What Size Hot Water System Do You Need? 50L to 400L Explained

21 August 2026

What Size Hot Water System Do You Need? 50L to 400L Explained

The quick rule: allow about 50 litres of storage per person on an electric system, or size a continuous flow unit by how many outlets run at once. A couple does well on a 125L to 160L electric tank or a 16 to 20L/min continuous flow unit. A family of four typically needs 250L to 315L electric, 135L to 170L gas storage (gas reheats faster, so the tank can be smaller), or a 26L/min continuous flow system. Five or more people, go up again.

Get the size wrong and you pay either way. Too small means someone showers cold every morning, and it’s usually the person who runs last. Too big means you’re paying to keep water hot that nobody uses, every day, for a decade.

Sizing by Household

The reason electric tanks run bigger than gas: most heat overnight on off-peak power, so whatever’s in the tank at 7am has to last the day. Empty it by dinner and you’re waiting for the next cycle. Gas storage reheats continuously, which is how a 135L gas tank keeps up with a household that would drain a 250L electric.

Add a margin for teenagers, a deep bath habit, or two bathrooms that genuinely run at once. We’ve met plenty of “family of four” households that shower like a family of seven.

About That 50L System

The 50L electric unit gets searched more than any other size, and it’s the most misunderstood product in the category. It is not a whole-house system. It’s the right answer for an ensuite, a kitchenette, an office tearoom, a granny flat or a studio with one occupant. In use, it gives you one proper shower, roughly 15 to 20 minutes of hot water, before it needs time to recover. Its service life is the same as bigger electric tanks, 10 to 15 years.

If you’re pricing a 50L unit for a family home, the kindest thing we can tell you is: don’t. You’ll be back at the decision within a month, this time with an audience of cold, unhappy people.

Storage, Continuous Flow, Solar or Heat Pump?

Storage costs less upfront and shrugs off simultaneous use. Continuous flow never runs out, provided the gas supply or electrical capacity is there, though very heavy simultaneous demand can outrun a single unit. Solar suits homes with good north-facing roof and daytime use. Heat pumps draw roughly a third of the power of a standard electric tank and currently attract generous rebates in Victoria, and they’re where more Melbourne homes are heading, not least because from 1 March 2027, end-of-life gas hot water systems in existing homes must be replaced with electric alternatives. Sizing a new system today with that rule in view is just sensible planning. Before you commit to a heat pump, read our guide to heat pump rebates and the installation mistakes to avoid; the traps in that post are all ones we’ve been called to fix.

What Installation Costs in Melbourne

General guide, installed: a 50L electric unit usually lands between $900 and $1,500, a 250L electric tank between $1,600 and $2,800, gas storage between $1,700 and $3,000, continuous flow between $2,000 and $4,000, and heat pumps between $2,500 and $5,500 before rebates. When a 250L quote comes in at the top of the range, it’s nearly always the changeover, not the unit: tired valves, non-compliant pipework or awkward access that has to be brought up to standard while we’re there.

Whatever the number turns out to be, you’ll have it in writing before we start. Fixed pricing is how O’Shea Plumbing has quoted for years, and one line from a customer review says why better than we can: “I knew the cost before he started work.” Every installation is handled by our licensed hot water plumbers, and if a dishwasher or fridge connection is happening at the same time, the appliance installation crew folds it into the same visit.