Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context:
- Total rate of energy transfer; SI units: W = J⋅s−1.
- Specific rate of energy transfer (total normalized per unit area);  SI units: W⋅m−2 = J⋅m−2⋅s−1:
- This is a vector quantity, its components being determined in terms of the normal (perpendicular) direction to the surface of measurement.
- This is sometimes called energy flux density, to distinguish it from the second definition.
- Radiative flux, heat flux, and sound energy flux are specific cases of this meaning.