Fossil fuel is stored chemical energy. Burning it releases the energy. Photovoltaic solar power is converting one type of energy (light) into another (electrici

Fossil fuel is stored chemical energy. Burning it releases the energy. Photovoltaic solar power is converting one type of energy (light) into another (electricity), with the waste heat being the same waste heat the light would've created by shining on something else. Solar thermal energy is using the heat from the sun to do work (which may include heating oils, air, or water to drive a turbine to generate electricity) or to generate electricity (using, say, thermal-piezo conversion). The only effective ways we have to store solar power are batteries, supercapacitors (once they're viable), large heat masses (such as stone or water), growing plants, or putting energy into chemicals for later storage (electrolysis and storing the hydrogen, forming artificial hydrocarbons, recycling aluminum for aluminum/air power cells, pumping water uphill to power a turbine later, pressurizing gases to later drive a turbine, stuff like that). Not all of the things I mentioned work at every scale. Some of them are more efficient than others. They may all have advantages and drawbacks.