His solar calculations IIRC were off by a factor of 10 in terms of what amount of energy could be extracted from a given piece of land. I do not have the book now otherwise I could've pointed it out. I think he was calculating the footprint of different forms of energy. Thumb rule is that at 20% efficiency a PV panel can get 200 Wp energy per sq m which translates to 200 MW per sq km. Leaving aside space for equipment, gaps, roads etc one can easily have a thumb rule of 100 Mw peak per sq km. I believe his book had this calculation as 10-20 MW per sq km.
This assumes 1 KW peak irradiance. Off course things will change based on latitude, season etc and one has to adjust for these factors however Solar is a no brainer in tropical and temperate climates alike at this point in my opinion.
So definitely he is purposely understating the potential of solar. Because it does not include future growth of tandem solar cells (Si + perovskite) taking it to more than 25-30% efficiency, or the fall in costs if they continue (albeit at a slower pace). Co-located Wind & solar power plants at suitable sites can be even more efficient with their reliability and capacity factors increasing if batteries added to the mix.