Anyone flying by plane over Marl in the Ruhr area can now marvel at Germany's largest solar system on a roof: photovoltaic (PV) modules on a gigantic area of almost 100,000 square meters - that's about 14 football pitches - are spread over two logistics halls of the wholesaler Metro. The 43,000 modules have a total output of 18 megawatts and are expected to generate 16.4 megawatt hours of electricity per year. According to the metro, this could theoretically supply a good 5000 households a year with electricity. For Mona Neubaur, Deputy Prime Minister of North Rhine-Westphalia, the plant is a "key on the way to Europe's first climate-neutral industrial region," as she said on Monday at the official commissioning of the plant. Metro has been renting the two properties from real estate developer Goodman for several years, and the solar system was built and installed by renewables developer Bayware. According to the company's own information, the electricity generated is sufficient to cover its own needs, and the surplus electricity is fed into the grid.

Hanna Decker

Editor in the economy.

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Solar systems on commercial roofs are becoming increasingly popular. According to the German Solar Industry Association (BSW), more than 200,000 solar systems are installed on company roofs. Aldi Süd, for example, has now equipped almost 70 percent of its stores with photovoltaic systems. The trade fair in Frankfurt also generates electricity from solar radiation on some hall roofs. However, with 12 modules, the latest system on the roof of Hall 5300 is significantly smaller than that of the metro in Marl.

Costs for PV modules have fallen sharply

The German government is stepping up the pace when it comes to solar expansion. So far, 67 gigawatts of PV capacity have been installed in Germany, and by 2030 this figure is expected to rise to 215 gigawatts. For this to work, 2026 gigawatts are to be added every year from 22 at the latest, half of it on open spaces, the other half on roofs. For this to work, at least 6 to 7 gigawatts of it must be installed on commercial roofs, estimates the German Solar Industry Association. The annual expansion will therefore have to roughly triple from the current 2 gigawatts per year. The BSW assumes that so far only one tenth of the suitable roof areas of commercial enterprises are used for electricity generation.

For tradespeople, there are basically several ways to finance a solar system on the roof. The vast majority of plants do not run in "island operation", i.e. self-sufficient, but are also still connected to the public power grid. The operation of a rooftop PV system is particularly worthwhile for companies with a lot of self-consumption, i.e. those that also consume a large proportion of the electricity they generate themselves. This is because your own electricity is much cheaper than electricity from the energy supplier. Every kilowatt hour that is generated on the roof reduces the demand for electricity that has to be drawn from the public grid and for which the full levies, levies and grid fees are due.

Energy-intensive companies that produce in shifts, possibly even on weekends, can achieve self-consumption rates of up to 60 to 70 percent. Because the cost of PV modules has fallen sharply in recent years, the operation of a solar system is becoming increasingly attractive for such traders, especially since the EEG levy has been abolished since the beginning of the year.

Solar systems with several megawatts are still very rare

The calculation is not quite so simple for tradespeople who have large roof areas but do not have any significant electricity requirements apart from lighting, such as logistics halls. Here, the so-called simultaneity effect is only slightly pronounced, i.e. the times in which the electricity is generated or consumed do not necessarily coincide. These companies therefore feed a large part of the electricity they generate into the grid. The installation of storage systems can theoretically increase the self-consumption rate. Operators of plants with a capacity of less than 1 megawatt receive a fixed market premium for each kilowatt hour generated.

Solar systems on roofs with a capacity of several megawatts are still very rare. The operators of such mega-plants must participate in tenders issued by the Federal Network Agency, which, to put it simply, determine the amount of the market premium granted. In the most recent, significantly oversubscribed tender in June, 79 bids for an average of 10.2 cents per kilowatt hour with a total output of 192 megawatts were awarded. A total of 650 megawatts of capacity will be auctioned in this segment this year.