Solar Green roofs

Here you will find information about Solar Green roofs.

Useful publications:

Solar Energy Handbook

Guidance of city of Vienna on Combining Solar Technology with Green Roofs & Vertical Greening Systems

Solar Energy Handbook's cover

1. Using solar energy and building surfaces in the city – Now and in the Future

Using solar energy and building surfaces in the city - Now and in the Future

The last few decades have seen significant changes in the demands placed on urban built environments. At the same time, climate-related problems in new and existing buildings are being aggravated by more and more surface sealing, the urban heat island effect and less water evaporation from green spaces, including parks, roofs and building facades. The number of very hot days in Vienna is expected to double over the next 100 years, causing the amount of energy needed for cooling to increase threefold over the next 50 years. Alongside the creation of green spaces, the surface areas of existing and newly built structures can be used to generate energy. Photovoltaic systems and solar thermal systems placed on walls and roofs can generate electricity and heat. As space and surfaces are limited resources, it is important to integrate different needs in designs for multi-purpose use.

Figure 1: Potential effects of building greening on the building, its immediate surroundings and the wider urban space
Potential effects of building greening on the building, its immediate surroundings and the wider urban space

Meeting future climate and energy challenges in urban environments requires solutions that combine and draw upon synergies from solar and photovoltaic technologies, “Green City” solutions and nature-based systems.
Under the headline theme “Using Solar Energy and Building Surfaces in the City – Now and in the Future”, this handbook presents the technologies that are available to harness solar power as we look to the future. By describing in detail how electricity and heat generation technologies can be applied in combination with vegetation on buildings and which synergies can be expected from this design approach, the authors hope to provide a tool that will help building designers to optimise the use of solar energy. The combination of technology and nature enables us to use solar energy most effectively and to improve the urban climate, thus raising the quality of life in the city.

The roofs of existing buildings offer great potential for the installation of solar technology, and we should make use of it. Nearly two-thirds of the total roof surface area of the City of Vienna are at least theoretically suitable for retrofitting with solar technology. The surface area suitable for greening has been calculated at about 5,800 hectares of rooftops and 12,000 hectares of wall surfaces (net figures).

And the relevant technologies are evolving all the time. In photovoltaics, for example, solar cells are now available in different colours. They no longer have the typical look of a PV array and can be integrated as design features in building facades. Thanks to these advances, solar technology has become very versatile and can be integrated in urban settings with hardly any constraints on design. Likewise, coloured glass for solar heating systems has been on the market for several years now, enhancing the appearance of solar thermal collectors on building walls.

Vegetation on roofs and walls is a valuable asset in densely built-up areas, providing benefits in terms of microclimate, energy, economy and ecology as well as prolonging the useful life of buildings. Moreover, greening of buildings creates amenity value; providing a more pleasant environment for people, greenery contributes to human well-being and quality of life.

The combination of building greening and solar technology gives rise to synergy effects which help to increase the efficiency and performance of existing surface systems. Greening/cooling the rear of a PV facade has a positive effect on the performance of the PV modules. Combining PV and solar heating with a green roof creates shaded areas on the roof that function as new habitats for diverse plant and animal species.

Technologies that can be used on building
surfaces – photovoltaics, greening with vegetation and solar thermal systems

2. Technologies

The following section presents the three technologies that can be used on building surfaces – photovoltaics, greening with vegetation and solar thermal systems, and describes their design and how they function. Additionally, there are sections on community installations and the Renewable Energy Development Act.

