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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.
You gain plenty of postivies with that expense. Individual energy independence, system resilency, price stabalization, on and on. Lifespan of solar panels is 30+ years, and payback is often in the 4-7 year range. The numbers work out in your favor for a long time.
But okay, leaving out rooftop solar, and just going grid scale. With the renewable mix your city has now, and assuimg your intial power usage numbers are correct, you need 30% of the 150km to have the city running on 100% renewables. Thats 45 kilometers of nearby land. Seems way more doable than 150km, especially if its 10km here, 15km there, etc. Using a mix of agrivoltaics (farm + panels) and distressed land (old mining/industrial/military/etc) should make that even more viable.