COP, or coefficient of performance, is the number used to compare heat pump technologies. For a heat pump water heater, it tells you how much heat goes into the stored water compared with the electricity used by the appliance. A COP of 3 means the unit delivers about three kWh of heat to the water for one kWh of electricity consumed under the test conditions.
That definition is simple. The practical reading is more subtle. A model with a strong catalogue COP can disappoint if it is installed in a cold, badly ventilated room. A slightly lower catalogue number can work well if the air source is stable and the tank is correctly sized.
The formula
The formula is:
COP = useful heat delivered / electricity consumed
For a domestic hot water heat pump, useful heat means the heat transferred to water in the tank. Electricity includes the compressor, fan, control electronics and sometimes part of the test cycle. The official value is measured in a laboratory, often under EN 16147 conditions for heat pump water heaters.
This matters because every model needs a shared comparison basis. Without a standard, one manufacturer could quote performance with warm air and another with cold air. A standard does not predict your home perfectly, but it gives a fair starting line.
Lab COP versus real COP
Real COP changes across the year. The appliance performs better when the air source is warm and stable. It performs less well when the air source is cold, the water setpoint is high, the filter is dirty or the tank is undersized and calls on the electric back-up.
In Belgian conditions, the gap between lab COP and real seasonal COP is normal. The aim is not to chase a perfect number. The aim is to choose a setup that keeps the heat pump doing most of the work instead of letting the resistance take over.
Air source effects
Ambient air from a cellar or utility room can work well when the space is large enough and not too cold. If the appliance cools a small room sharply, the COP falls and comfort nearby may suffer.
Outdoor air can be convenient through ducts. In mild Belgian weather it can perform well. In winter, low outdoor temperatures reduce COP. The design must allow for frost, airflow and condensate management.
Extracted ventilation air can be very stable because it comes from inside the home. It may give a better seasonal COP, but it depends on having a suitable ventilation system and correct balancing.
COP and Wallonia grants
For Walloon grant files, the administration checks technical eligibility. The heat pump water heater must be a compliant appliance, and the installer must provide the required certification evidence. RESCert appears here because heat pump work in grant files has certification requirements.
This does not mean EcoChaleur should be described with a blanket RESCert claim. The safe statement is that EcoChaleur works with installers holding all legally required certifications for the relevant works. The certificate evidence is then attached where the grant file needs it.
COP and cost thinking
Avoid comparing appliances only by their maximum lab COP. A good comparison includes storage volume, air source, noise, maintenance access, tank protection, control options and local water hardness. A high COP unit placed badly can consume more than a balanced unit installed correctly.
Think in scenarios. A family with regular morning showers needs enough stored hot water so the resistance does not take over. A couple in a well-ventilated apartment may prefer a smaller tank. A home with solar panels may schedule heating when self-produced electricity is available.
How to protect performance
Set the water temperature sensibly. Keep filters clean. Make sure airflow paths are not blocked by storage boxes in the utility room. Schedule maintenance. Check limescale in hard water areas. Keep the anti-legionella cycle active, but do not use unnecessarily high temperatures every day.
The best COP is not printed only on a product sheet. It is the result of a correct model, correct source air, correct tank size and correct settings.