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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.
However, seasonal variations are a thing. You’d need a lot more in winter. June makes over 6x the power that January makes. Batteries aren’t really a total solution to that.
Check out https://pvwatts.nrel.gov/pvwatts.php
I suspect, though I don’t know for sure, that a 100% coverage of Vienna roofs is going to be close to that 400m2. Walkways, parks, roads, railways, and all that add up. Your square is over Prater, and Augarten for example, and the Danube.