2.1. Photovoltaics

2.1.1. How does a photovoltaic system work?

Photovoltaic (PV) technology uses solar cells to transform the energy from sunlight intoelectrical energy. PV installations can be mounted directly on a building (on the roofarea) or integrated into the building facade. There are also free-standing PV systemsmounted on the ground.The components of a PV system are: the PV modules, wiring, safety devices and acurrent inverter. A power storage unit (battery pack) can be added to store energy andthus improve the PV system’s effectiveness.The sunlight that is intercepted by the PV modules is transformed into electricity, withboth direct and indirect solar radiation being used. The PV modules produce directcurrent, which has to be converted to alternating current to be usable in the building.This is done by means of a current inverter. The solar-generated electricity is now readyfor use, to run electrical appliances in the building, power vehicles and generate heat.PV systems are designed to use most of the electricity generated on site, reducing the need to buy electricity from the grid. When the PV system generates more power than is needed, the surplus may be either stored in a battery or fed into the grid, i.e. sold to a utility company.

Figure 2 - Design of a grid-connected PV system
Design of a grid-connected PV system

2.1.2. Structure of a PV module
– Solar cells:
The smallest unit in a PV module is the solar cell, which converts sunlight to electrical energy. The module is made up of a number of …

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Solar panels and green roofs – how to plan for the best combination?

Article of Over Easy Solar

Modern urban buildings must take maximum advantage of all spaces, including rooftops. In many cities, there are regulations that require green roofs, and often architects and planners believe that it rules out solar energy. But the good news is that you can make green roof and solar a great combination!

Types of green roof

There are different types of green roofs, and many ways to build them up. Generally, a layered structure consisting of a root barrier, a drainage system, a substrate (soil) and plants build up the green roof.

Most green roofs are so-called extensive green roofs, planted with sedum plants. Extensive means that they are optimized for large area coverage, low weight and low maintenance needs. Sedum plants are succulent plants that can hold a lot of moisture and therefore resist long periods of little rain and the strong sun and heat that rooftops are exposed to. The sedum plants typically grow 5-15 cm over the rooftop, depending on the season, the substrate thickness, climate and maintenance. Extensive green roofs are designed to be light-weight, and typically weigh 50-150 kg/m2 when soaked in rainwater. Extensive green roofs are good for combination with solar panels.

Photo of sedum rooftop with flowers; an extensive green roof, sedum with flowers
An extensive green roof, sedum with flowers

The other main type of green roof is intensive green roofs, which offers thicker soil substrate and more biodiversity. The weight is high (150-800 kg/m2), and therefore put high demands on the building structure. Plants include bushes, weeds, grasses and sometimes even trees, and the typical plant height is in the range of 30-60 cm. Intensive green roofs are difficult to combine with solar panels.

Checklist for combination roofs

To combine green roofs with solar energy, you need to plan well. There are many examples of installations that don’t work very well. We here want to propose a set of design principles and guidelines for planning a good combination:

  • Know your type of green roof

To plan a working solar installation on a green roof it is important to know the type of green roof to ensure that you select a suitable solar solution. The substrate thickness, plant height and maintenance needs are crucial to know.

  • Plan for the right roof load

While green roofs in themselves add substantial weight to the roof, adding a solar installation may end up with doubling the roof load. For existing buildings, this is important to know in order not to compromise the safety of the building, and new buildings may need to be reinforced with respect to original plans, adding cost and consuming more concrete and other materials in the construction process.

  • Ensure low conflict between plants and solar panels

It is important to plan for a solution where the green roof and the solar panels will not be “fighting against each other”. Solar panels may alter sunlight, evaporation and water flow on the roof, and might create unexpected negative consequences. It is important not to completely cover the green roof with solar panels. A conventional east-west solar installation may provide a lot of kWp in theory, but the fact is that plants will end up covering the solar panels to a smaller or larger degree. In general, a green roof solar installation will have less power (kWp) than a completely covering flat roof solar installation. We recommend planning with 50-70% of a the power of a conventional solar installation for flat roofs, otherwise it will be a suboptimal solution both for the green roof and the solar panels.

  • Optimize for easy maintenance

Although extensive green roofs require very little maintenance, they need more than traditional dead materials. You must make sure that the green roof is accessible through the lifetime of the building. If planned wrong, as we will see below, the combination of green roof and solar may also generate a much higher need for maintenance than what each solution separately would have.

