The German Building Energy Act, the Gebäudeenergiegesetz revised in autumn 2023 and in force from the first of January 2024, was meant to make residential heating in Berlin look like residential heating in Stockholm: an electrified, low-carbon, distributed network of heat pumps quietly humming through every neighbourhood. The legislation was passed. The subsidies are flowing. The pumps are being installed. And the low-voltage distribution grid that they all connect to is, in an unusually polite way, asking everyone to slow down.
The grid operator for Berlin’s distribution network is Stromnetz Berlin, a subsidiary of the city-state since the 2021 buyback from Vattenfall. The company manages roughly thirty-five thousand kilometres of low-voltage and medium-voltage cables that carry electricity from the substations to the apartment blocks. None of those cables were sized for the load profile that a heat pump produces.
The arithmetic of a typical winter morning
A modern air-source heat pump in a German Altbau apartment, on a still and cold January morning at minus eight degrees Celsius, draws between three and seven kilowatts of electrical power. Multiply that by every flat in a Wilhelminian-era block in Friedrichshain and the local transformer, sized in the 1970s for an aggregate load that assumed gas heating and minimal electric demand, runs at the edge of its rated capacity. Stromnetz Berlin’s engineers can map the failure modes precisely. They have done so.
The result, in 2024 and 2025, has been a quiet rationing of new heat-pump connections in certain districts. In Charlottenburg, parts of Schöneberg, and pockets of Kreuzberg, the connection request from a homeowner triggers a network study, which can take months and sometimes years to clear. The applicant receives the subsidy paperwork from the federal Bundesförderung effiziente Gebäude programme but cannot install the pump until the local cable is upgraded. Stromnetz Berlin has accelerated its capital-expenditure plan, but the underlying constraint, that a buried cable cannot be dug up faster than the construction season allows, is structural.
The federal-municipal handshake
The federal subsidy regime under the Habeck-era Bundesministerium für Wirtschaft und Klimaschutz, continued under the Merz government, covers up to seventy per cent of the cost of replacing a gas boiler with a heat pump. The subsidies are popular. The administrative bottleneck is now the grid, not the homeowner. Stromnetz Berlin’s 2024 annual report quietly notes that it is processing more capacity-upgrade requests in any single quarter than it processed in any year before 2022.
The harder political fact is that grid investment is partly recovered through network charges added to every Berliner’s electricity bill. Federal regulators at the Bundesnetzagentur are still deciding how the cost of the heat-pump-driven expansion will be allocated. The 2025 ruling, expected in the autumn, will determine whether the grid uplift is paid by all electricity users equally, or whether heat-pump owners pay a tariff premium for several years.
What is actually happening in the basements
The technical fix, in the meantime, is a layered one. New installations increasingly include thermal storage tanks, which let the heat pump run during off-peak hours and discharge during morning peaks, smoothing the load on the local cable. The federal subsidies now favour systems with at least three hundred litres of buffer storage. Berlin’s housing cooperatives, the Wohnungsbaugenossenschaften, are also rebuilding their boiler rooms with hybrid systems: a small gas boiler retained for peak-shaving, the heat pump doing the base load. It is not the pure-electric vision that the 2023 legislation imagined. It is the pragmatic version that the cables permit.
What Berlin will probably not do, in the next five years, is reach the headline targets that the GEG anticipated. What it will do is build the grid out at the speed that grid investment allows, which is roughly the speed of urban excavation. The pumps will follow. The order of operations matters more than the rhetoric. Berlin is learning, again, that the slowest part of an energy transition is not the technology. It is the cable.
Sources: Gebäudeenergiegesetz revision 2023, Bundesförderung effiziente Gebäude programme documentation, Stromnetz Berlin annual reports 2022-2024, Bundesnetzagentur consultation papers on grid-charge allocation.