Albedo

In the context of solar energy, albedo refers to the share of incident shortwave solar radiation that a surface reflects in total. It is dimensionless and is expressed between 0 and 1 or as a percentage. Zero means no reflection, one means complete reflection. However, an albedo of zero results in complete absorption only for an opaque surface: transparent materials can also let radiation pass through.

A ratio rather than power

For a horizontal surface: albedo = upward reflected irradiance / downward incident irradiance. Both quantities are given in watts per square metre. Their units cancel out. Fresh snow can reach values above 0.85. However, such figures are not immutable material constants.

A worked example: if 800 W/m² strike a ground surface and 160 W/m² of this is reflected, the albedo is 0.20, or 20 percent. With the same illumination and an albedo of 0.40, it would be 320 W/m². That is a doubling of the radiation reflected by the ground, not automatically a doubling of the solar power yield. The calculation does not yet say what share reaches a module.

For photovoltaic systems, albedo thus changes a boundary condition of irradiation. It is neither electrical power in kilowatts nor generated energy in kilowatt-hours. Nor does it denote the efficiency of a solar cell: more available light and a more efficient conversion of that light are different matters.

Why system geometry is decisive

Even monofacial modules can use ground-reflected light on their front side. The share depends, among other things, on the tilt and the visible ground area. In the simple model of a uniformly illuminated, diffusely reflecting ground, this contribution to the front side increases with a greater tilt relative to the horizontal. The statement that particularly flat modules generally benefit more would therefore be wrong.

In bifacial photovoltaics, the rear side is additionally active. Its irradiation depends on albedo, ground clearance, tilt, row spacing and shading. A bright surface helps only insofar as its reflected light reaches the cells. The spectral composition also matters: visible brightness does not reliably describe reflection across the entire wavelength range relevant to solar cells.

Three questions should therefore be kept separate: How much solar radiation reaches the ground? What share is reflected? How much of that then reaches an active module surface? Even a highly reflective surface may contribute little if it is permanently shaded or largely hidden from the cells' point of view. Albedo answers only the second question.

Simplified reflection models often assume uniform scattering in all directions. Real surfaces may deviate from this. Albedo alone therefore describes neither the spatial light distribution nor possible directional reflections. For a reliable system assessment, the model assumptions must match the actual surface and installation.

Variability and measurement

Soil moisture, vegetation development, roughness and snow cover change the albedo. The solar position and the share of direct or diffuse illumination also play a role. A table value or a single measurement may therefore describe the annual course inadequately. Diffuse sky radiation arises from atmospheric scattering; ground reflection is to be distinguished from it as a separate radiation contribution.

Measurement is often carried out with an albedometer consisting of two horizontal radiation sensors: one faces the sky, the other the ground. The ratio of their readings gives the albedo. Representative ground areas, suitable sensor positions and the avoidance of disturbing shadows are decisive. For bifacial yield assessments, it must also be clarified whether the measuring device and the model adequately account for the spectral sensitivity of the solar cells.

A sensible planning question is therefore not only: "What albedo value does the site have?" but also: "For which season, which area and which later management condition does it apply?" A value determined before construction begins, for example, is not necessarily representative if a different ground surface is created afterwards. Measurement, assumption and expected operating condition should be clearly distinguished in the yield assessment report.

Bright roofs and ecological limits

White roofing membranes and reflective coatings can reduce a roof's solar heat gain. For its temperature, thermal emissivity, i.e. the ability to radiate heat, matters in addition to albedo. High solar reflection and good heat radiation are different properties. Moreover, soiling and ageing can degrade reflection; for long-term assessments, aged surface conditions are more informative than factory-new values alone.

No blanket cooling bonus for PV modules mounted above can be derived from this. A cooler roof surface is initially a statement about the roof, not about the cell temperature. For the electricity yield, additional rear-side illumination and possible temperature effects must be assessed separately. Even a bright roof surface does not replace checking whether sufficient light reaches the cells under the specific mounting structure.

For ground-mounted systems, maximum reflection is not a sufficient planning goal. Vegetation cover and adapted maintenance can promote habitats and soil functions. Whether changing the ground surface makes sense therefore also depends on the original biotope, water balance and management. Light-coloured gravel is not automatically a sealed surface; possible soil impairments must be assessed on the basis of the specific construction.

For ground-mounted PV in Germany, project-specific permit conditions and funding conditions may contain requirements for vegetation cover, maintenance or soil protection. A technically favourable albedo value is not permission to circumvent such requirements. The local admissibility of a reflective ground cover therefore cannot be derived from the albedo value.

Significance for profitability and the power system

Economically, the additional net yield is decisive, not reflection alone. As a simple comparison calculation: 20,000 additional kWh per year yield, at an assumed revenue of 0.06 euros/kWh, initially 1,200 euros of additional annual income. Additional costs for materials, installation, maintenance and replacement would have to be deducted from this. The figures are a worked example, not a yield or price forecast.

For forecasts, realistic and cautious albedo assumptions should therefore be compared. A higher annual yield also does not automatically mean more electricity at times of particularly high demand. Albedo is an optical influencing variable, not a temporal control of generation and not a substitute for flexibility. Used precisely, the term explains one part of the solar yield – not its entire profitability, environmental compatibility or grid integration.

Sources

Best Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications: Fourth Edition — National Renewable Energy Laboratory (NREL), in Zusammenarbeit mit IEA PVPS Task 16; 2024-09. Abschnitte 2.5.3, 2.5.5.1 und 3.3.2: diffuse Strahlung, Definition und Veränderlichkeit der Albedo
Best Practices for the Optimization of Bifacial Photovoltaic Tracking Systems — IEA Photovoltaic Power Systems Programme; 2024-08. Abschnitte zu bifacialer Ertragsmodellierung sowie Abschnitt 3.6 zur Albedomessung, insbesondere S. 40–42
POA Ground Reflected — Sandia National Laboratories, PV Performance Modeling Collaborative; unbekannt. Berechnungsgleichung für bodenreflektierte Bestrahlungsstärke und unmittelbar folgende Modellannahmen
Guidelines for Selecting Cool Roofs — U.S. Department of Energy; 2010-07. Abschnitte „How Cool Roofs Work“, „Not All Cool Roofs Are White“ und „Cool Roof Economics“
Photovoltaik-Freiflächenanlagen — Umweltbundesamt; unbekannt. Abschnitt „Stärkere Ausrichtung der Flächenkulisse an Umweltbelangen“

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Edited by eCMS on 11.11.2026 13:50