Irradiation at high altitudes for alpine photovoltaic systems

^{Created by V.G., BFH, on 14.01.2026}

The irradiation power of alpine photovoltaic (PV) systems can be very high for short periods, unlike in the midlands. Values of up to 2400 W/m² for instantaneous values are reported. This is mainly due to pressure-induced thin atmosphere as well as high ground reflections in the snow due to strong module inclination lead to significantly higher maximum irradiation at module level compared to the midlands. As a result, the corresponding short-circuit currents in PV modules can be up to 2.4 times higher than specified in the data sheet according to STC. By increasing the DC voltage, inverters can shift the operating point of PV modules outside the maximum power point (MPP) and thus reduce the current and power of the module array. This current regulation works much faster than the fastest changes in irradiation can be expected. Therefore, from this point of view, any amount of PV power can be connected to an inverter. If the irradiation is too high, the inverter simply reduces PV production (Figure 1). From the inverter’s point of view, reducing the power is therefore more a question of optimization than a safety function.

Figure 1: If the power of the PV modules is above the maximum input power of the inverter (1), the inverter reduces the power by increasing the voltage from the MPP to the maximum permissible power of the inverter (2) (origin of the graph [1])

The situation is different with the maximum short-circuit current. If a fault occurs in the inverter that leads to a short circuit in the module array, the inverter must be able to safely carry this short-circuit current or disconnect it. It is tested for this capability in accordance with IEC 62109-1 [2]. However, if the maximum short-circuit current exceeds the tested value, the inverter’s reaction is no longer deterministic. For this reason, the short-circuit current of the PV modules, like the open-circuit voltage, must at no time be higher than that for which the inverter has been tested.

The maximum non-concentrated radiant power of the sun from the distance of the earth is 1361 W/m². On earth, values above 1250 W/m² are practically impossible without ground reflections, which is why electrical installation guidelines typically stipulate a safety factor of 1.25 to laboratory test values at standard test conditions (STC) of GSTC = 1000 W/m².

The highest levels of irradiation are seen through the phenomenon of ‘cloud enhancement’. A cloudy sky is always significantly brighter than a blue sky, known as a clear sky. If the clouds at a location break up briefly and the sun shines through the clouds, the irradiation at the corresponding location accumulates and values above GSTC can be measured. Naturally, this condition does not last long, as clouds are not stationary (Figure 2).

Figure 2: On cloudy days the peak irradiance is low (left), on clear sky days it is high (right). However, the highest irradiance peaks are seen on slightly cloudy days, with clouds breaking up (center) [1]

Measurements that include ground reflections concentrate the sunlight to a certain degree and can therefore be significantly higher than the extraterrestrial irradiance. But such high-resolution irradiance measurements are generally not available.

These irradiation peaks were analysed using high-resolution measurements taken at two locations. One is Mont Soleil, 1289 meters above sea level, and the other is the test facility for the originally planned Schattenhalb alpine PV plant, 2049 meters above sea level. Global horizontal radiation measurements are available for both locations. At Mont Soleil 2024, these measurements showed a maximum value of 1564 W/m². At Schattenhalb 2025, the maximum irradiation was 1655 W/m². Both values are significantly above the extraterrestrial radiation constant of 1361 W/m².

At the Schattenhalb site, irradiance was also measured at module level (inclination 60°/azimuth -58°). Both in the middle row of modules and in the rear row, this resulted in a front-side irradiance of 1799 W/m². Looking at the bifacial yield of the two measurement points, the maximum irradiation was 2108 W/m² on the middle row and 2411 W/m² on the rear row. The rear row has the higher value because it is not shaded by other installations. These values are more than double the values for which the inverters are normally tested.

As already mentioned, these high-resolution values are not usually available, but rather an aggregation to, for example, 15 minutes. In order to estimate the effect of aggregations on the height of the irradiation peaks, an aggregation to 15 seconds, 1 minute, 5 minutes, 15 minutes and 1 hour was carried out using the high-resolution measurements from Schattenhalb.

