How does it work:
During harsh sunlight, Solar Power raises the ability-bearer's Special
Attack by 50%, but it also loses 1⁄8 of its maximum HP after each
So basically you get a huge power boost for your special attacks but you lose health each turn.
How do you get it on charizard/Heliolisk:
You can obtain a Hidden Ability Charizard by finding Charmeleon in a Fire type Friend Safari, while you can catch a Hidden Ability Helioptile from Electric Type Friend Safaris.