Actually, https://en.wikipedia.org/wiki/Solar_furnace ... The furnace in Odeillo, France can reach up to 3500 °C, and it's cross section is just about 50x50 meters. Of course, your typical skyscraper won't create those temperatures accidentially, but if you have the surface area of a skyscraper side in mirrors and can arrange them like you want... it gets pretty pretty hot.

That's the temperature, about the energy: Steel melts at 1370 °C, specific heat of carbon steel is 490 J/kg K. I can't find the actually melting energy. But this means, roughly, that you need 168.5 kWh to raise a ton of steel to melting temperature. The Wikipedia article says the Odeillo device creates 3200 kW (although that paragraph seems to be cut off)., so in principle it should be possible to melt a car. Scary thing.