The transition towards a 100 % fossil-free energy system, while achieving extreme penetration levels of intermittent wind and solar power in electricity generation, requires demand-side technologies that are smart (intermittency-friendly) and efficient. The integration of Smart Grid enabling technologies has been widely acknowledged by the industry and academia as the key to achieve a more reliable, efficient and secure grid. However, there is a lack of information about real flexibility with an active participation from customers. In this paper, we combine techno-economical modelling results with an empirical study in order to achieve a number of recommendations with respect to technology concepts and control strategies that would allow residential vapor-compression heat pumps to support large-scale integration of intermittent renewables. The analysis is based on data gathered over a period of up to 3 years for 283 residential heat pumps installed and operating in Denmark. The results are used to assess the flexibility of domestic heat pumps and their ability to follow production.