I have my quite positive experience with solar too. But "90% population" claim is unrealistic.

For starters, solar needs batteries to be truly effective. Batteries need rare-earth metals which is a narrow bottleneck. It's fine to buy 4 batteris for yourself but procuring a million will raise prices and probably break backs of many projects. A trillion, and you probably igniting wars for resources.

Also we can't multiply a homesteader's optimum a million times and expect it to be close to what a million people industrial city really needs.

At a large enough scale everything gets different.