA Switzerland 2020, a historically extreme low-loss year
- **292 million CHF** — Storm damage 2021, record year; the largest single driver was the hailstorm in Ticino on 28 June
- **115 million CHF** — 30-year average of storm damage at AXA Switzerland

In between lie 2022 at CHF 143 million, 2023 at CHF 239 million after further severe hailstorms, 2024 at CHF 132 million and 2025 at CHF 72 million. The trend follows no line but fluctuates from year to year around a high average.

The hot August of 2026 ended with a hailstorm: on 28 August 2026 a cold front brought hailstones of up to 6 cm in diameter and gusts of 142 km/h to Schaffhausen (MeteoSwiss). The climate research network NCCS expects fewer hailstorms in Switzerland but more large stones: hail becomes rarer but more severe. AXA adds from the insurer's perspective that storms discharge at shorter notice and more locally, often combined with downbursts.

> **Hail resistance class HW3 as the baseline, HW4 in hail regions**
>
> The hail resistance class states which hailstone a module withstands without damage: HW3 corresponds to 3 cm, HW4 to 4 cm stone diameter. SIA standard 261/1 and the VKF hail register make HW3 the baseline for components of the building envelope; in designated hail regions, Swissolar fact sheet 17 recommends modules of class HW4. There is no cantonal legal requirement for PV modules. Insurers and building owners increasingly demand HW3 as standard, however, and module manufacturers state the class in the test certificate.
>
> Source: [Swissolar fact sheet 17, hail damage (German)](https://www.swissolar.ch/01_wissen/fachwissen/photovoltaik/merkblaetter/21017d_merkblatt_hagelschaeden.pdf)

After a storm or hail event we recommend three steps: first check the monitoring for error messages and power drops, then a visual inspection for mechanical damage to modules, mounting structure and cabling, and a measurement if anything looks suspicious. Damage at cell level, the microcracks, stays invisible in the visual check and only shows in thermography or electroluminescence images.

## How much yield do soiling and vegetation cost?

Soiling reduces yield in central Europe by 3 to 4 percent per year; without cleaning the loss adds up to more than 10 percent (BSW-Solar/Fraunhofer ISE).

The evidence is clearer than many operators assume ([Bundesverband Solarwirtschaft, April 2026](https://www.solarwirtschaft.de/2026/04/15/studien-belegen-verschmutzung-verursacht-relevante-ertrags-und-wertverluste-bei-pv-anlagen/)). For Switzerland, IEA PVPS documents a case next to a railway station with losses of up to 10 percent. And partial shading has a disproportionate effect: according to measurements by HTW Berlin, 10 percent shaded module area leads to 15 to 20 percent loss on the affected string.

Vegetation is the underestimated case. A tree at the edge of the roof that cast no shadow at commissioning reaches the first row of modules within a few years. Because the modules of a string are connected in series, the weakest module throttles the whole row.

Our operating experience confirms the order of magnitude. Systems near motorways soil so heavily within one to two years that a single cleaning pass no longer suffices; even with cleaning robots several passes are then needed, which raises the cost considerably. One example system with moderate vegetation at the roof edge already showed around 25 percent power loss in our monitoring.

What that means in kilowatt-hours is shown by a calculation for the system of [Schenker Storen AG](https://www.ampere-dynamic.ch/en/references/schenker-storen/) in Schönenwerd: 1'847 kWp on more than 8'800 m² of roof, planned annual production of more than 1.8 million kWh. A soiling loss of 3 to 4 percent there corresponds to 54'000 to 72'000 kWh per year, electricity that would otherwise have replaced grid purchases at commercial tariffs. The consequence for cleaning schedules: the heavier the soiling, the higher the cost per pass. Cleaning early costs less than cleaning late.

## What does a maintenance regime for a PV system from 100 kWp look like?

Training the responsible staff, regular inspections and continuous monitoring are the basis for lasting high system performance.

