The characteristics of thermoplastic films, like low conductivity and high transparency, allow simplifying the prediction of the necessary engineering parameters in CO2 laser lap welding by the application of an energy balance approximation. The energy density evaluated by computation is experimentally applied to the welding of transparent high- and low-density polyethylene and polypropylene samples, with thicknesses between 10 and 100 μm. The comparison between prediction and experimental results is also made with an existing thermodynamic model. The latter is developed in accordance with the characteristics of transmission lap welding of the thermoplastics films considered in this work. Results show, for circa 80% of the analyses, a deviation lower than 20% between experimental and predicted values, highlighting the relevance of the energy balance approximation. Also, the similarity between predicted and experimental values confirms the main assumptions made in the development of the thermodynamic model. However, the action of thermal forces in the weld pool prove to be an important factor, restricting the minimum laser beam diameter over the sample. This limits one of the parameters to be evaluated by the energy balance approximation, but from an engineering point of view, its relevance and precision in the process modeling is preserved.