That'll help, but the sun is never in the right position...The midday sun, which gives the highest energy density irradiation, sadly shines on the roof.....

The midday sun only shines on the roof between the tropics. In the farther nothern hemisphere (and presumably southern, though it is less populated and consequently less energy demanding), the winter months bring a large amount of sun to southern exposed buildings. This extra sun is already used to help heat homes. My aunt's house was built to absorb the winter sun through small, but numerous windows while large eaves block the summer sun. A large skyskraper with a good south facing facade of solar panels could significantly decrease it's heating oil requirements. If it only did this well, it could save millions of barrels. But I think it could produce more than just that. Plus, if done well, it could give a very pleasing face lift to some unsightly archtecture.