Key Takeaways
- There are guides to help you choose the right tank size - 50L per person per day and the number of bedrooms in the home
- The right size tank for you will depend on your personal choice - a smaller unit will be more efficient but may carry the risk of running out of hot water; a larger unit will be less likely to run out, but will be less efficient to run
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Sizing your heat pump hot water tank is a balance. On one hand, you want a tank big enough to avoid running out of hot water. On the other hand, you don't want to a tank too big that your unit is inefficient, heating too much water that is not used.
Below I highlight how you should think about the size of your heat pump hot water unit.
One Cycle
The first key to understand about heat pump hot water is we want to the units to run one cycle per day. As I explain in Free Hot Water - Optimising Your Unit, the units work by sucking heat out of the air, amplifying the heat and pumping it into the water. We therefore want to run the units during the day, ideally for one cycle. This is different to the way gas or electric units work, which heat water continuously, it doesn't matter if they are run at night time or day time.
It's for this reason that heat pump hot water units tend to be larger than gas or electric storage units. We want to run the units for one cycle during the day.
If you feel you're not getting enough hot water with this setup, you can of course adjust the timers.
50L Per Person Per Day
The general guide for hot water usage is 50L of hot water per person per day.
Sustainability Victoria suggests 50L for a household with one person, 90L for two people, 120L for 3 people and 150L for 4 people.
We should also note that many modern appliances like washing machines and dishwashers heat water in the appliance. These appliances draw cold water, not hot, which decreases our need for hot water.
Number of Bedrooms
The Victorian Energy Upgrades Program recommends sizing the unit on the number of bedrooms.
This makes sense - if you have visitors or if you sell the house, you are sizing the hot water unit for the property rather than the number of residents.
However, I think it can be overkill, for two reasons.
First, if we apply the 50L rule above, clearly a 3 bedroom house with one resident doesn't need 225 litres. If they're only using 50L, they are possibly heating 175 litres of water each day that they don't need.
Second, if you run out of hot water, it's not the end of the world.
My family of four has run out of hot water once in the six years we have lived in our house. We had the washing machine running, the bath running for my young girls and my wife was having a shower. All of a sudden the water coming out of the bath tap turned lukewarm. My wife had to cut her shower short but otherwise life continued as normal - we survived.
Personal Choice
I feel some of the expert recommendations, like above, can be a little too much when you consider the consequences are usually trivial.
In my view, you're better off getting a smaller system and running the risk of running out of hot water, than getting a larger system that means you're heating too much water redundantly.
I realise a lot of people will disagree with this, but really it's up to you. If you agree with me, you'll be fine with a smaller tank. If you feel the consequences of running out of hot water are not trivial, then get a larger tank.
