where K is the thermal conductivity of the refrigerator wall (W/mK) A is its area (m2), d its thickness (m) and Tint and Text (both in K) are the temperatures at its internal and external surfaces, respectively.
The formulation presented in Equation (3) is convenient because it allows the determination of the heat flow in terms of system constants (K and A) and variables (Tint and Text) that can be easily measured experimentally.