The COP of a heat pump or refrigerator operating at the Carnot efficiency has in its denominator the expression TH - TC. As the surroundings cool (TC reducing) the denominator increases and COP reduces. Therefore, the colder the surroundings, the lower the COP of any heat pump or refrigerator. If the surroundings cool, say to 0 °F (-18 °C), COP falls in value below 3.5. Then, the same system costs as much to operate as an efficient gas heater. The yearly savings will depend on the actual cost of electricity and natural gas, which can both vary widely.