Hydroelectricity is very limited in its maximum instantaneous output, you can't feed half a grid with it, there is just nowhere to put all that water to without

Hydroelectricity is very limited in its maximum instantaneous output, you can't feed half a grid with it, there is just nowhere to put all that water to without massive flooding, so it's a bit of a moot point. Rooftop solar doesn't matter for the top down approach in that paper, they don't make any assumptions about PV location and assume perfect grid, so rooftop PV won't make it any better. So seeing a top down, intentionally optimistic approach arriving at a number much higher than that book's author's, I am very sceptical of the book's results.