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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.
Uhm 30MWh? That can't be right...
In the past 5 years on average my home has used around 3.5MWh per year. We live here with two persons so that would be around 1.75MWh per person per year. Now let's say we are very efficient and say the average person would need about 2.5MWh per year, that's still one order of magnitude off from your number.
I can't believe the average person in Vienna uses that much energy, especially not those living in apartment buildings.
Edit:
Ah they include everything in that graph, so the entire energy requirements of the nation divided by the population. So this includes agriculture, industry, transportation (including gas for personal vehicles) etc.
This is a pretty unfair comparison, as the people in the city would only need to generate the power for living there. Industry can have its own power sources. And in my experience many farmers have huge solar installations on top of their huge ass barns. They produce so much energy they often have their own energy infra building next to their property.
So if you include all of the energy for stuff that isn't living, you should also include all of the surface area for stuff that isn't living. In other words, way more than just the city itself.
And getting 100% of that energy from solar probably isn't the best idea, it should be a mix of sources to make sure power is always available.
Yeah my reasoning was that big industrial (manufacturing, not agriculture) centers are often close to cities so the energy is consumed close to the city :) So it counts as "city consumption" to me.