Key Takeaways
- I think efficiency in heat pump hot water units is overrated
- I don't think people should choose units based on their perceived efficiency
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Heat pump hot water is the cheapest way to heat water for your home. Why? Efficiency. Heat pump hot water uses heat pump technology which is simply the most efficient means available. For every $1 we spend on energy going into a unit, we get out $3, $4 or even $5 worth of energy out.
So it may seem a little counterintuitive that I then say that efficiency is overrated in the heat pump hot water discussion.
I say this for two clear reasons. First, all heat pump hot water units are so much more efficient than alternatives, that arguably it doesn't really matter if one unit is more efficient than another. Secondly, it's almost impossible to tell which units are the most efficient.
So if it doesn't really matter and it's almost impossible to tell, let's not dwell on it. Instead, as I outline in How to choose a heat pump hot water unit, I recommend people focus on the three key factors when choosing between units: cost, reliability and refrigerant.
Below I go through some of the data points people use to measure heat pump hot water efficiency and explain their shortcomings.
Coefficient of Performance (COP)
The Coefficient of Performance is a number that describes the effectiveness of heat pumps, refrigerators or air conditioners. Simply put, it is calculated by dividing the heat output divided by the energy used to run the system. For example, if a heat pump hot water unit takes 1kW to run, but puts out 3kW worth of heat, the COP is 3.
Heat pump hot water manufacturers report the COP of their units by measuring the amount of electricity required to heat water from say 20 degrees to 50 degrees.
Alan Pears, Senior Industry Fellow at RMIT and pioneer of energy efficiency policy in Australia since the 1970s, encourages us to ignore the efficiency claims by heat pump hot water manufacturers.
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.
STC Ratings
Some instead recommend using the number of Small-Scale 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.
I have gone through every unit and listed the number of STCs they attract in our Comparison Tool. You can see the number doesn't vary a great deal.
In Melbourne, the number of STCs awarded varies between 21 and 25 for a typical (~300L) unit. I don't think we can conclude from this that a unit which receives 25 STCs is significantly more efficient than a unit which receives 21. I don't think there's enough evidence to suggest it will lead to material savings over the life of the two units. As mentioned consistently throughout this website, I think it's more important to pick a unit which is likely to last longer.
VEEC Ratings
To further highlight the point, even the regulators cannot agree. If we look at the two Aquatech units, the Rapid X6 and Dynamic X8, according to the VEU the Rapid X6 is more efficient (7 VEECs vs 6) but according to the Clean Energy Regulator, the Dynamic X8 is more efficient (23 STCs vs 22).
If they can't work it out, I don't think we as humble consumers can work it out.
Summary
But, I don't think it really matters. Remember instead:
- Heat pump hot water units are all much more efficient than gas and electric counterparts
- The gains from choosing a 'more efficient unit' are likely to be minimal
- If you have solar and set up the units to run during the day (see Smart Hot Water - Optimising Your Unit), the units are mostly being powered for free anyway.
Picking a unit that will last and is within budget is much more important than picking one that is more efficient. We want to avoid choosing units that appear super efficient, but only last 3 years. I'd much prefer a unit that appears less efficient but will last 10 years and save me the headache and worry of having to replace my hot water unit frequently.
