Hot Water Cylinder Size Calculator

Need a new hot water cylinder? We replace and install them across Hastings, Napier and Hawke's Bay, from like-for-like swaps to mains pressure upgrades. Getting the size right matters: too small and the hot water runs out, too big and you pay for a cylinder you don't need. This calculator works out the right size for your home in six quick questions.

A 3 to 4 person home usually needs a 180 or 250 litre hot water cylinder. As a rough guide, 1 to 2 people need 135 to 180 litres, 3 to 4 people need 180 to 250 litres, and 5 or more need 250 to 300 litres or more. Mains pressure, a bath most days, night-rate power or a heat pump all push you towards the bigger end. Electric cylinders come in standard sizes of 135, 180, 250 and 300 litres.

How we size a cylinder: 50 to 60 litres per person a day

We work out how much hot water your home uses in a day, then pick the smallest standard cylinder that holds at least that much.

1. Daily use. The number of people × 50 litres on low pressure, or × 60 litres on mains pressure.
2. Bath. Add 60 litres if someone has a bath most days.
3. Slow or night-only heating. Add 25% for night-rate power or a heat pump.
4. Round up to the next standard size: 135, 180, 250 or 300 litres. For a heat pump, start at 180, the smallest common heat pump size.
5. Morning rush. Long showers, or everyone showering at once: go one size up.

For example, a family of four on mains pressure uses about 4 × 60 = 240 litres a day, so the next standard size up is 250 litres.

Where the numbers come from

• 40 to 60 litres per person a day. That's typical hot water use in New Zealand homes, stored at 60°C (BRANZ Level; Building Code G12/AS2).
• Mains showers use more. A study of New Zealand homes measured mains-pressure showers at 12.5 litres a minute, against 7 on low pressure (BRANZ HEEP).
• A bath adds about 60 litres. A bath takes 80 litres half full and 140 full (SmartWater). When you mix 60°C cylinder water with 15°C cold to get 42°C, about 60% of the bath comes from the cylinder.
• The 25% buffer. EECA's heat pump installation guide treats a 250 litre cylinder as a safe size for 200 litres a day. Heat pumps reheat more slowly than an element (1.5 to 2 kW against 2 to 3 kW, EECA). On night rate the cylinder only heats from 11pm to 7am (Unison's night option), so nothing tops it up during the day, and we use the same buffer.
• Controlled power doesn't change it. Unison can switch controlled hot water off for up to 7 hours in any 24. Across New Zealand, switch-offs usually last under 3 hours (EECA). A cylinder sized for a day's use rides through that.
• Standard sizes. Electric cylinders commonly come in 135, 180, 250 and 300 litres, and bigger ones are made up to about 400 litres (Rheem's range).
• Showers and reheat times. Shower counts assume 7.5-minute showers (BRANZ water use study), mixed to 42°C from 60°C stored water and 15°C cold (Rheem's and Rinnai's test conditions). Reheat times assume a 3 kW element, which heats 51 to 54 litres an hour (Rheem, Rinnai).
• Night rate and the morning rush. Plumbers' and retailers' sizing guides say to go up a size for long or back-to-back showers, and we do the same. No source gives a figure for night rate, so using EECA's 25% buffer there is our own call.

Numbers last checked 2 October 2026. The sources are listed at the foot of the page.

Hot water cylinder sizes by household

The same formula, worked out for the most common homes. A bath most days or a morning rush can push any of these up a size.

Cylinder size by number of people

Showers and reheat time from a full cylinder

Showers back to back from a full cylinder, at 7.5 minutes each and 42°C. Shower counts and reheat times are approximate. Reheat is from cold with a 3 kW element.