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Vienna would need about 150 km² of solar panels to produce enough electricity
(discuss.tchncs.de)
Yes, i did think about this in the shower.
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.
It depends on the size and geography of your installation. Everywhere short of the arctic circle, solar is more dependable than wind for a single home size or smaller installation. Wind and tidal power come in when you can throw up a bunch of 300m windmills to power a town. Wind in particular needs a very large area, and multiple disbursed areas are even better.