A heat pump maintains a temperature of 25°C inside a house. The house leaks 0.8 kW per degree temperature difference between the inside and outside of the house. Assuming that this is a real heat pump having a coefficient of performance equal to only 70% that of a Carnot heat pump, and assuming that the heat pump requires 1.5 kW of power, what is the minimum temperature outside that can exist to allow the heat pump to work?