In heat transfer, q represents the heat flux, which is the thermal power per unit area. It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient, which has SI units in watts per square meter per kelvin (W/m2/K), is defined as the ratio of the heat flux to the temperature difference between the solid surface and surrounding fluid area. The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, U.