What Does COP Mean on a Heat Pump Water Heater?
What does COP mean on a heat pump water heater? COP on a heat pump water heater is the Coefficient of Performance – a measure of how efficiently the heat pump converts electricity into heat for filling the hot water tank. If we take an example of a COP of 3, that means the heat pump can deliver 3 kilowatt hours of heat for every single kilowatt hour of electricity it uses under the test conditions.
How Do We Calculate COP?
To calculate COP, we compare how much useful heat the heat pump produces versus how much electrical energy it uses. In simple terms, the amount of useful heat is compared to the electricity it needs to work.
For example:
- if a heat pump has a COP of 2, that’s 2 kilowatt hours of heat for every single kilowatt hour of electricity used – a pretty straightforward calculation
- or if the COP is 3, that’s approximately 3 kilowatt hours of heat for every single kilowatt hour of electricity used.
A higher COP therefore indicates the heat pump is more efficient.
To put things into perspective – a conventional electric immersion heater has an effective COP of close to 1, which means it produces around 1 kilowatt hour of heat for every single kilowatt hour of electricity it uses.
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Is a Higher COP Really Better?
The answer is yes – the higher the Coefficient of Performance, the less electricity your heat pump needs to heat your water. That said, COP is not fixed and will change based on operating conditions, especially the incoming air temperature and the desired water temperature.
A heat pump may start off with a high COP when operating with warm air and moderate water temperature, but that COP will drop when the air gets colder or you need the water to be hotter than usual.
What Affects the COP of a Heat Pump Water Heater?
Several factors affect the COP, including:
- the temperature of the incoming air
- the temperature of the water in the tank
- the type of refrigerant
- the efficiency of the compressor
- how well-insulated the tank is
- your hot water usage
- defrost operation (where applicable)
- if you also use an electric immersion heater
The bigger the difference between the source air temperature and the desired water temperature – known as the temperature lift – the harder your compressor has to work.
That’s why heat pumps usually work more efficiently when they’re not being asked to heat the water to really high temperatures.
COP and R290 Heat Pumps
A lot of modern air source hot water heaters use R290 (also known as propane), which is a great choice for domestic hot water applications because it’s environmentally friendly and can operate at high water temperatures. However, it’s worth noting that even with R290 the COP will still vary depending on a number of factors, including the temperature of the air and the water.
Remember: COP is Not the Same as Annual Energy Use
When you see the quoted COP, it’s usually measured under very specific test conditions. In reality, the actual electricity you’ll use will depend on a number of factors including:
- how much hot water you use
- how big your tank is
- How much heat is lost when the tank is standing idle
- how many times a day the tank needs to be reheated
- your storage temperature
- how you’re controlling it
- if you use a backup immersion heater
So, two heat pumps with the same quoted COP will not necessarily have the same running costs.
What is a Good COP for a Heat Pump Water Heater?
A good COP can be anywhere from 2.5 to 4.0, which is typical for many modern heat pump water heaters under the right operating conditions. If you’ve got a unit with a COP of 3, for example, that means it can use around a third of the electricity you’d need for direct electric resistance heating to get the same amount of heat out.