The abbreviation ESCo stands for Energy Service Company.
SUN Energy esco model
An ESCo company is a comprehensive provider of broad range of energy related services. These services include design, implementation, risk management, energy efficiency assessment, financing of projects, energy production and management of such systems.
ESCo companies generate profits and return on their investments solely through achieving energy savings. ESCo companies operate on long term basis and often involve re-investments in installed systems to further improve energy efficiency. 
With development of advanced information systems, ESCo companies are able to introduce real time management of systems, metering substations and other asset management applications.
Parallel to this, companies gain automated real time analysis and reporting on key performance indicators (KPI). This way, operations are even more efficient allowing companies to further exceed energy saving requirements.
This relatively new breed of ESCo companies is called ESCo 2.0 and ZON Energie and its subsidiaries are part of this wave.