Photovoltaic (PV) systems use solar cells to convert sunlight into electricity. Solar hot water systems can be used to heat buildings by circulating water through flat-plate solar collectors.
Passive solar heating is a system designed to use solar energy (energy from the sun) for the purpose of providing electricity or heat. This system is used in households and offices to absorb, retain and supply heat to the desired location.
This system is termed "passive" because it does not require any external device for conversion of solar energy rather it exploits sun's energy directly. Hence, according to this question, PASSIVE SOLAR HEATING is the form of renewable energy that uses the sun's energy most directly.
