Solar Power is an ability introduced in generation 4.
8 Pokémon can have this ability, such as Sunkern, Sunflora, Tropius, Charmander and Charmeleon. For 5 of them it is the hidden ability, usually obtained from events, raids or breeding.
Official effect
Increases Special Attack to 1.5× but costs 1/8 max HP after each turn during strong sunlight.