According to the philosophy of the German energy transition, heat pumps are the future.
They generate heat from electricity - ideally green electricity from wind and sun.
In many new buildings, the devices that function roughly like an inverted air conditioning system are now standard.
It is estimated that the number of heat pumps will increase from one million today to four million in 2030.
But experts see various hurdles and contradictions - and therefore point to an obvious alternative.