The ECA approach to analyse energy consumption of systems controlled by software will be discussed. Applications include thermostats, fridges, cars, and they can be analysed for their electricity, gas or water usage. This can be used during design of the systems to reduce the energy consumption. This approach need to incorporate details of the whole software stack (including libraries, operating system and language used), and include models of the hardware that is controlled. This talk focusses on the foundation, validity, scope and identifies challenges for the future.