The overall heat transfer coefficient represents the total resistance experienced as heat is transferred between fluids or between a fluid and a solid. The overall heat transfer coefficient is calculated by dividing heat flux by the temperature difference between the two materials where heat is being transferred.
The two materials refers to solid and fluid where a phase transition is involved or between two fluids where convection is involved. Heat flux is the thermal power per unit of area, and its SI unit is watts per square meter. The coefficient‘s standard unit is watts per square meter kelvin, while the temperature difference is measured in kelvin. The heat transfer coefficient is a reciprocal of thermal insulance.