Nowadays, an increasing focus is placed on the development of smart energy systems (SESs) that combine a variety of heterogeneous sub-systems with largely varying applications. Due to their multidisciplinary nature, there are no universally accepted standards assessing their performance. Previous efforts typically define Key Performance Indicators (KPIs) only for specific SES types and application areas. This paper presents a robust KPI framework that can be applied to any SES type regardless of the application area. The proposed framework consists of four layers following the top-down approach. The layers define the application area, SES requirements, main stakeholders involved specifying their objectives and finally, the KPIs for each of the stakeholders' objectives. The proposed KPI framework is applied on the use cases of a European research project with different application areas showing its features.