How much does the use of back-up heaters reduce the efficiency of heat pumps?

Episode 5 of a series on heat pumps in existing buildings Heating systems with heat pumps are usually equipped with a direct-electric back-up heater. Usually, the electric back-up heater takes over the heat supply from a defined outdoor temperature (for example, -5°C, also called the “bilvalence point”) – either alone or together with the heat …