Types of solar installations for green roofs

A solar installation consists of solar panels, a mounting system, cabling and an inverter. The type of solar panels and mounting system is extremely important to make a good combination roof.

There are three main ways used to build a solar installation on a green roof:

  • Conventional solar panels and conventional flat roof mounting structures

For flat roofs without green, regular solar panels that measure about 1 by 2 m in size are mounted into a mounting system in an east-west configuration. The solar panels will cover nearly the entire roof, except for some walkpaths in between the panels for most mounting systems. While this is an efficient and well-established way to build a solar installation, it is not a good idea for combination with a green roof. Why? Because the large solar panels will cover the green roof almost completely. Below the panels there will be a humid and dark environment, which favors the growth of plants that can grow fast to reach the light above the solar panels. These plants will shade for the solar installation and generates a massive maintenance need to remove them continuously through the growth season. The large solar panels will also make the maintenance very difficult, as maintenance personnel will need to lean over the large solar panels without putting much weight on them to remove the weeds in between. This combination is therefore not recommended.

Photo of weeds that grow at the low point when conventional solar panels are combined with sedum roofs
Weeds grow at the low point when conventional solar panels are combined with sedum roofs
Photo of sedum that shades the solar panel
Sedum shades the solar panel
Photo of example of bad combination of solar panels and green roof, solar panel completely shaded
Example of bad combination of solar panels and green roof, solar panel completely shaded
  • Conventional solar panels with a specialized mounting structure

There are several providers of specialized structures for usage of conventional solar panels on green roofs, for example the well-established solutions from Bauder and Zinco. These solutions provide an elevated mounting of the solar panels at an angle of 15-30 degrees, with ample space between, for plants to grow and maintenance personnel to move. The elevated and sparse installation of solar panels make them highly exposed to winds and winds can generate substantial lift forces on the structure. Therefore, they need high ballast and may add up to 100 kg or more per square meter of rooftop. When planning such an installation it is crucial that the building is designed for such loads, and, for retrofitting of existing buildings with green roofs, it might not be a viable solution. The roof is relatively easily available for maintenance. However, since the tilted panels will concentrate rainwater on the lower end of the panel, there will often be accumulation of plant growth at the lower end of the panel. Good planners of such installations know this, and intentionally divide the thickness of the green roof substrate on the roof, so the lower end of the solar panel has thinner substrate, in order to avoid growth of tall plants that shade the solar panels on the lower edge. In the end, these installations are well-established in the market, but the installation may be complex, the weight might be high, and therefore it is not always a possible solution.

Photo of specialized green roof solar allows for space and light to sedum
Specialized green roof solar allows for space and light to sedum
Photo of specialized mounting systems for PV installations on green roofs that allow for space to the plant growth
Specialized mounting systems for PV installations on green roofs allow for space to the plant growth
Photo of solar mounting systems for green roofs taht require a lot of ballast
Some solar mounting systems for green roofs require a lot of ballast
  • Specialized vertical bifacial solar panels in a specialized mounting structure

To address the challenges that arise when using conventional solar panels (weight, maintenance, etc), you might consider a third way to install solar panels on green roofs. An example of this is the system offered by Over Easy Solar. Use of vertical solar panels in combination with green roofs were first tested in Switzerland in 2015, and it offers a solution that may give the best of both worlds. The vertical solar panels do not cover the green roof, so all of the plants get access to rainwater, light and evaporation. Small vertical solar panels mounted at a suitable distance over the rooftop require little or no ballast, making it a very light-weight and easy system to mount. The VPV solution from Over Easy Solar normally only adds 11 kg/m2 to the rooftop. However, you still need to know which type of green roof is under, and for intensive green roofs this is not a recommended solution since tall plants will overgrow the solar panels.

