Hot water system sizing

Last updated 2026-08-28 · Last checked 2026-08-28 · 381 words · 2 min read

Tell the tool how many people and bathrooms you have, your climate and the type of system you are considering, and it returns a tank size in litres or a continuous-flow rating in litres per minute, using Rheem's and Rinnai's own sizing guidance. "Help me choose" shows all types side by side with price bands from CHOICE.

How the sizes are chosen

Storage tanks are matched to household size using Rheem's January 2024 guide (basis: seven-minute showers at 42 °C in a moderate climate): off-peak electric 125 L for up to three people, 160 L for four, 250 L for five, 315 L for six, 400 L continuous tariff for up to nine; gas storage 90 L for one or two, 130–170 L for larger homes; heat pump 178 L for two, 270 L for two to five; solar 160–180 L for one to three, 250–315 L for three to four. CHOICE's four-person benchmarks (electric peak 125–160 L, off-peak 250–315 L, gas 135–170 L, heat pump 270–315 L, solar 300–360 L) cross-check those rows.

Continuous flow is sized by simultaneous outlets: 16 L/min for one bathroom, 20 for one to two, 26 for two (up to three in warm climates), 32 for three or more. Rinnai's brochure gives the warm and cool climate columns; the tool moves cool-climate homes up one size. If your showers run back to back, the tool also steps up.

The basis figure of about 50 L of hot water per person per day comes from YourHome and CHOICE.

Rules that affect the choice

Storage water must be held at 60 °C to control Legionella and delivered at 50 °C through a tempering valve — YourHome states that all new systems require one. Victoria requires all-electric new homes needing a planning permit from 1 January 2024 and has announced electric replacement for end-of-life gas hot water from 2027. Queensland's current QDC MP 4.1 permits electric-resistance water heaters. NSW BASIX scoring is not modelled — check with your certifier.

Limitations

Manufacturer tables assume moderate climates and average use; a household with a spa bath or long showers needs the next size. Dux and AquaMAX sizing charts were not retrieved for this version; Rheem's table is used as representative because AquaMAX is a Rheem Australia brand.

Test vectors (for the build)