Sizing a hot water system properly is the difference between always-warm showers and running out mid-shampoo. Undersized systems are a daily frustration, oversized systems waste money on standing losses and install cost. We size at quote stage based on household occupancy, daily peak usage pattern, and the type of system you choose.
The basic rule, by household size
For storage-type systems (gas storage, electric storage, heat pump), tank volume is the key spec, you need enough hot water to cover your daily peak without depleting the tank.
- 1-2 person household: 125-160 L storage tank, or small continuous flow (16-20 L/min for gas)
- 3-4 person household: 250-315 L storage tank, or 26 L/min gas continuous flow
- 5-6 person household: 400 L storage tank, or 26-32 L/min gas continuous flow
- 7+ person household: 400+ L storage with high recovery, or 32 L/min gas continuous flow (dual unit sometimes)
Daily hot water usage averages roughly 50 L per person per day, but peak usage in the morning shower window matters more for sizing than total daily volume.
Storage vs continuous flow, which sizing approach
Storage systems (gas storage, electric storage, heat pump, solar) heat a fixed volume and store it for use. If demand exceeds tank capacity in a short window, you run out and wait for the tank to reheat (1-3 hours typical). Size the tank to handle your peak window.
Continuous flow systems (gas instant hot water, also called tankless) heat on demand. They are sized by flow rate (L/min) rather than tank volume. A 26 L/min unit can supply two simultaneous showers comfortably. A 32 L/min unit handles three simultaneous showers. You never run out, but if you exceed the flow rate, the temperature drops.
Heat pump sizing specifically
Heat pumps are storage systems with a slower recovery rate than electric or gas storage. The tank needs to be sized to handle the daily peak demand because the heat pump cannot reheat fast enough to recover mid-shower. For most Gold Coast families, 270-315 L is the standard recommendation.
- Sanden Eco 250 L: suits 3-4 people typically
- Reclaim Energy 300 L: suits 3-5 people
- iStore 270 L: suits 3-4 people, fits in tighter installations
Solar hot water sizing
Solar hot water is sized by tank volume and panel area. Standard 4-person Gold Coast install is 300-315 L tank with 2-4 panels. The panels generate the heat, the tank stores it for use. Boost element (electric or gas) tops up on cloudy days.
Other sizing considerations
- Bath users. A full bath uses 80-120 L. Households that take frequent baths size up.
- Premium showerheads. Rainfall and body-jet showers use 12-20 L/min versus 7-9 L/min for standard. Sizing must account for higher per-shower draw.
- Outdoor hot water demand. Outdoor shower, outdoor kitchen sink, pool top-up to spa, all add to total demand. Canal-front and acreage homes often need larger systems for this reason.
- Future-proofing. If you expect family size to grow (more kids, parents moving in), size for future demand not just current.
What happens if you undersize
Cold showers in the morning peak. Constantly waiting for reheat. Frustration. Eventually you replace earlier than planned because the system is undersized for actual use. Worth getting the sizing right at install time.
What happens if you oversize
Higher install cost ($300-800 extra for the next size up). Slightly higher daily standing losses on storage systems (negligible on heat pumps and continuous flow). Largely a cosmetic over-spec, no real downsides beyond the upfront cost. Better to be slightly oversized than undersized.
How we size your specific install
On-site assessment, household size now and projected, daily usage pattern (morning peak, evening peak, both), shower spec (standard or rainfall), bath users, any outdoor scope. We then quote 2-3 options at different sizes so you can see the cost/performance tradeoff.
The morning shower test
The single most useful sizing test for storage systems is the morning shower test. Time-stack three back-to-back showers (one after another with 5-minute gaps). If the third shower goes cold, your tank is undersized for your peak window. If all three are comfortable, sizing is right. If the fourth is cold but third is fine, sizing is borderline.
For continuous flow, run two showers simultaneously plus a kitchen tap on hot. If any of the three goes lukewarm, flow rate is undersized. Comfortable across all three = right sizing.
If you have a household with shift workers (showers spread across the day rather than concentrated in morning), sizing requirements relax significantly. The peak window matters more than the daily total.
Litres per minute by fixture, real numbers
Understanding actual flow rates helps size the system properly:
- Standard showerhead (3-star WELS): 7-9 L/min
- 4-star WELS showerhead: 6-7 L/min
- Rainfall shower (over 200mm): 12-18 L/min
- Body-jet shower system: 18-30 L/min
- Standard basin mixer: 4-6 L/min hot when in use
- Kitchen mixer: 6-8 L/min hot
- Dishwasher (hot fill): 6-12 L/min during fill cycle
- Washing machine (hot fill): 8-15 L/min during fill cycle
- Bath fill: 12-20 L/min, takes 5-10 minutes for a full bath
For a continuous flow unit rated at 26 L/min, peak simultaneous demand under typical Gold Coast usage (one rainfall shower + one standard shower + kitchen tap = 25-30 L/min total) is right at the limit. Add a dishwasher fill cycle on top and the system starves. Worth knowing before specifying.
The growing household scenario
Sizing the hot water system for current household = bare minimum approach. Many young families size for current 2-person occupancy then add kids and find themselves with daily cold-shower problems. The cost of going one size up at install is $300-800. The cost of replacing an undersized unit early is $2,000-4,000.
If you are sizing for a household that may grow (adult kids moving home, parents moving in, expanding family), spec one size up. Better to have spare capacity for a decade than to be replacing in 3 years.
The Gold Coast specifics that affect sizing
Some Gold Coast-specific factors that push sizing decisions:
- Beach showers after surf: coastal households often shower extra times per day after beach visits. Push sizing up.
- Pool users: pool families shower more frequently to rinse off chlorine or salt. Push sizing up.
- Summer heat patterns: Gold Coast summer means more showers and cooler shower temperatures (less hot water per shower). Net effect typically neutral.
- Winter cold mornings: coastal winter mornings (June-August) push everyone to longer hot showers. Sizing must accommodate the winter peak.
- Acreage outdoor shower: hinterland properties with outdoor showers (livestock care, gardening) add demand.
- Holiday letting: short-stay units sized for peak occupancy (4 adults in a 2-bedroom), not average.
Right-sizing for replacement vs new build
On replacement, you have an existing data point (was the old unit adequate?). If yes, like-for-like sizing works. If you remember morning cold-shower issues, go up a size. If the old unit always had spare capacity, you might be able to go down a size on the new (especially when switching to a more efficient type).
On new build, sizing is based on projection and best-practice ratios. We size to AS4234 minimums plus a 15-20% comfort margin for Gold Coast usage patterns.
Pricing implications of sizing
- Going from 250L to 315L heat pump: add $300-500
- Going from 315L to 400L: add $400-700
- Going from 26 L/min to 32 L/min gas continuous flow: add $200-400
- Going from single unit to dual gas continuous flow (very high demand): add $1,800-3,500
Small upgrades at install are far cheaper than replacement later. Worth getting right at quote stage.