Growing shares of renewable intermittent energy in power grids together with changing energy consumption and price patterns increase the need for energetic flexibility to help balance the power grid. A variety of systems have been identified as being flexible in their electric energy prosumption. However, quantitatively describing the flexibility available from individual resources can be challenging, time-consuming, and costly. This paper shows how the energetic flexibility of domestic buildings and their heating systems can be estimated autonomously based only on energy meter data and outdoor air-temperature measurements. The fact that this approach requires only limited, readily available measurement data and can easily be automated makes it well-suited as an inexpensive and highly scalable tool to estimate the flexibility available from large populations of buildings.