What Size Hot Water Heat Pump Do I Need? (NZ Guide)

The right cylinder size depends mostly on how many people use hot water daily and how they use it. A practical starting point:

These are guidelines. Actual usage depends on shower length, baths, dishwasher habits and laundry. A household of three with short showers can use less than a couple who love long baths.

The golden rule: when in doubt, size up. A slightly larger cylinder costs a little more upfront but avoids the daily frustration of running out. The running cost difference between adjacent sizes is small because a well-insulated cylinder loses little heat.

New Zealand spans mild subtropical north to frosty inland south, and heat pump recovery slows as the air gets colder. Adjust your sizing accordingly:

Northland, Auckland: mild. Standard sizing applies year-round.

Waikato, Bay of Plenty, Gisborne: moderate. Standard sizing is fine; winter mornings just mean slightly longer run times.

Taranaki, Hawke's Bay, Manawatu-Whanganui, Wellington, Nelson and Tasman, Marlborough: cooler. Standard sizing usually works, but lean toward the larger option if you are between sizes.

Canterbury, West Coast: cold winters. Size up one step from the household guide: winter mornings slow recovery noticeably.

Otago, Southland: the coldest zones, with Central Otago the toughest. Size up one step and favour a CO2 model (such as Reclaim Energy), which holds its efficiency in hard frosts. The combination of a bigger buffer and cold-rated hardware keeps hot water reliable through winter.

A good installer factors your location in automatically; if a quote does not mention climate at all in Queenstown or Invercargill, ask why.

The critical sizing factor is your peak demand window, usually the morning rush when everyone showers, or the evening when showers, dishes and laundry coincide.

A typical shower uses roughly 35-50 litres of hot water over 5-7 minutes with a standard showerhead; water-efficient heads use less. Build your worst-case scenario:

• 2 showers back-to-back: roughly 70-100L
• 3 showers back-to-back: roughly 105-150L
• 4 showers plus a dishwasher cycle: roughly 160-220L
• Add a bath and you can add 120-180L in one hit

Stored water at 60-65C is mixed down with cold at the tap, so each litre in the cylinder delivers roughly one and a half litres of usable warm water. A 300L cylinder therefore covers a string of showers, the dishwasher and a load of washing before it runs cool, provided it started full and hot.

Recovery time is how long the heat pump takes to reheat the cylinder after heavy use. It is the key behavioural difference from an element or gas burner: a heat pump trickles a large amount of heat efficiently over a few hours rather than blasting it in quickly.

As a rule of thumb, a full reheat of a family-size cylinder takes a few hours in mild weather and longer on cold winter days. That is fine in practice, because the cylinder acts as a buffer: it is reheating through the day while nobody is using hot water.

Most systems include an electric element boost that can speed up recovery during unusually heavy demand (guests, consecutive baths). It costs more per kWh of heat, so treat it as the spare tyre, not the daily driver.

Tip for solar homes: set the heat pump to run in the middle of the day. The cylinder stores that cheap solar-heated water for the evening and next morning, effectively turning your cylinder into a thermal battery.

Situations where choosing a larger cylinder than the standard recommendation pays off:

• Solar PV on the roof: more storage means more of your hot water can be heated on daytime solar and saved for later. With export buyback rates at only 7-12c/kWh, self-consumed solar is worth far more than exported solar.
• Cold climate: as above, Canterbury, the West Coast, Otago and Southland should size up to ride through slow winter recovery.
• Teenagers: shower habits change the maths. Plan for the worst case, not the average.
• Regular guests or a future bathroom: size for where the household will be in five years, not just today.
• Bath lovers: a full bath is one of the largest single draws in a home; account for it in your peak scenario.

The price step between adjacent cylinder sizes is small relative to the total $6,000-9,000 installed cost of a typical replacement, and far smaller than the cost of replacing an undersized system early.

Pairing a hot water heat pump with rooftop solar is one of the most effective ways to cut hot water costs to near nothing.

How much solar do you need? A hot water heat pump draws roughly 0.5-1.5kW while running. Even a modest solar system usually has that much surplus in the middle of the day, and a 5kW system (around $11,000-13,000 installed in NZ) has comfortable headroom.

Timer settings: set the heat pump to run during peak solar hours, roughly late morning to mid afternoon. It heats the full cylinder on your own generation and stores it for the evening.

The economics: every kWh of solar you use yourself displaces electricity at around 35c/kWh; every kWh you export earns only 7-12c. Routing solar into your hot water cylinder is therefore one of the highest-value uses of your surplus, ahead of exporting and without the price tag of a battery.

Diverters and smart controllers: some setups route surplus solar to the heat pump automatically. A simple timer captures most of the benefit; a diverter polishes it.