Photo of vertical solar panels - a good combination with sedum roofs
Vertical solar panels is a good combination with sedum roofs
Photo of sedum that gets light and water with vertical PV panels on green roofs
Sedum get light and water with vertical PV panels on green roofs
Photo of vertical solaar pannels on a Green roof

In sum, you need to use a specialized system for green roofs. Over Easy Solar’s solution with the VPV Unit is a good option, and there are also other alternatives that makes a high-quality and low-maintenance green roof solar installation possible.

BuGG Focus „Solar Green Roof”

Excerpts from the BuGG technical information “Solar Green Roof”

Bundesverband GebäudeGrün e. V. (BuGG),
German Association of Building Greening

Photo of a Solar Green roof

Basic key factors for „Solar Green Roofs”

The following basic principles must be considered for a long-term implementation of Solar Green Roofs:

• Avoid shading of the solar modules

• Position modules and module rows in such a way that maintenance is easily possible

• Regular, professional maintenance

• Early communication and coordination of the involved parties

• Preferential use of load-supported systems (0 – 5° roof pitch ) to avoid roof penetrations.

Photo of a Solar Green roof

Avoid shading due to plant growth

• Sufficient distance between the substrate surface and the lower edge of the module of at least 20 – 30 cm. Depending on the plant selection, the distance should be even larger if necessary

• Use of suitable plants with low growth and dense surface closure

• Low substrate height (of about 5 – 8 cm ) in front of the solar modules to exclude taller-growing species. If necessary, ensure that the minimum load is applied to secure the stand.

Photo of a Solar Green roof

Substrate height

The vegetation support layer can be installed continuously over the entire roof surface at the same construction height of approx. 8 – 10 cm, depending on the type of greenery, vegetation goal and minimum load to be achieved (for load-supported systems). Or it can be modelled in waves with a construction height of 6 – 15 cm so that there is less substrate in front of the solar modules than under the modules.

Photo of a Solar Green roof

Schematic representation of a load-supported Solar Green Roof structure (deviations possible due to manufacturer and system)

Schematic representation of a load-supported Solar Green Roof structure

Solar Green Roof with south orientation

Solar Green roof with south orientation

Solar Green roof with South orientation

Solar Green roof with South orientation

More information and the complete technical information “Solar Green Roof” can be found on the website!

Technical information about Solar Green roofs

Analysis of innovative system designs for a power market optimised PV portfolio in Germany

By enervis energy advisors GmbH

Communication on the potential of applied PV in the European Union: Rooftops, reservoirs, roads (R3)

Georgia Kakoulaki, Nigel Taylor, Sandor Szabo, Robert Kenny, Anatoli Chatzipanagi and Arnulf Jäger-Waldau (2024)


Photovoltaics (PV) is a cost-competitive and scalable technology for electricity generation that plays a crucial role to accelerate the European energy transition and achieve carbon neutrality. Large-scale installation of rooftop PV, as well as innovative PV applications such as floating PV coupled with hydropower and bifacial PV along roads and railways, offer multi-benefits, not least in reducing competition for land. In this study, we present a geospatial approach to assess the pan-European technical potential of these three applications, using publicly available datasets. The findings reveal that the PV total installed capacity could exceed 1 TWp, which is far larger than the total PV capacity for 2030 in the EU Solar Energy Strategy (720 GWp) and would be a significant contribution to the several TWs needed for the overall transition to net-zero by 2050. The evidence presented is a useful starting point for policy-setting at national and regional level, as well as for research and detailed analyses of location specific solutions.

