What Size Hot Water System Do I Need?
Choosing the wrong size means either running out of hot water mid-shower or paying to heat water you never use. Here is exactly how to calculate the right capacity for your home, based on household size and usage patterns.
1-2 people: 125-160 L tank or 16-20 L/min continuous-flow. 3-4 people: 160-250 L tank or 20-24 L/min. 5+ people: 250-315 L tank or 24-28 L/min. If you have a bathtub, add 50-100 L to the tank recommendation. Continuous-flow gas units are sized by flow rate (litres per minute) and never run out, making them ideal for larger households. Electric heat pumps are sized the same as tank storage but cost less to run.
- • 1-2 people: 125-160 L tank OR 16-20 L/min continuous-flow gas.
- • 3-4 people: 160-250 L tank OR 20-24 L/min continuous-flow gas — the most common Northern Beaches install.
- • 5+ people: 250-315 L tank OR 24-28 L/min continuous-flow gas — consider twin units for large homes.
- • Homes with 4+ bathrooms: 315-400 L storage or twin continuous-flow units.
- • Bathtubs add 50-100 L to your tank requirement — factor this in.
- • Continuous-flow gas never runs out — better for families with overlapping peak usage.
- • Heat pump tanks use the same sizing rules but cost 60-70% less to run than standard electric.
Sizing guide by household
These recommendations are based on what we actually install in homes across Mosman, Manly, Dee Why, Brookvale, and Frenchs Forest. Adjust up if you have a bathtub or high usage.
| Household size | Electric storage (tank) | Continuous-flow gas | Heat pump | Note |
|---|---|---|---|---|
| 1-2 people | 125-160 L | 16-20 L/min | 160-200 L | Smallest available for most brands works fine. |
| 3-4 people | 160-250 L | 20-24 L/min | 200-270 L | Most common size we install across the Northern Beaches. |
| 5+ people | 250-315 L | 24-28 L/min | 270-315 L | Large families need continuous-flow or a big tank. |
| Large home (4+ bathrooms) | 315-400 L | 28+ L/min or twin units | 315+ L or twin | Consider twin continuous-flow units for peak demand. |
Tank vs continuous-flow sizing
The two main system types are sized differently:
Storage tanks (electric, gas, heat pump)
Sized by tank capacity in litres. A full tank of hot water is your available supply. Once it is used, the system needs time to reheat (recovery time — typically 30-60 minutes for electric, 15-30 minutes for gas). Choose a tank large enough to cover your peak morning usage without running out.
Continuous-flow gas units
Sized by flow rate in litres per minute (L/min). These units heat water on demand and never run out — the trade-off is they can only heat so much water at once. A 20 L/min unit can run one shower (9 L/min) plus a kitchen tap (6 L/min) simultaneously. A 26 L/min unit can run two showers plus a tap. They are ideal for homes where multiple people need hot water at the same time.
Heat pump considerations
Heat pumps recover more slowly than gas but faster than standard electric elements. They are best paired with a correctly sized tank rather than oversized, because reheating a large tank takes longer. The 60-70% running cost saving means the payback period is 3-5 years on a correctly sized system.
Bathtubs and special considerations
If you have a bathtub, sizing changes significantly. A standard bath uses 120-180 L of hot water (mixed with cold). A deep spa bath uses 200-300 L. If one person fills the bath and another wants to shower within an hour, you need:
- A 250-315 L storage tank (recovery time 45-60 min for electric), OR
- A continuous-flow gas unit rated at 24-28 L/min (heat on demand, no recovery needed), OR
- A heat pump with 270-315 L tank and a booster element for faster recovery
Other factors that affect sizing: rainwater tanks (lower inlet pressure can affect continuous-flow units), solar pre-heat systems (smaller boost tank needed), and apartment living (smaller tanks typically suffice due to less usage).
What you should do first
Before deciding on a size, audit your actual usage:
- Count peak demand. How many people shower in the morning, and do any overlap? Count the maximum number of showers in a 60-minute window.
- Check your current tank. Look at the label on your existing system — it will show the capacity in litres. Has it been enough? If you run out, go up one size. If you never use it all, go down one size.
- Factor in future changes. Are you planning a bathroom renovation, adding a granny flat, or expecting a child? Size for your peak needs over the next 5 years, not just today.
- Talk to a plumber. We see the sizing mistake most often: people undersize for their actual usage. A 10-minute phone call with our team can save you years of cold showers or wasted energy.
When to call a plumber
Sizing is not complicated, but getting it wrong is expensive — either ongoing cold showers or years of wasted energy heating unused water. If you are replacing an existing system, our team can look at your current setup, ask a few questions about your household, and recommend the right size in minutes.
Contact our team for free sizing advice and a fixed-price quote. We install all major brands across the Northern Beaches and will not upsell you to a larger system than you need.
Northern Beaches considerations
A few local factors affect sizing on the Northern Beaches. Properties in Manly, Dee Why, Brookvale, and Narrabeen with outdoor continuous-flow gas units need a model rated for the coastal environment — look for stainless-steel heat exchangers and corrosion-resistant cases. Standard units in salt spray zones can fail within 3-4 years.
Winter incoming water temperature on the Northern Beaches drops to 8-12°C, which reduces the effective flow rate of continuous-flow gas units by roughly 15-20%. If you are on the borderline between two sizes (e.g., a 20 L/min or 24 L/min unit), choose the larger one for winter performance.
Many older homes in Mosman and Frenchs Forest have limited space for a larger tank. In these cases, continuous-flow gas is often the best solution — small wall-mounted unit with unlimited hot water. Our team can advise on what physically fits in your space during a free onsite assessment.
Frequently asked questions
What happens if my hot water system is too small?
The most common complaint we hear: the last person in the queue gets a cold shower. An undersized system runs out of hot water during back-to-back showers, filling a bath, or running the dishwasher while someone is showering. Upgrading to a larger tank or switching to continuous-flow gas solves this.
Can a hot water system be too big?
Yes. An oversized electric storage tank wastes energy keeping water hot that you never use. You are paying to heat and reheat a full tank when you only use half of it daily. Oversized continuous-flow systems are less of an issue because they only heat on demand, but the upfront cost is higher than necessary.
Do I need a larger system if I have a bathtub?
Yes. A standard bathtub holds 120-180 litres. A deep spa bath can hold 250+ litres. If you fill the tub and someone else wants a shower shortly after, you need either a 250+ litre storage tank or a continuous-flow gas unit that never runs out. Factor bath usage into your sizing calculation.
Does the Northern Beaches climate affect sizing?
Not significantly for indoor units. For outdoor units, cold winter mornings mean incoming water temperature drops to 8-12°C instead of 18-22°C in summer. Continuous-flow gas units may need a larger capacity model to maintain flow rate during winter. Heat pumps also work harder in winter but are designed for this.
Not sure what size you need?
We will help you choose the right system and size for your home — no obligation, just honest advice. Same-day install available for urgent replacements.
Related reading
- Hot Water System Maintenance Checklist— Keep your new system running efficiently for years.
- No Hot Water? 7 Common Causes and How to Fix Them— Troubleshoot issues before calling a plumber.