Transaction costs are perceived as one of the main barriers in achieving energy efficiency. Hence, the omission of transaction costs in the evaluation (and preparation) of energy efficiency policies leads to suboptimal decision-making. However, empirical evidence on the main factors influencing transaction costs of energy efficiency programmes remains insufficient. By investigating two cases of major energy efficiency subsidy programmes in the Czech Republic, we analyse the role of two factors influencing the transaction costs: size of the projects and type of actors. The results show that while the dependence between the size of the projects and the size of transaction costs is rather straightforward, the role of actors is more complex. On one hand, no significant difference has been found between total transaction costs of the two types of actors entering the analysed programmes (private companies and public entities). Our results imply the potential for optimization of transaction costs in energy efficiency subsidy programmes lies in streamlining the internal processes (especially in the preparatory phase and in public tenders) and a clear legal environment. On the other hand, differences between the two entities were found in the costs of external services, indicating a room for optimization for public bodies.