Overview of the potential and challenges for Agri-Photovoltaics in the European Union

Anatoli Chatzipanagi, Nigel Taylor and Arnulf Jaeger-Waldau (2023)

Agri-Photovoltaics (Agri-PV) consists in the simultaneous use of land for both solar photovoltaic power generation and agricultural production. It is an innovative form of PV deployment that has attracted attention worldwide and now also in the EU. It is highly relevant to a range of policies, including those related to the energy transition, agriculture, environment and research & innovation (R&I), and directly supports the goals of the European Green Deal (EGD). This report investigates the technical potential of Agri-PV systems in the EU ed Agricultural Area with Agri-PV systems could allow 1 TW of PV capacity, for instance well above the 590 GW foreseen by 2030 in the recent EU Solar Strategy communication. The study also maps the current situation in relation to the definition of Agri-PV and to related standards and guidelines, and draws attention to the challenges faced by developers for implementing projects. Furthermore, it explores the synergies between the agricultural, environment and energy policies and identifies the R&D challenges. Last but not least, the report makes recommendations regarding future steps to support the expansion of Agri-PV in the EU. The main points include a clear and concrete definition of Agri-PV, potentially as part of a European standard for Agri-PV systems, the promotion of Agri-national energy strategies (financial support, dedicated capacity targets, etc.), the simplification of permitting and grid connection procedures, the engagement of the rural communities to the planning and decision making and the assurance of the welfare of the farmer (economic benefit, security of property, etc.). Continued research and development, in particular cross-cutting studies that take into account energy, crop yield and biodiversity aspects, will be essential to overcome technical challenges and ensure fully sustainable solutions for the future.

Photovoltaic systems with vertically mounted bifacial PVmodues in combination with Green roofs

Thomas Baumann, Hartmut Nussbaumer, Markus Klenk, Andreas Dreisiebner, Fabian Carigiet and Franz Baumgartner (2019)


Dependent on the specific conditions flat roofs can be well suited for the installation of large photovoltaic systems in urban areas. For urban designers also other aspects, such as the insulation of buildings, cooling, air purification and water retention play an important role besides the ecological energy generation. The combination of photovoltaics and roof greening can therefore be an interesting fusion. It combines the advantages of a green roof with the local electrical energy production at the place of consumption.

However, using a conventional photovoltaic system with tilted modules in south or east-west direction on a green roof causes problems, as typical low tilt angels and high ground coverage rates result in an almost complete coverage of the roof surface. Plants, growing in between the covered areas provoke undesirable shading of the collector surface. Only a frequent maintenance procedure, complicated by dense PV system layouts, can avoid a reduction of the energy yield in the course of time.

Vertically mounted specially designed bifacial modules are an option to realize photovoltaic power generation in combination with a functional green roof at low maintenance costs. In this paper, we report on the layout and the energy yield of a corresponding system. Custom-made bifacial modules with 20 cells were produced and vertically installed in landscape orientation. The narrow layout of the modules lowers the wind load and reduces the visibility. The enhanced power in the morning and evening of vertically east-west installed modules can additionally lead to higher self-consumptions rates.

Despite having some shading and undergrounds with albedo factors of less than 0.2, the bifacial installation with a rated power of 9.09 kWp achieved a specific yield of the 942 kWh/kWp in one year (11.08.2017–10.08.2018). This is close to typical values of 1000 kWh/kWp achieved for south-facing PV systems in the same region.

The impact of the greening on the albedo and the system performance is investigated in more detail with two smaller sub-systems. The energy yields of the two bifacial sub-systems are compared to a monofacial, south-facing reference module. The use of silver-leaved plants in this system resulted in higher albedo values and a more resilient roof greening.

Performance Analysis of Vertically Mounted Bifacial PV Modules on Green Roof System

Thomas Baumann, Fabian Carigiet, Raphael Knecht, Markus Klenk, Andreas Dreisiebner, Hartmut Nussbaumer and Franz Baumgartner (2018)


