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Vienna would need about 150 km² of solar panels to produce enough electricity
(discuss.tchncs.de)
Just to put things into proportion:
This is vienna. Vienna has an area of around 400 km².
And the blue area is the area that would have to be covered by solar panels to produce enough energy for the whole city:
Source: I did the maths myself. I assumed that per person around 30 MWh of energy/year are needed. Data for this: our world in data, energy usage per person. It's well known that 1 m² of solar panel produces around 200 Wp and that's 200 kWh/year. So you need about 150 m² of solar cells per person. Vienna has about a million inhabitants, so that makes 150 km² of solar panels approximately.
Because putting solar panels on parking lots is more expensive than on fields. On a field you can just put stuff somewhere. For a parking lot you have to build raised platforms which have to have supports in places cars don't have to park/drive through etc, maintenance is more expensive due to being high above ground, and construction is more complicated.
Solar panels also aren't necessarily bad for nature, they actually help many plants and animals. Most fields weren't always fields, but used to be forests. The partial shade introduced by the solar panels is good, because the ecosystem is still adapted to that.
To put it simply: they're putting it in the places that are cheapest first. Once those are full or become more expensive, other places will see solar panel installations.