Heat pump hot water size guide: what tank size do you need?

Heat pump tanks usually come in sizes from about 150 to 315 litres, with some up to 400 litres or more. The ACT Government says standard tanks "usually range from 150 - 315 litres" (ACT Government). Too small and you run out after the third shower. Too big and you pay to keep extra water hot. Here is how to land in the right range.

What size heat pump do I need?

The Victorian buyers guide recommends sizing by bedrooms "because the number of people in a household can change over the 10-15 year lifespan of a hot water system" (VEU buyers guide). CHOICE gives 270 to 315 litres as the heat pump tank size for a 4-person household (CHOICE).

How much hot water does a household use?

Your Home says "one person uses about 50 litres of hot water per day but assess your own circumstances" (Your Home). As a rough guide to daily use at that rate:

That daily figure is not the tank size. Tank water is stored hotter than the water at your tap (it is mixed with cold to reach a safe temperature), and the heat pump reheats during the day. That is why the bedroom table above, not a simple litres-times-people sum, is the better starting point.

What pushes me up a size?

• Running on a timer. Victoria's buyers guide warns that "a larger tank may be required for households using timers to reduce the risk of hot water running out between reheating times". If you plan to heat only in a 3-hour solar or free power window, the tank has to carry you through the evening and next morning.
• Slow recovery. At low air temperatures, the federal review says heat pumps can take "significantly longer to reheat" (Decision RIS). In a cold climate a slightly larger tank covers the gap.
• Everyone showers at once. Morning rushes and teenagers drain tanks faster than a daily average suggests.
• A big bath or spa, or a lot of hot water for laundry.

What lets me go smaller?

• Efficient showerheads. Your Home says showering uses most of a household's hot water and a 4-star WELS showerhead "can reduce this usage by about half".
• A heat pump that runs whenever it needs to, rather than only in a short window.
• Heat loss. Your Home says about 30% of a storage system's energy is lost as heat from the tank and pipes, so an oversized tank costs you every day.

Is there a size limit for rebates?

Yes, for STCs. The Clean Energy Regulator register covers "air source heat pumps with a capacity of no more than 425 L" (CER register). Almost all household units are well under that. State programs follow their own product lists.

Replacing a gas system: what size?

Don't just copy the litres on the old gas tank. A continuous flow (instantaneous) gas unit has no tank at all, so use the bedroom table. Heat pumps can be slow to reheat, especially in cold weather, so a household used to a gas tank recovering quickly may want a slightly larger heat pump tank. My guide to replacing a failed gas system covers the rest.

Tank sizes each brand offers are listed in the brand overview.