Solar roof system website for the city of Bentheim - Calculation basis


Originally developed at the University of Applied Sciences Osnabrück, the method for determining solar potential makes it possible to automatically check the solar suitability of the roofs of each building of an entire city or a district on the basis of high-resolution laser-scanned data and to calculate the solar energy potential. PublicSOLAR by IP SYSCON GmbH calculated the solar energy potential of all buildings in the city of Bentheim. This is based on high-resolution laser-scanned data by the district government of Lower Saxony, LGLN. The method for calculating solar energy potential is based on geographic information systems (GIS). First the location factors roof pitch, exposure and surface size are determined for each homogeneous roof surface area. Highly accurate annual irradiation analysis is then used to precisely determine the solar radiation and the shading, caused by roof structures or vegetation, and included in the calculation of potential. The potential power output, CO₂ savings and installable KW capacity are calculated for each suitable roof.

Calculation method of solar potential analysis


The solar potential analysis is based on laser-scanned data collected in 2010 for the city of Bentheim with a total area of 88 km². The building footprints of the Automated Real Estate Map (ALK) were used for the pinpointing the 36,757 buildings. The building footprints show the exterior walls of the house. Roof overhangs are not included. Newly constructed buildings after the date of recording of the laser-scanned data have not yet been calculated and therefore not shown in the cadastre.

Irradiance analysis

The irradiance analysis determines the level of direct insolation and solar irradiation. Solar irradiation is crucial for the efficient use of sunlight. An annual irradiance analysis involves calculating the sun's position in minute-rhythm throughout the year, making it possible to precisely determine the annual level of solar irradiation. Direct insolation is used to calculate the shade factor. A sharp reduction in the direct insolation suggests strongly shaded regions. This can be caused by trees, adjacent buildings or roofs. In addition, depending on the angle of inclination, north-facing roof sections receive no direct sunlight. Heavily shaded roofs are deemed unsuitable and therefore excluded from the calculation. Minor shade reduces solar radiation and is included in the solar potential calculation. This is based on the average global radiation value of 976 kWh/m²/a measured on a horizontal surface. Roof areas suitable for solar energy installation indicate an irradiation of 70% or higher for thermal use and 75% or higher for PV use in the cities.

PV module efficiency

The evaluation of the potential to generate electricity is based on the efficiency of the PV modules. These modules have a 15% efficiency. The calculation of the potential power output is based on the assumption that the modules use a fixed tilt mounting system for flat roofs. The elevated installation allows 100% utilisation of sunlight, but it can only be used effectively on about 40% of the roof area.

CO₂ savings PV

The calculation is based on a CO₂ equivalent value of 0.633 kg/kWh derived from Germany's energy-mix (as of 2010). The production-related CO₂ emissions are also taken into account, as evaluated by GEMIS 4.6 for mono-crystalline systems. Accordingly, the CO₂ savings for a system with a 15% efficiency and 0.530 kg/kWh was calculated. The results of the calculation for power output form the basis for potential CO₂ savings.

kWp capacity PV

The rated output of photovoltaic systems specified in kilowatt power (kWp) is based on 6,7 m² per KWp. This corresponds to the capacity of mono-crystalline systems. For flat roofs, the potential kWp capacity is based on the assumption that a fixed tilt mounting system is installed. Assessment of potential and suitability classification, PV Sites assessed with potential for PV systems are evaluated for potential power output, the associated CO savings for PV systems of each roof in kg per year, the estimated volume of investment (euro) and the options for the type of KWp capacity to be the installed. The calculation of these underlying variables to determine the individual parameters for the use of photovoltaic systems represents a snapshot of the market situation. Efficiencies, prices and installation costs for PV modules are subject to change during the project phase due to factors such as technological innovation, production costs, demand and supply, and regional price disparities. The calculation of these system parameters provides an opportunity to conduct a cost-effectiveness analysis for each roof at a later date, with little effort, taking into account the then current values for module efficiency, investment costs, feed-in tariffs and financing terms. The result indicates the areas that have a solar energy potential of 100 % to 75 % of the maximum radiation energy. Roof areas suitable for PV use require at least 10m² of module area (3D surface) for pitched roofs. Flat roofs with a fixed tilt mounting system must have at least 40m² of suitable roof area for PV use. Classification of suitability levels for photovoltaic systems:

  • Highly suitable, >95% usable solar radiation
  • Well suitable, 80–95% usable solar radiation
  • Conditionally suitable, 75–80% usable solar radiation

Assessment of potential and suitability classification, solar thermal In principle, all areas that are suitable for PV systems are also suitable for solar thermal systems. Roof areas suitable for thermal use have a solar potential of 100% to 70% of radiation energy. Roof areas suitable for thermal use have a solar potential of 70% to 100% of radiation energy. The use of thermal systems requires a minimum area of 5m² (pitched roof). Flat roofs with a fixed tilt mounting system must have at least 10m² of space for solar thermal installation. There are two levels of classification:

  • Highly suitable, > 85% usable solar radiation
  • Well suitable, 70% – 85% usable solar radiation