As a buyer, of course you want to know how effective a heat pump is. Most manufacturers choose to audit the COP in only one given operating situation. The heat pump's capacity to produce heat in relation to the energy required to extract it has been established in specific test conditions.
If a certain heat pump has a COP 3 it means that in the specific test conditions it can produce three times more energy than it consumes. In other words the net surplus is two thirds.
Reality must apply
It pays to be on your guard when discussing efficiency. A high COP value measured in special conditions without all the system components - such as circulations pumps - can give high values. But the measurement should not be tailored to the manufacturer's marketing plan. It should give you, the homeowner, a complete picture of how the heat pump works over time.
Annual efficiency is a far more honest measurement of the heat pump efficiency. The whole year is taken into account, both the hot summer months and the cold winter periods.
The size of the house, the geographic location and how many occupants it has are all conditions that affect the overall picture.