Note: this is only true for air-source heat pumps. Ground source-heat pumps aren't affected by ambient air temperature, but they're also much more expensive and

Note: this is only true for air-source heat pumps. Ground source-heat pumps aren't affected by ambient air temperature, but they're also much more expensive and require suitable ground. Also, I hear air-source heat pumps have made significant advances lately in how well they handle sub -15 °C temperatures.