Heat pumps are considered an important building block for the heating system of a decarbonized future, as they heat and cool efficiently with electricity. They are particularly useful in an energy system based on renewable energies, as their consumption is easy to plan and the heat can be stored. This allows the heating system to react flexibly to energy generation from the sun and wind and thus support the power grid. This is the result of a transnational research project of the International Energy Agency (IEA). The Fraunhofer Institutes IEE (Kassel) and ISE (Freiburg) were involved from Germany.