A combination of PV and green roof is an ideal fusion in terms of ecology. The green roof improves the water retention in the city, whereas the PV system produces electric power at the place where it is consumed. Flat tilted modules in south or east west direction on green roofs generally require intensive maintenance to prevent them from being shaded by plants and often cover the roof area to a large extent. Because of the space requirement conflict between PV on the roof and green roofs, it is essential to combine these two systems in a smart way. Vertically mounted bifacial modules can be an option to combine PV and green roof and to also allow a cost-effective maintenance.
In this paper we report about the layout and the performance of a corresponding system, subdivided into two groups with differing albedo. Custom made bifacial modules with 20 cells were produced to reduce the wind load and to improve the general appearance. This 9.09 kWp bifacial plant achieved a specific yield of 942 kWh/kWp in one year (11.08.2017 to 10.08.2018). High quality DC power measurement systems are installed to monitor two modules in each bifacial test field and a reference south-facing module. This allows an energy yield comparison between the vertical bifacial test system with east-west orientation and the monofacial south-facing reference over four months of outdoor measurements. The use of plants with good reflective properties, which are also well suited two the ambient conditions on flat roofs, resulted in a yield increase of 17 % compared to a standard green roof planting. The vertically installed bifacial modules obtained an almost identical specific yield (-1.4 %) compared to a stand-alone monofacial south-facing reference module. Due to the increased yield in the mornings and afternoons, the vertical bifacial modules can achieve higher self-consumption depending on the load profile.

Biosolar Roofs: A Symbiosis between Biodiverse Green Roofs and Renewable Energy

Chiara Catalano and Nathalie Baumann (2017)


In a world where: the global population is doomed to count 10 billion by 2050, more than the 80% of the European population will live in cities, the energy consumption of the residential sector alone is expected to increase by the 48% from 2012 to 2040, the current vertebrate extinction rate is up to100 greater than the natural one; there is a need to conserve both the biological diversity and the natural resources within the built environment. In this scenario of densely urbanised areas, the economic conflict is clear when the increasing cost of the land makes less remunerative for municipalities the implementation of open green spaces. Moreover, the exclusive integration of photovoltaic panels on the roof tops is challenging the systematic adoption of green roofs as an environmental mitigation measure. However, researchers proved that green roofs optimise the energy production in different climates due to their cooling effect while providing habitat for wild flora and fauna and still reducing the stormwater run-off. In the last years (2011-2015), the green roof competence centre (now urban ecology research group) of the Zurich University of Applied Science (ZHAW, Switzerland) together with other partners from Austria (BOKU), UK (Onsite Training), Sweden (SGRI), France (INIT), Spain (Link) and Hungary (Sound Garden) cooperated together to develop a common strategy to promote among professionals (designer, firms, etc.) the combination of solar energy and biodiverse green roofs. This project named biosolarroofs ( was funded by the EU-Life Long Learning Program with a total amount of 250.000 €. The aim of this paper is to provide backgrounds, outline the strategies and the lessons learned from the biosolarrofs project but also the call for future research needed to support the symbioses of biodiverse green roofs and photovoltaic.

Performance Analysis of PV Green Roof Systems

Thomas Baumann, Daniel Schär, Fabian Carigiet, Andreas Dreisiebner and Franz Baumgartner (2016)


The common assumption is that plants underneath PV modules will cause a cooling effect resulting in a higher energy yield due to the negative temperature coefficient of PV power. The PV module temperature is measured to verify this thesis on a flat roof PV plant in Winterthur, Switzerland. Thirteen different mounting test field types with two different mechanical PV mounting systems are used, 20° or 15° tilted angles. Each type of mechanical mounting system is combined with planting beneath and with and without irrigation in the different test fields. The difference of the module temperatures, weighted according to the energy production, of all 13 analysed test fields is between ± 1.8 °C. This temperature difference results in a calculated energy yield difference of about ± 0.7 % for the used crystalline silicon modules. This value is in the range of the measurement uncertainty of the power measurement (± 1.2 %). Therefore the green roof has only negligible influence on the temperature reduction of the PV modules on the base of the used system components. However, a combination of PV and green roof is absolutely doable and recommendable provided that the mounting system is optimized for green roofs. Other more powerful benefits of such combination of green roofs and PV are the improvement of water retention in the city.