What Size Hot Water System Do I Need?
Get this wrong and you're either freezing mid-shower or paying to heat water you never use. Here's exactly how I size systems for homes across Beacon Hill, Frenchs Forest, and Allambie Heights.
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. Bathtub? Add 50-100 L. Continuous-flow gas never runs out — ideal for larger households. Heat pumps use the same sizing as storage tanks but cost 60-70% less to run.
| 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. |
I had a call from a family of five in Beacon Hill who needed a new hot water system. They'd been running a 125L tank and wondering why the last person always got a cold shower. "My teenage daughters can shower for an hour each," the mum told me. I upgraded them to a 250L tank with a heat pump. They've never run out since, and their energy bill dropped by more than half.
The two main hot water system types are sized differently, and it catches people out all the time.
- Storage tanks (electric, gas, heat pump) are sized by litres. A full tank is your available supply. Once it's empty, it needs 30-60 minutes to reheat. Choose a tank big enough for your peak morning usage.
- Continuous-flow gas is sized by litres per minute flow rate. It heats on demand and never runs out, but it can only heat so much water at once. A 20 L/min unit runs one shower plus a tap. A 26 L/min unit runs two showers plus a tap.
- Heat pumps recover slower than gas but faster than standard electric. Size the tank correctly — an oversized heat pump tank takes too long to reheat and cancels out the efficiency savings.
Bathtubs change everything
A standard bath uses 120-180 litres of hot water. A deep spa bath uses 200-300 litres. If someone fills the bath and another person wants a shower within the hour, you need:
- A 250-315 L storage tank, OR
- A continuous-flow gas unit rated at 24-28 L/min (no recovery needed), OR
- A heat pump with 270-315 L tank and a booster element
How to check if your current system is the right size
- Count peak demand. How many people shower in the morning? Any overlap? Count the max showers in a 60-minute window.
- Check your current tank label. It'll show the capacity in litres. If you run out of hot water, go up a size. If you never use it all, go down.
- Plan for the next five years. Renovating? Adding a granny flat? New baby? If you're planning bathroom renovations, size for that, not today.
- Call me. A ten-minute phone call saves years of cold showers or wasted energy. I'll ask a few questions and tell you the right size.
Northern Beaches reality check
Outdoor continuous-flow units in Manly, Dee Why, Brookvale, and Narrabeen need a model rated for coastal conditions — stainless-steel heat exchanger and corrosion-resistant case. Standard units in salt spray fail within 3-4 years.
Winter incoming water temperature drops to 8-12°C on the Northern Beaches, which cuts effective flow rate of continuous-flow gas units by 15-20%. If you're on the borderline between two sizes, pick the larger one for winter performance.
Many older homes in Mosman and Frenchs Forest have tight cupboard spaces that won't fit a larger tank. Continuous-flow gas is usually the answer — wall-mounted, compact, unlimited hot water.
Frequently asked questions
What happens if my hot water system is too small?
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?
Free sizing advice, no obligation. I'll recommend the right system for your household. Same-day install if it's urgent.
Related reading
- Hot Water System Maintenance ChecklistKeep your new system running efficiently for years.
- No Hot Water? 7 Common Causes and How to Fix ThemTroubleshoot issues before calling a plumber.
- Why Is My Hot Water System Leaking?Check for issues with your current system.
- How Long Should a Hot Water System Last?Is it time to replace your old system?
- Gas vs Electric Hot Water: Which Is Right for You?Choose the best energy source for your new system.