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Glossary · Energy industry & energy transition

Irradiance (Solar Irradiance and Irradiation)

Irradiation solar energy photovoltaics Global radiation Direct Radiation Diffuse radiation yield

In solar energy, the German term Einstrahlung refers to the solar radiation that strikes a given surface. In photovoltaics it describes the supply of solar energy from which a module can generate electricity. In everyday usage the term stands for two different quantities. One is the instantaneous radiant power per unit area, the other is the radiant energy per unit area accumulated over a period of time. Confusing the two often leads to calculation errors.

The instantaneous quantity is technically called irradiance (Bestrahlungsstärke) and is given in watts per square metre (W/m²). It shows how strongly the sun acts on a surface at a particular moment. The quantity accumulated over time is called irradiation (Bestrahlung) or total irradiation (Einstrahlungssumme). It is usually given in kilowatt hours per square metre, for example kWh/m² per month or year. One kilowatt hour per square metre corresponds to 3.6 megajoules per square metre. By way of illustration: an annual total of around 1,086 kWh/m², averaged over all 8,760 hours of the year, corresponds to only about 124 W/m², because nights, winter and cloud cover all count. Peak values at midday, by contrast, are many times higher. For yield calculations, the accumulated quantity is the central one.

Direct, diffuse and global radiation

Solar radiation reaches a surface by different paths. Direct radiation comes from the direction of the solar disc without significant scattering. It is high under a clear sky and produces sharp shadows. Diffuse radiation arises from scattering by air molecules, aerosols and clouds. It comes from many directions of the sky and is also present under an overcast sky. Global radiation is the sum of direct and diffuse radiation on a horizontal surface. Most weather service and climate data refer to this quantity.

For photovoltaic systems, horizontal global radiation alone is not sufficient, because modules are usually tilted and oriented in a particular compass direction. What counts for electricity generation is the irradiance on the module plane, known in English as plane of array (POA). It is made up of three components: the direct component, the diffuse sky component and the radiation reflected from the ground or surroundings. Planning tools such as the European Commission's PVGIS therefore convert the horizontal data to the selected tilt and orientation. As a result, they output the irradiance in the module plane. In Central Europe, a south-facing tilted module receives a different amount of irradiance over the year than a horizontal surface, an east-west roof or a vertical façade.

With bifacial modules, that is bifacial PV, the rear side of the module also uses light. The reflectivity of the ground then becomes important, which is described by the albedo. Light gravel, snow or light-coloured roofing membranes reflect significantly more light than dark soil or grass.

Orders of magnitude in Germany

According to data from the German Weather Service (Deutscher Wetterdienst, DWD), the mean annual total of global radiation in Germany for the period 1991 to 2020 was around 1,086 kWh/m². Individual years deviate considerably from this. The lowest-radiation year since the start of reliable records in 1983 was 1987 with about 950 kWh/m², the highest was 2022 with 1,227 kWh/m². For 2025, the DWD reported an area average of 1,187 kWh/m², fourth place since 1983 and around 100 kWh/m² above the 1991 to 2020 mean. Since the early 1990s the series has shown an upward trend.

The spatial differences are also pronounced. Global radiation increases from the northwest towards the south. In 2025 the lowest values were in the north, particularly in Schleswig-Holstein. Regions such as the Lake Constance area, the Breisgau or Lower Bavaria, by contrast, reached peak values of over 1,300 kWh/m². These figures apply to the horizontal surface. On a favourably tilted, unshaded module surface in Central Europe, typically more radiation falls.

Irradiance is not electricity generation

Irradiance is the solar input variable. The electricity generated arises only after several conversion and loss steps. Modules convert only part of the incident radiation into direct current. The actual yield is influenced by module efficiency, cell temperature, angle of incidence, light spectrum, soiling, ageing, shading, cable losses, losses in the inverter and operating limitations. For system losses excluding temperature and spectral effects, PVGIS by default applies a flat 14 percent. This value is a guideline and does not apply to every system.

A common mistake is to equate high irradiance with high electrical output. The rated power of a module in watt peak is determined under standard test conditions (STC). These specify 1,000 W/m² of irradiance in the module plane, 25 °C cell temperature and a standardised reference spectrum. These conditions serve to compare modules and do not describe normal operation. In practice, high irradiance often coincides with high module temperatures. Crystalline silicon modules lose efficiency as temperature rises, so output then increases less strongly than the radiation values would suggest. How much a module heats up depends on the mounting: according to PVGIS, modules without rear ventilation can become up to 15 °C hotter at 1,000 W/m² than freely mounted ones. Conversely, cool, clear days, for example in spring, often bring particularly high instantaneous output. For the annual yield, however, it is not the individual peak value that counts, but the sum of usable irradiation over time.

