She has collected data to answer the question: how much electricity could be produced if every suitable roof in the city were equipped with photovoltaic solar panels? Up to now, the balance sheet for solar electricity in Germany has been sobering. Only 0.3% of electricity comes from photovoltaic devices. That led the researcher to inspect the entire city area of Osnabrück from an airplane with an on-board camera – centimeter by centimeter. Every shadow cast by every chimney was registered and included in the calculation. The result was a highly precise map of all the city’s roofs. In a further calculation, Klärle only included roofs that could accommodate at least 20 square meters of solar cells. The result showed that 70% of the city’s private electricity needs could be generated using photovoltaic cells. The researcher says the investment involved could be written off in 10 years. From then on, the power would be essentially free of charge – apart from maintenance – and free of CO2 emissions. The idea appears to have convinced city officials. This year, many public buildings in Osnabrück are being equipped with photovoltaic devices.

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We're at the airport that serves Osnabrück in northwest Germany. It is set to become a truly solar city. Geomatics expert Martina Klärle is heading off on a surveillance flight. Laser scanners will be measuring every square meter of the town and its outlying areas. Klärle wants to make huge, polluting power plants a thing of the past. So she’s collecting data about Osnabrück’s roofs and the intensity of sunlight is that shines on them: "My vision is to use all suitable roof surfaces to make solar electricity."

The key to that vision is a complicated mathematical formula derived by her work at the University of Applied Sciences in Osnabrück. Called SUN-AREA, the project uses the laser scanner data to precisely calculate every roof’s potential as a solar collector. The path of the sun across the sky is included in the calculation – and so is the shadow cast by a chimney in the course of the day. The seasonal change in hours of sunlight is also included. The first computations have been carried out for Osnabrück. Engineer Martina Klärle: "The SUN AREA project has proved that 70 percent of the city's total demand for electricity can be covered using only the roofs that are already present in Osnabrück. In other words, if all the roofs that are especially suitable were now fitted with photovoltaic devices, we could meet 70 percent of the electricity needs of all of Osnabrück."

And the "especially suitable" rooftops make up only 20 percent of the city's roofs. Costs would amount to three billion euros. The project head says that expenditure is worthwhile, both in environmental and in economic terms. Geomatics Engineer Klärle: "The question of whether a photovoltaic collector pays off or not has two answers. Environmentallly speaking, a photovoltaic system will pay for itself after a year. Economically, a photovoltaic system will only become profitable for the operator after about 10 years."

After that, the city would be getting its electricity for free – apart from the maintenance costs. Now Martina Klärle is trying to turn her vision into reality. She gave city officials her map of Osnabrück – with an exact catalogue of all the suitable roofs. The data has been made public. And it's already met with a positive response. The city is stepping up efforts to equip public buildings - like this school - with solar collectors. Osnabrück has doubled its solar energy installations in the past year alone. And it's only the beginning.

This is what most roofs here still look like. The fraction of electricity produced with solar energy in Germany is still less than one percent – despite the steadily rising prices of fossil fuels like coal or oil. Professor Klärle: "We’re at the point where we can’t afford to get our electricity from coal-fired power plants, and we don't want to get it from nuclear power plants. I just won’t accept that we have all this potential on our roofs, and we don’t use it."

And – surprisingly – Germany is very well suited to generate electricity using photovoltaic systems. The reason is that the majority of the population is not concentrated in the urban centers, but spread out over rural areas. That means more space per person, and more roof area. That's reason enough to utilize the power of the sun more fully. Martina Klärle explains: "In the countryside, the ratio would be so good that we could have a 100 percent yield from solar energy. We could meet 100 percent of those electricity needs with photovoltaic devices. "

Klärle’s aerial laser scanner and her formula have helped her prove convincingly that a change in energy production policy is possible. Up to now, complete power coverage by renewable energy systems has been a dream. Now people are waking up -- and making it happen.