Heat pump hot water systems are generally highly energy-efficient compared to traditional electric resistance water heaters.
The efficiency of a heat pump water heater is typically measured by its coefficient of performance (COP), which is the ratio of the heat output to the electricity input.
It’s a pity though that heat pump manufacturers use coefficient of performance instead of percentage efficiency.
A heat pump with a COP of 3 has 300% efficiency, but many people won’t know how to compare this COP to claims of the ‘amazing 100% efficiency of an electric fan heater or radiator’!
Also, few realise the significance of even a small change in the temperature difference across a heat pump or aircon.
Unfortunately, manufacturer claims about high COPs must be carefully evaluated, as we have no effective Standards for reporting.
The efficiency of a heat pump hot water system (HWS) is higher when it operates in a warmer environment. For just one degree reduction in temperature difference, efficiency can improve by 2 to 3%. Some manufacturers state COPs at over 30C operating conditions while others use more realistic ambient temperatures of 15 to 20C.
A product with a COP of 6 at 30C could have a much lower COP at 15C. Some products maintain higher COPs at low temperatures than others. So COP comparisons involve apples and oranges.
The consumer group Choice recently(16 May 2023) published a table of 51 heat pump hotwater products using publicly available data. This showed that different manufacturers made COP claims at very different operating temperatures. So claims are not comparable.
Choice also did not include information on the number of Small Technology Certificates (STCs) each product is eligible for.
Buyers of heat pump hot water units receive an up-front incentive payment based on the number of Small Technology Certificates (STCs) they receive from the Clean Energy Regulator. The number of STCs awarded is based on fairly sophisticated computer modelling of their energy savings based on a typical daily hot water usage pattern and historical hourly temperature data, and an assumed daily usage of around 200 litres of hot water.
While we can debate how realistic this method of assessing efficiency is, it is at least consistent and representative of the outcome for a 4-person household in each of 5 climate zones.
I selected 10 products from the Choice table and compared the manufacturers’ claimed COPs for their products with the number of STCs awarded in each of 5 climate zones, trying to represent the range of refrigerants and performance.
Although my choices aren't necessarily representative, it does show that manufacturers who made claims for high COPs at high operating temperatures looked much more impressive than the outcomes suggested by their Small Technology Certificates.
This analysis suggests that you should ignore claims about COPs, and use the (imperfect) number of STCs awarded when comparing performance.
A COP measured at 30C may look a lot better than a COP measured at 20C, but it does not necessarily lead to bigger energy, emission, or cost savings. And some products maintain higher performance at colder temperatures than others.
Many manufacturers do not state the number of STCs their product scores. So consumers have to dive into the user-unfriendly database of the Clean Energy Regulator to access STC data.
Governments have failed consumers.