**Four levels of system maintenance**

1. **Monitoring: detect deviations daily** — The monitoring compares production with the target value from irradiation and temperature. Power drops of individual strings or inverters are noticed before they show up in the annual balance.
   - Performance ratio per string and inverter
   - Error messages with a recommended action, at Ampere Dynamic within 2 working days
2. **Visual inspection: after every storm and once a year** — Trained facility management staff check modules, mounting structure, cable routing and vegetation. External inspections on the four-eyes principle complement in-house responsibility.
   - Glass breakage, displaced modules, loosened clamps
   - Vegetation at the roof edge and degree of soiling
3. **Cleaning: early rather than thorough** — The timing of cleaning follows the site and the monitoring, not the calendar. One pass at light soiling costs less than several passes at heavy soiling.
   - Site factors: motorway, agriculture, industrial emissions
   - Robotic or manual cleaning depending on roof type
4. **Specialist testing: measure what you cannot see** — Specialist firms carry out insulation and power measurements, thermography and electroluminescence. Thermography shows hotspots and cell failures from the air, electroluminescence reveals microcracks on the individual module.
   - After hail and storm events
   - For unexplained power drops in the monitoring

Training is part of the regime. Facility management staff who know what glass breakage, a loosened clamp contact or a shaded string looks like in the monitoring report earlier and more accurately. External specialist tests build on that report.

Organisationally there are two paths: the business relies on its own responsibility, trains facility management and keeps control in-house. Or it outsources monitoring and inspection to a specialist firm. Under [Service & Operations](https://www.ampere-dynamic.ch/en/services/service-operations/), Ampere Dynamic looks after around 100 systems; the packages range from digital remote monitoring to operational management with documented condition reports.

## How do you get more out of the existing asset?

Maintenance secures the planned yield. Anyone who needs more electricity or wants to make better use of the surplus has four further options.

Market conditions keep changing, and with them the value the system creates. With a large surplus it is worth examining a joint [self-consumption](https://www.ampere-dynamic.ch/en/knowledge/glossary/eigenverbrauch/) arrangement as a ZEV (self-consumption community), vZEV (virtual ZEV) or LEG (local electricity community). Battery storage raises self-consumption and avoids feed-in at negative spot prices. An [energy management system](https://www.ampere-dynamic.ch/en/knowledge/glossary/energiemanagementsystem/) controls storage, charging stations and heat pumps by the electricity price. The figures are in the article [Electricity market 2026 and dynamic tariffs](https://www.ampere-dynamic.ch/en/knowledge/electricity-market-2026-dynamic-tariffs/).

Once all roof areas are used, facades, fences and car parks remain. Vertical PV on facades and perimeter fences uses the surface twice, produces more winter electricity and qualifies for the tilt angle bonus and the [Winterstrombonus](https://www.ampere-dynamic.ch/en/knowledge/glossary/winterstrombonus/) (winter electricity bonus). PV car park canopies earn the car park bonus, integrated systems with bifacial modules an additional subsidy, and they shelter vehicles and charging stations from the weather. The requirements and subsidy amounts are explained in the article [Winterstrombonus and vertical PV](https://www.ampere-dynamic.ch/en/knowledge/winterstrombonus-vertical-pv/), the technical implementation on the solution page [Vertical photovoltaics](https://www.ampere-dynamic.ch/en/solutions/vertical-pv-for-industry-and-commerce/).

The common thread stays the same: from the single asset to a flexible energy system that is maintained, monitored and controlled according to market conditions.

## Frequently asked questions

**How often does a commercial PV system need maintenance?**

Monitoring runs daily and reports deviations in production. A visual inspection follows every storm or hail event, and at least once a year. Cleaning depends on the site: next to motorways or industrial emitters it is due after one to two years in our experience. Specialist tests with insulation measurement, thermography or electroluminescence complete the cycle when anomalies appear.

**How do you detect hail damage on solar modules?**

Visible glass breakage shows up in the visual inspection. Damage at cell level, so-called microcracks, stays hidden from the eye and still reduces yield. It becomes visible in aerial thermography images or in electroluminescence measurements on the module. After a hail event we therefore recommend checking the monitoring for error messages first and then measuring selectively.

**Which hail resistance class does a PV system need in Switzerland?**

Under SIA 261/1 and the VKF hail register, hail resistance class HW3 is the baseline; in designated hail regions, Swissolar fact sheet 17 recommends HW4. There is no cantonal legal requirement for PV modules; insurers, planners and building owners increasingly demand HW3 as standard, however. The class is stated in the module manufacturer's test certificate.