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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.
Vienna isn't even the worst place for solar power. Consider places like Ireland, Scotland or Sweden for example. No wonder why wind, hydro or even tidal power suddenly begins to make a lot more sense. IMO, solar really begins to make sense in Spain, Italy, Greece and Turkey. Those places actually get some decent sunshine. Using solar power in Austria should be ok, but it's not great by any means.
Nearby mountains probably get in the way of building large solar facilities. Maybe you could combine solar power with farming. The panels can provide some shade to more sensitive plants.
See also: Solar Atlas
Sidenote: Iceland and the northern half of Norway, Sweden and Finland aren't even on the map. I guess Solar Atlas is trying to tell you that you don't need a fancy map to know that building solar in places like that is a waste of money.
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.