What Size Hot Water System Do I Need? A Simple Guide

Working out what size hot water system you need can feel like a guessing game, and it's one of the most common questions we hear. Get it wrong and it stings either way: too small and you run out mid-shower, too big and you pay to keep water hot that you never use.

The right size for your place depends on how many people live there, how you use water, what time of day you use it, and what type of system you're running (electric, gas, solar or heat pump). Here's how to narrow it down.

1. Why size matters

A badly sized system costs you money. Undersize it and you're halfway through a shower when it runs cold. Oversize it and you're paying higher energy bills to keep a big tank hot all day for no reason. Once you've got the basics, picking the right size gets a lot easier.

2. Start with household size and habits

Headcount is the obvious starting point, but how you use hot water matters just as much. Long showers? Dishwasher running daily? Teenagers who treat the bathroom as a second bedroom? It's about how the household behaves, not just how many of you there are. As a rough guide, by average daily use in litres:

• 1 to 2 people: 100 to 160L
• 3 to 4 people: 160 to 250L
• 5 or more: 250 to 400L and up

3. Different systems, different sizing rules

Electric storage, gas storage, continuous flow, solar and heat pump systems all have different output capacities, so they're sized differently.

• Electric storage heats water and holds it in a tank, so you size it for peak-time demand.
• Gas storage recovers faster, so the tank doesn't need to be as big.
• Continuous flow systems don't store water at all, they heat on demand.
• Heat pumps are more efficient to run.
• Solar can cost very little to run once it's up, though there's an up-front cost to get it in.

In other words, the technology changes how size is calculated, which is why there's rarely one exact answer.

4. Typical sizes in Australia

The most common sizes you'll see on the market:

• Electric storage: 25L to 400L
• Gas storage: 90L to 300L
• Gas continuous flow: rated in litres per minute, commonly 16, 20 and 26 L/min
• Heat pump: usually 160L to 315L
• Solar (with booster): 250L to 400L and up

Storage systems are quoted by tank capacity in litres, continuous flow by flow rate.

5. Matching size to the household

For a family of five, a 250 to 400L storage system or a 26 L/min continuous flow unit usually keeps up with back-to-back morning showers, dishwashing and laundry. For a family of four, a 160 to 250L electric or gas storage tank, or a 20 to 24 L/min continuous flow unit, is generally enough. For a couple using water efficiently, a 100 to 160L electric tank or a compact continuous flow unit can be all you need.

6. Recovery time and off-peak tariffs

Here's where electric sizing trips people up. The tank has to hold enough hot water to get through peak use, especially on an off-peak tariff where it heats overnight. So even if you only use 200L a day, you might need a 250L tank to avoid cold showers in the evening. Gas storage recovers faster, so it can often be smaller. If you're switching from electric to gas or to a heat pump, make sure you understand how recovery rate affects your supply.

7. Sizing tools don't tell the whole story

Online size calculators give you a rough range, but they don't account for everything that affects real output, like pipe length, climate, fixture flow rates and water pressure. They all play a part, so treat the online numbers as a starting point, not the final word.

8. Think about the future

Planning to grow the family, add a second bathroom or put in a spa bath? That changes what you'll need. Going slightly larger than your current needs can save you the cost of upgrading again later, but don't overdo it, because excess size just means unnecessary running costs. It's a balance.

9. Size isn't everything

Efficiency, reliability, installation cost and energy source all matter too. A smaller, high-efficiency system can easily outperform a larger, less efficient one, especially in a mild climate or a home with low to moderate use.

How we do it at Fix-It Right

If you'd rather not guess, we'll size it with you. We look at your household, your usage and your energy source and recommend a system that actually fits, not just the biggest tank we can sell you. We keep it simple: there's $0 call-out, we give you three written quotes so you can pick what suits your budget, and you'll always know the price before we start. If the number's a stretch, Fix-It Now Pay Later gives you 12 months with no interest and no repayments. Every job is backed by our 10-year workmanship guarantee, because the whole idea is to fix it properly the first time. Plumbing fixed for good, not fixed for now.

We install hot water systems across Melbourne, Geelong, Ballarat and Canberra. Book online or give us a call and we'll help you get the size right.