When the weather is mild, a standard heat pump uses less energy than other heating systems, so it costs less to run.
A heat pump doesn’t actually produce heat: it extracts and concentrates heat in the outside air and transfers it to the home’s interior. That’s why it uses less energy. Since the available heat diminishes as the temperature goes down, each system has a thermal balance point at which temperature the heat pump’s capacity is equal to the heat loss of the rooms the heat pump supplies. This temperature is usually between -10°C and -5°C.
Above this temperature, the heat pump can meet the home’s heating needs without any auxiliary heating source. For example, the heat pump can supply 3kWh of heat for each 1 kWh it uses, for a ratio of 3:1 (exact performance depends on a number of factors). However, below the balance point, auxiliary heeting is required. It is usually made up of electric elements that use 1 kWh for every 1 kWh of heat supplied (performance ratio of 1:1).
The savings provided by a heat pump are generated when it operates at temperatures above the balance point.