I was ressearching about how would a humanoid be able to photosynthesize through their skin, but it seems there isn't nearly enough surface area, so this idea came to mind:

an unicellular photosynthetic organism in a symbiotic relationship with an alien race. The symbiotic cells started by living just under the skin of the aliens millions of years ago, gaining protection from the environent and providing energy through their photosynthesis to the hosts. Through evolition, the host develops long hair-like fillaments in their body, especially the head, to increase the surface area of where the symbiotic photosynthesizers live, improving efficiency of the process. In this world, the energy to cast magic would come from carbohydrate mollecules similar to glucose and glycogen, so this creature would basically have a constantly regenerating energy source, as long as they stayed in the sunlight.

Does this sound plausible? what would be the disadvantages/consequences to this? any suggestions?