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That is very interesting info. Thanks Anne.

We might estimate that 1/4 of all roofs/slopes face east and 1/4 face south, so deduct 20% of 50% from the total.

Also, 1/4 of all roofs/slopes probably face north, so we loose 25% here.
That would be a grand total of 35% loss from the “ideal” condition of all roofs facing due south. (Although one could argue that extending the solar day has great value.)

Of course there has to be some adjustment for shading from trees and other buildings. Some adjustment for skylights and dormers.

I’d guess that at least the equivalent of 50% of rooftops are potential power producers.

Another thing might have been cost. Perhaps the authors assumed that east and west facing roofs were unsuitable due to lower yield. But PV has gotten incredibly cheap. That’s one point that a report from 2008 definitely paints the wrong picture. Btw, the yield on east and west facing roofs is about 80% of an ideal system, so that is very doable.

Your option that only 1/5 of the houses has enough suitable roof space to generate 100% of consumption is also a possibility. But to me the 4/5 would be in a category called ‘suboptimal’, not ‘unsuitable’.

Just my 2c.

i was assuming that it was bcs they were including to multi-family buildings, in which the roof would never support enough solar for everyone in the building.

The report that this claim is based on is from 2008, sort-of prehistoric times in the PV sector.

That is a bold claim. Is there a source for that?