The paper describes the operation and thermal analysis of a new prototype to determine thermal balance of radiative cooling. The aim of the study is to establish a simple but accurate procedure to calculate the radiative heat exchange between two bodies to be used in the determination of sky temperature, clear sky index or plate emissivity. The radiative transfer calculation has been based on the power required to maintain a constant temperature at the radiative plate of the prototype, provided that the convection and conduction terms are well known. The methodology is applicable to a further advanced system, which avoids convection and reduces conduction to maximise radiative effects, minimising errors and providing more accurate results. Tests have been carried out at a plate temperature of 40 °C, which was optimum for the prototype size and operating conditions. Higher or lower temperatures have lead to a reduction of the fraction of radiative transfer or to the use of very low external power, which complicates the prototype design and makes the temperature control system very costly. Tests carried out in the prototype have shown a perfect matching between total heat transfer and supplied power with an error of less than 5%. This result allows high precision determination of sky temperature, clear sky index or plate emissivity from the use of the proposed methodology.