Choosing the most suitable heat pump hot water tank size is one of the most important decisions you’ll make about heat pumps.
Too small, and you’ll be left without enough hot water.
Too large, and you’ll be wasting energy needlessly heating water you’ll never use.
Like everything about heat pumps, we’ve done a great deal of research on this topic.
Below is a comprehensive guide on how to make sure you choose the right size tank for you and your family.
And as always, we’re here to help if you would like to speak with an expert (there’s a link to our contact form at the end).
How You Use Your Hot Water
Before I set out general rules of thumb, I want to stress one point – the most important determinant of tank size is how you use your hot water.
And how you use your hot water mostly comes down to how you shower/bathe.
I’ve sold a heat pump to a 2-person home that I thought would provide plenty of hot water. They called me shortly after installation saying that they were running out. It turns out both people washed their hair every night, showering for 30 minutes each! We fixed this by altering the settings (more below), but it taught me the importance of asking these questions early.
Teenagers and pre-teens are also the ones to watch out for. I’ve had numerous customers tell me that their teenage son or daughter has no concept of a short shower. I vividly remember one customer asking me if a heat pump could be shut off from delivering hot water – the only way she could get her teenage son out of the shower was by going to the old unit and shutting it off!
Our Guide to Heat Pump Hot Water Tank Size
This table illustrates how we can think about choosing the right tank size. We look at the number of people in the home and the typical usage of those people.
I class “Light” usage as showers for 5 mins or less, “Medium” usage as 5-10 minute showers and “Heavy” usage as greater than 10 minute showers.
A key omission from this table is the number of bedrooms in the house. In addition to the number of people living in the home, we ask how many bedrooms are in the home as that can indicate guests, growing families, tenants or future-proofing the house for the next owners. If you have 2 people in the home, but it’s a 4-bedroom house, you might like to make your decision based on 3-4 people instead of 1-2.
Let’s use an example. Let’s assume we have a 270L unit and our shower head uses around 9 to 10 litres of water per minute. We can assume that 50% of this water will be hot water, so 5 litres of hot water per minute. A 270 litre tank allows us 54 minutes of showering time. If one person has a shower for 30 minutes, the remaining residents will need to shower quickly!
Running Your Unit
Another key factor in determining your tank size is how you run your unit. As I explain in Setting up and optimising your heat pump hot water unit, we want to run heat pumps during the middle of the day for one cycle. Ideally we want to run the unit on a timer, let’s say between 11am and 3pm. This means we need a tank big enough so that by the end of that heating cycle, there is enough hot water to last us until the next cycle the following day. This is the ideal configuration, but it doesn’t have to be setup this way, so we have levers we can pull (extending the timer) if we need more hot water.
Visiting guests is a great example. If you have guests visiting infrequently, it makes more sense to have a unit that caters just for your day-to-day needs and then change the settings when guests arrive. It’s sub-optimal to choose a larger unit that will mostly be too large if it only caters for a few days a year.
Your Old Tank Size Is Irrelevant
We often have customers call saying “my old tank was 135L, so I’ll need something of a similar size.” This is a trap to avoid.
Heat pumps take longer to heat than old electric or gas storage units. Gas storage units heat water at around 4 litres per minute and electric storage units at around 3 litres per minute. You’ll remember what I said above about using 5 litres of hot water per minute during shower. The old gas units can almost keep up, so they don’t need as large a tank.
But for the reasons I explained above about running the unit during the day, and because heat pumps take longer to heat water, we need a much larger tank. The size of the previous unit is largely irrelevant.
Heat Pump Hot Water Tank Size Survey
In early 2025 I surveyed our customers and followers of our monthly newsletter. What better way to see if people are happy with their tank size than by surveying those that already have a heat pump? Pleasingly, the results were overwhelmingly positive. Nearly 83% responded that they were happy with their tank size. 14% responded saying they are mostly happy, but would have chosen a larger unit. Only 3 responses were negative either for running out of hot water or for having too much.
My thinking around tank sizing has changed slightly over the years. Now, I err on the side of larger tanks. We’ve had one or two customers run out of hot water, particularly in winter. Mid range heat pumps don’t work very well in cold temperatures, so if customers need to run their heat pump during a cold winter’s night, they might find they don’t heat enough and run out of hot water.
I’ve found this particularly relevant for families of 4 people. Previously we would have recommended a tank size of around 270L, but now with more questioning and information, if their hot water usage is significant, I recommend looking at larger tanks.
Manufacturer Nuances
Once we have an idea of how our customers use hot water, we then zero in on the most suitable tank size.
Some manufacturers have better offerings for different tank sizes.
For example, we like and install iStore heat pump hot water units but currently their largest unit is 270 litres, which I wouldn’t recommend to families of 5 or more people.
There are other nuances from each of the manufacturers. A few examples include:
- Reclaim – Reclaim’s most common tank size is 315 litres. They also sell 160L, 250L and 400L tanks, but the difference in tank price is generally only around $100-$300. This isn’t a huge difference, especially when we consider the total system price varies between $5,000 and $7,500. If a customer is tossing up between a 250 litre and 315 litre, we’d recommend the larger tank as it only costs an extra $100 or so.
- Haier – heat pumps come in 200L, 250L and 330L sizes. The 200L unit actually costs more to install than the 250L. This is because we have negotiated special pricing for Powrhouse customers for the 250L (as we do lots of volume) and also because the 250L unit is eligible for more government rebates.
- Aquatech – currently come in 225L and 270L, with a $400 price difference. On an installation price of say $1,500 to $2,000, the $400 price difference is meaningful.
Arggghhh! No Hot Water
The last point to highlight is that some people are highly sensitivity to running out of hot water. Or put another way, they simply cannot tolerate running out of hot water. If this is you, don’t overthink it. Follow our guide but always go for the bigger unit. As I often say “no one ever complained of having too much hot water.”
Summary
I hope this has been helpful to guide you to choosing the right size heat pump hot water unit. Given we plan on using our new heat pump for a long time to come, it’s worth investing the time to understand the factors that affect your decision.
As always, we are very happy to help. If you are interested in a general discussion about your heat pump options and/or a quote, we’d love to hear from you.
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