[DennisP]

Due to axial tilt, for most of the year geosynchronous satellites are in full sunlight 24/7. Twice a year, around each equinox, there's a period of 44 days during which the satellite goes into shadow briefly each day, for a time ranging from 2 minutes, to 72 minutes at the maximum.

https://corpblog.viasat.com/how-satellites-are-affected-by-t...

This compares pretty favorably to capacity factors of other power sources.

Illumination per square meter averages quite a bit higher than ground solar, and except for the brief times in shadow is 30% higher than ground solar at noon.

The book evaluated other locations including Lagrange points. Everything had pros and cons; the main disadvantage of Lagrange points was the greater distance, requiring bigger antennas. I think more delta-v in deployment was a factor too, though I forget by how much.