Renewable energy
Definition
Energy from sources that nature replenishes — sunlight, wind, flowing water, Earth's heat, and plant matter. Unlike coal or oil, using them does not drain a finite reserve.
Technical definition
Energy harvested from naturally replenishing flows: solar irradiance, wind, hydropower, geothermal heat, and biomass. All except geothermal and tidal ultimately derive from solar energy; conversion physics spans the photoelectric effect (solar cells), electromagnetic induction (turbines), and thermodynamics.
Example
A modern wind turbine converts the wind's kinetic energy into electricity with about 45% aerodynamic efficiency. A rooftop solar panel turns roughly 20% of the sunlight hitting it into electrical power via the photoelectric effect in silicon.
'Renewable' does not mean 'free of environmental impact' or 'always available.' Solar and wind are intermittent — the Sun sets and winds calm — which is why storage and grid design are central engineering challenges, not afterthoughts.