The book used a very pessimistic 10% efficiency for panels installed in large scale solar farms:
https://www.withouthotair.com/c6/page_41.shtml
If a breakthrough of solar technology occurs and the cost of photovoltaics
came down enough that we could deploy panels all over the countryside,
what is the maximum conceivable production? Well, if we covered 5% of
the UK with 10%-efficient panels, we’d have
10% × 100 W/m2 × 200 m2 per person ≈ 50 kWh/day/person.
I assumed only 10%-efficient panels, by the way, because I imagine that
solar panels would be mass-produced on such a scale only if they were
very cheap, and it’s the lower-efficiency panels that will get cheap first.
Utility scale panels today are commonly a bit over 20%.
