Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context:

1. Total rate of energy transfer (not per unit area);^[1] SI units: W = J⋅s⁻¹.
2. Specific rate of energy transfer (total normalized per unit area); ^[2] 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 first definition.
   • Radiative flux, heat flux, and sound energy flux are specific cases of this meaning.