Getting the tank size right matters more with a heat pump than it does with a standard electric or gas system, mostly because heat pumps reheat more slowly. Buy too small and you’ll run short of hot water during busy periods. Buy too large and you’re paying for capacity you don’t need. Here’s how to work out the right size for your household before you get a quote.
Why getting the size right matters
A heat pump heats water using a compressor rather than a high-wattage element, so it takes longer to reheat a full tank than an electric or gas system does. That’s not a downside as such, it’s just a different way of working, but it means the tank needs to hold enough stored hot water to comfortably cover your household’s demand between reheats.
Undersizing is the more common mistake. A tank that’s too small can run out of hot water during a busy morning, even though the system itself is working exactly as it should.
Heat pump tank sizes available in Australia
Heat pump hot water systems in Australia are generally available in these size bands:
- 180 to 200 litres, suited to 1 to 3 people
- 260 to 270 litres, suited to 3 to 4 people
- 325 to 330 litres, suited to 4 to 6 people or higher usage households
A 250 to 270 litre tank is the most commonly installed size for a typical Australian household of 3 to 4 people.
Sizing by household: how many people do you have?
Household size is the starting point for sizing, but it’s not the only factor. Two adults living alone use hot water differently to two adults with three teenagers, even though both are technically small households by some measures. Use the household size bands above as your starting point, then adjust up if your usage pattern or fixtures suggest you’ll need more.
Does your usage pattern affect what size you need?
Yes. A household where everyone showers within the same hour or two each morning puts more simultaneous demand on the system than one where hot water use is spread through the day. If your mornings are a rush of back-to-back showers, it’s worth sizing up slightly to give yourself a buffer, even if the household size band alone would suggest a smaller tank.
Baths, larger shower heads, and multiple bathrooms running at once all increase peak demand as well. None of these show up in a simple “how many people live here” calculation, which is why a proper assessment looks at usage pattern, not just headcount.
Sizing for solar: why a bigger tank can save you more
If you’re planning to run your heat pump during the day using rooftop solar, a bigger tank generally works in your favour. Heating a large tank to full temperature during solar generation hours means you’re carrying more stored hot water through the evening and overnight without the compressor needing to run again, which is exactly when you’d otherwise be drawing more expensive grid electricity.
A 330 litre tank is often recommended for solar households for this reason, even where the household size alone might suggest something smaller.
Replacing an existing system: should you upsize?
If you’re replacing a 250 litre electric system, a similarly sized or slightly larger heat pump is usually the right like-for-like upgrade, since heat pumps benefit from a bit of extra buffer to account for the slower recovery time. Jumping up one size band from your existing system is a reasonable default if your household size and usage haven’t changed.
If your household has grown since the existing system was installed, replacement is a good time to reassess rather than automatically matching the old tank size.
Common sizing mistakes and how to avoid them
The most common mistake is choosing the smallest, cheapest tank to save on the upfront cost, then running short of hot water during peak demand. The saving on a smaller tank is usually modest compared to the ongoing frustration of running cold partway through a busy morning.
The opposite mistake, buying more capacity than the household will ever use, is less common but does happen, usually when a household downsizes or a system is being installed speculatively for a future renovation. Oversizing isn’t dangerous, but it does mean paying more upfront and running a larger unit than necessary.
How Hogan Hot Water recommends the right system for your home
Rather than sizing off a generic chart alone, Hogan Hot Water assesses household size, usage patterns, any existing or planned solar, and the property layout during the quoting process, then recommends the model and size that fits. Brands like iStore and Envirosun offer a range of sizes across these bands, so there’s usually a model that fits your household without over or under-buying.
Frequently asked questions
What size heat pump do I need for a family of four?
Most 3 to 4 person households are well suited to a 250 to 270 litre heat pump, though households with heavy morning demand or plans for solar may benefit from sizing up slightly.
Is it better to size up if I’m not sure?
Sizing up slightly is usually the safer choice if you’re genuinely unsure, since running short of hot water is a bigger day-to-day frustration than the modest extra cost of a larger tank.
Does a bigger tank cost much more to run?
Not significantly. Running cost is driven mainly by how much hot water you use, not the tank’s maximum capacity, so a correctly sized larger tank doesn’t cost dramatically more to run than a smaller one.
Can I upgrade to a bigger tank later if my household grows?
Not without replacing the unit. Tank size isn’t adjustable after installation, which is why it’s worth sizing for your medium-term household plans rather than just your current situation.
Who decides the right size for my home?
Hogan Hot Water assesses your household size, usage pattern, solar setup and property layout during the quote, and recommends a size based on your specific situation rather than a generic chart alone.
If you’re not sure what size suits your household, get in touch for a free quote or site assessment and we’ll work it out properly before you buy anything.