Why irradiance matters in the power system

In photovoltaics, irradiance determines the generation profile over the course of the day and with the weather. It therefore acts directly on the residual load, on electricity prices, on grid loading and on the need for flexibility. On sunny days, the simultaneous feed-in from many solar systems can sharply reduce demand from the grid. If generation, consumption, storage and grid capacity do not match, this can lead to low or negative exchange prices, local grid bottlenecks or curtailment.

Irradiance fluctuates strongly in space and time. Passing cloud fields can change the generation of a region within minutes. For grid operators, direct marketers and balancing group managers, the short-term radiation forecast therefore matters alongside the annual value. It is the basis of the feed-in forecast and relies on satellite data, weather models and measured values. Forecast errors must be absorbed through intraday trading, balancing energy or control reserve. With a growing PV share, irradiance thus changes from a variable for site assessment into a weather variable that matters for the whole system.

Location, orientation and shading

Annual irradiance depends on the geographical location, but location alone does not explain the yield. Roof pitch, orientation and shading can cause differences within a single place that are similar to or larger than the north-south gradient in Germany. A heavily shaded south-facing roof can deliver less than an unshaded east-west roof. In its calculations, PVGIS also takes into account, for example, the terrain profile at the horizon, that is, shading by hills or mountains.

In photovoltaics, shading has a disproportionate effect because the cells of a module are connected in series. If part of it is shaded, this can limit the current of an entire cell string. Bypass diodes, power optimisers or microinverters reduce such losses, but do not eliminate them completely. Orientation also shapes the generation profile. In Central Europe, south-facing systems have high midday peaks. East-west systems spread generation more across the morning and afternoon. For self-consumption, grid connection and marketing, this profile can matter more than the maximum annual yield per kilowatt peak.

Typical oversimplifications

  • Sunshine duration is not irradiance: According to the definition of the World Meteorological Organization, sunshine duration counts the time in which direct solar radiation perpendicular to the direction of the sun is at least 120 W/m². It says nothing about how strong the radiation is above this threshold, and it does not capture diffuse radiation. Photovoltaic systems also generate electricity under cloud cover, although less. For yield models, radiant energy is therefore more meaningful than the number of sunshine hours.

  • The same rated power does not mean the same yield: Two systems of 10 kilowatt peak each can generate very different amounts of electricity, depending on tilt, orientation, shading and technology. The specific yield in kWh per kWp serves as a key figure for comparison, and it corresponds arithmetically to the full-load hours.

  • High irradiance does not automatically mean high value: A high annual total improves the prospects of yield. But it does not replace checking the grid connection, area, permit, curtailment risk and price profile. Where a lot of solar power is generated at the same time, the market value of the electricity generated can fall if storage, flexible demand and grids do not grow along with it.

Classification in planning and operation

For planning, irradiance data from ground measurements, satellite datasets, reanalyses and simulation programs are used. In Europe, PVGIS relies, for example, on hourly, satellite-based radiation data and, besides the mean annual yield, also states the year-to-year variability. Long-term means serve to estimate the annual yield. Hourly or quarter-hourly profiles are needed to calculate self-consumption, storage operation, connection capacity and marketing revenues realistically. Because individual years can deviate considerably from the mean, yield assessments often work with exceedance probabilities. A P50 value is statistically reached or exceeded in half of the years, a P90 value in nine out of ten years. The P90 value is correspondingly lower.

In operation, irradiance measurements in the module plane, for example with pyranometers or reference cells, are used for performance monitoring. If the measured yield is set in relation to the irradiance, key figures such as the performance ratio are obtained. This makes it possible to distinguish defective inverters, soiled modules, string faults or new shading from merely poor weather. Without reference to irradiance, a low daily yield can conceal a technical fault.

Irradiance shows what solar supply is available at a place and at a time. But it does not by itself explain how much electricity is generated, fed in, consumed or used economically. For that, module technology, system layout, weather, grid connection, market rules and consumption profile must be considered together. Used precisely, the term separates the natural energy supply from the technical conversion and from the use of the electricity in the system.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:43

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