The reason is actually very simple. Lhasa's altitude is not high, averaging around 3,600. Additionally, Lhasa is surrounded by mountains, which blocks most of the cold air, even in winter. In Lhasa city, there is enough sunshine. Even in winter, it is light at 9 a.m. and it gets dark around 8 p.m.
The sunshine duration in Shigatse is longer than in Lhasa, so solar energy at Shigatse is expected to be more abundant
The average sunshine duration in Lhasa is about 3005 hours per year, and Shigatse has about 3183 hours
A factor in the duration of 'sunshine may be linked to altitude
The altitude of Shigatse is almost 200 meters higher than that of Lhasa