Heat Pump Efficiency
There are four different measurements used to describe heat pump efficiency. Since you might see any of these measurements used, we'll explain them all here. Here is a simple rule of thumb: No matter which measurement is used: the higher the number, the higher the efficiency of the system.
Heat pump efficiency is determined by comparing the amount of energy delivered by the heat pump to the amount of energy it consumes. Heat pump efficiency is dependent on "outdoor" temperature. In the heating mode, performance drops as the air's "outside" temperature drops. In cold climates, heat pumps require a supplemental heating system for when they cannot produce enough heat to maintain the desired indoor temperature.
Electric resistance "backup" is most common; but gas, oil and propane back-up heating systems can also be used. The efficiency of the heat pump when the back-up heat is running is really just the efficiency of the back-up system. If it is resistance electric, the efficiency of the heat pump falls to about 100%, which is the efficiency of electric resistance heat. If a gas furnace backs-up the heat pump, when it operates, the heat pump's efficiency is that of the gas furnace.
Coefficient of Performance (COP) is the most common measurement used to rate heat pump efficiency. COP is the ratio of the heat pump's BTU heat output to the BTU electrical input. Conventional electric resistance heaters have a COP of 1.0 meaning it takes one watt of electricity to deliver the heat equivalent of one watt. Air-source heat pumps generally have COPs of 2 to 4, meaning they deliver 2 to 4 times more energy than they consume. Water and ground-source heat pumps normally have even higher COPs of 3 to 5.
Energy Efficiency Ratio (EER) is used for evaluating a heat pump's efficiency in the cooling cycle. It is the same rating system used for Air Conditioners, making it easy to compare different units. EER is the number of BTUs of cooling provided per watt of electricity consumed.
- EER ratings of greater than 13 are the most desirable.
COP and EER measurements are based on laboratory tests and do not necessarily measure how the heat pump performs in actual use. A heat pump's Performance will vary depending on the weather and how much supplementary heat is required. Therefore, a more realistic measurement, especially for air-to-air heat pumps, is calculated on a "seasonal" basis. These measurements are referred to as the Heating Season Performance Factor (HSPF) for the heating cycle and the Seasonal Energy Efficiency Ratio (SEER) for the cooling cycle.
HSPF is the estimated seasonal heating output in BTUs divided by the seasonal power consumption in watts. It can be thought of as the "average COP" for the entire heating system. An HSPF of 6.8 corresponds roughly with an average COP of 2.
- HSPFs of 8 or 9 are considered good.
- Energy Star rated heat pumps have a higher SEER and HSPF than standard models. This makes them more efficient and less costly to operate.