Figure 3: Irradiance values from the real measurement (1s) and the various aggregations

Looking at the rear row, it can be seen that the effective radiation peak of 2411 W/m² decreases only slightly when aggregated to 15-second or 1-minute values. However, it falls below 2000 W/m² when aggregated to 5-minute values and then even to 1500 W/m² when aggregated to hourly values. This graph also shows that the real peak occurs in the winter half-year, whereas the 15-second/1-minute/5-minute peaks occur in the summer half-year. A closer look at the data shows that the real peak occurred on 31 March (one day before the summer half-year). This is the period when the sun is already quite high above the horizon, but there is still snow on the ground, causing ground reflections.

Figure 4: Ratio of true irradiation peaks to aggregated irradiation peaks

If the 1s values are assumed to be correct, aggregation to 15 minutes results in an underestimation of the actual peak by around 30%, and for hourly values by as much as 60%. This means that when aggregating to one hour, the peak must be multiplied by a factor of around 1.6 in order to estimate the true peak.

Figure 5: Statistical distribution of irradiance (left) and true peak to aggregated peak ratios (right)

When looking at the statistical distribution of radiation levels and the ratios of true peak to aggregate peak, it becomes apparent that there are various upward outliers. If the ratios of the Max-1s values in the respective hour to the aggregate hourly value are calculated, this results in factors of up to 5.4. However, the median ratio is 1.36.

Looking at the middle row, it can be seen that the total irradiation is slightly lower (-13%) due to the rear shading caused by the other rows. However, the ratio between the 1s peak and the aggregated peak does not differ greatly from the ratio for the rear row.

Figure 6: Irradiance values and the ratio of 1s-Max to Aggregation-Max from the effective measurement (1s) and the various aggregations

Requirements for inverters

Discussions with two manufacturers have revealed further points that are relevant for the installation of inverters in alpine areas. First, a summary is provided, followed by more detailed explanation.

At high altitudes: derating, mechanical load limits and difficult installation conditions are important issues.
Common installation errors are moisture ingress, incorrect mounting position, untested system voltages
Planning according to manufacturer specifications and installations instructions is essential for safe and long-lasting operation

Electrical properties

As altitude increases, air density decreases, which reduces cooling efficiency; nevertheless, many inverters operate at altitudes of up to 2000 meters without any reduction in performance.
Above 2000 m, manufacturers begin derating depending on the type of inverter, with permissible DC voltages decreasing to varying degrees depending on the manufacturer (e.g. 87% at 3000 m; other manufacturers: 1000 V to 3000 m, 945 V to 3500 m, 909 V to 4000 m).
AC-side tolerances may be limited above 2500 m.
The maximum permissible short-circuit current load depends on the actual I_sc of the PV modules and the design of the DC switch; rapid current limitation/shutdown (<1 ms) by the inverter serves safety purposes.

Installation instructions

The device should not expose to direct sunlight during installation.
Slight oversizing of the inverter, as permanently high voltages in combination with high currents have a negative effect on the service life of the inverter.

Common installation errors

Faulty system design
- No complete preliminary check of system compatibility. For example, system voltage
Installation in unfavorable weather conditions (rain, high humidity)
- Moisture forming in the housing. Long-term corrosion and insulation damage possible.
Leaking housing ports
- Screw connections not tightened to the prescribed torque.Blank covers of the PV connectors removed but not refitted.
Inverter mounted horizontally (0°) instead of with the required minimum angle of 10°.
Improper replacement of PV fuses
Incompatible PV connectors from different manufacturers

References

[1] C. Bucher,» Photovoltaikanlagen», Zürich: Faktor Verlag, 2021.
[2] IEC 62109-1:2010, «Safety of power converters for use in photovoltaic power systems – Part 1: General requirements», 2010.