For a more realistic estimate of the maximum coefficient of performance of a heat pump, assume that a heat pump takes in heat from outdoors at 10°C below the ambient outdoor temperature, to account for the temperature difference across its heat exchanger. Similarly, assume that the output must be 10°C hotter than the house (which itself might be kept at 20°C) to make the heat flow into the house. Make a graph of the coefficient of performance of a reversible heat pump under these conditions as a function of outdoor temperature (from − 15°C to + 15°C in 5°C increments).