Reproduce:

Have solar panel in view of sky
Put 1 low light upgrade in.

Result

Turns to 40% efficiency @ day
Turns to 50% efficiency @ night

Expected

Turns to 100% efficiency @ day when optimal sun?
Turns to 50% efficiency @ night

This is an intended behavior.
Low light upgrades allow to generate energy in the dark at the cost of being less efficient when there is too much sun.
Note that I completely changed the way Low Light upgrades influence energy generation in the last version (0.7).