When I compare heat pumps, I tend to first look at the COP factor (how much energy you get out in relation to how much energy you put in). This COP factor efficiently tells you how much more efficent the heat pump is than an electric heater at producing heating and hot water (the electric heater has a COP below 1). It also tells you how much more energy you will get from a heat pump in comparison to another without needing to input more energy. Thus it is a very good starting point for comparing heat pumps.
There are a number of factors that makes the heat pump efficiency comparison as a result of COP factor a litle difficult. Some of these are differences making comparisons between geothermal and air to water heat pumps harder. Example of factors affecting the COP are:
- COP for an air to water heat pump is dependent on the outside temperature. In general, the lower the outside temperature, the lower the COP (the geothermal heat pump is not depending on outside temperature).
- COP is dependent on the heating water and the hot household water temperature. In general, the higher temperature you want on the heating or hot household water, the lower the COP will become.
- COP is dependent on the speed of the flow of water through the heat pump. In general, you want a speed through the heat pump that achieves the vendor recommended temperature increase.
- Heat pumps will have a limit for maximum water temperature they can handle. Above that temperature, electric heating is needed.
- COP will vary depending on how long it is since the heat pump was started. In general, every start of the heat pump starts with a lower COP than what is achieved when it has been running for a while.
- COP is measured differently by different vendors, for example including or excluding the power consumed by the circulation pump.
Since the heat pumps differ in COP factors, and since the COP factors differ due to a lot of environmental factors, you must express the COP factor as a function of at least the outside temperature in order to be able to compare between different heat pumps and compare between different COP factors (see an example diagram of the varying COP factor for NIBE Fighter 2020 and 2025 expressed as temperature curves).
This can be somewhat complicated. One reason being the different methods used to express them (to compare, you must compensate for the differences and compare all heat pumps using the same circumstances). Also, some vendors deliver very little data about their heat pumps, making the COP curve very hard to estimate.
After going through a lot of data, I tend to believe that a fair assumption is that the better the heat pump, the more data from the vendor (who want you to be able to compare the heat pump with any other vendor independent of how they have selected to specify their data). The vendors I have seen where it is really hard to get data about the heat pump has all so far turned out to have a COP factor that is below the better vendors, or a higher price for comparable COP efficiency.
To make it more easy to compare alternatives by testing different heat pump selections for your house, there is a form to compare and evaluate possible energy savings using heat pumps with different COP factors and power.