A couple in Edmondson Park rang me in July because the third shower of the morning was running cold by quarter to seven. Two adults, two teenagers, one 125 litre electric tank on off-peak. The tank wasn't faulty. It was never the right size for that house. The question I get most, from Liverpool out to Camden, is some version of "what size hot water system do I need?" Here is how I answer it. If the shower is cold right now, we are a hot water plumber, no call-out fee, across Liverpool, Campbelltown and Camden.
A family of four needs about 160 litres of electric storage on a continuous tariff, 250 to 315 litres on off-peak, 130 to 135 litres of gas storage, a 20 to 26 litre a minute continuous flow unit, or a 270 litre heat pump. One or two people can get by with 50 to 125 litres. If you are between sizes, go up.
The short answer: what size hot water system do I need?
For a family of four on a normal (continuous) electric tariff, a 160 litre storage tank does the job. Put the same family on off-peak and you want 250 to 315 litres, because the tank heats once overnight and has to last the whole day. Gas storage recovers much faster, so 130 to 135 litres suits four people. A continuous flow (instant) unit is sized in litres per minute, not litres stored: 20 to 26 litres a minute for four people and two bathrooms. A heat pump is usually a 270 litre tank, which covers two to five people.
The two types are sized differently. A storage tank is a bucket you size by how much the house draws between heating cycles. A continuous flow hot water system is a hose: it never runs out, but open more taps than it can feed and everyone gets a lukewarm dribble.
One thing people miss with storage. A 250 litre tank gives you far more than 250 litres at the shower. In NSW the tank holds water at 60°C or more to control Legionella, a tempering valve mixes it down to no more than 50°C for the bathroom, and your shower mixer pulls it down again to about 40°C. Every stored litre becomes roughly one and a half to two litres of shower water.
Hot water system sizes by household
These are the sizes I fit, and they line up with the manufacturers' own charts. Showers are the big variable. Four teenagers use far more hot water than four adults who are in and out in three minutes, so if that is your house, read the row below yours.
| People | Electric storage (continuous tariff) | Electric storage (off-peak) | Gas storage | Continuous flow | Heat pump |
|---|---|---|---|---|---|
| 1 to 2 | 50 to 125 litres | 125 to 160 litres | 90 litres | 12 to 16 litres a minute | 180 litres |
| 3 to 4 | 125 to 160 litres | 250 litres | 130 to 135 litres | 20 to 26 litres a minute | 270 litres |
| 5 to 6 | 250 litres | 315 litres | 135 to 170 litres | 26 to 32 litres a minute | 270 to 315 litres |
| 7 or more | 315 litres | 400 litres | 170 litres, or two units | 32 litres a minute, or two units | 315 litres, or two units |
Hot water system for a family of 4
Plainly: 160 litres electric on a continuous tariff, 250 litres on off-peak, 135 litres gas storage, a 26 litre a minute continuous flow unit for two bathrooms, or a 270 litre heat pump. Between two sizes? Go up. A slightly bigger tank costs little to run next to twelve winters of cold showers.
Off-peak hot water changes the maths
Most electric tanks around here sit on a controlled load tariff with Endeavour Energy, which the retailers call off-peak or controlled load 1. The network switches the element on for a block of hours overnight at the cheapest rate, and the tank has to carry the house through to the next night, which is the whole reason 250 and 315 litre tanks exist. Put a 125 litre tank on controlled load 1 in a four person house and you will be cold by breakfast, like that Edmondson Park job.
Controlled load 2 gives a longer heating window at a slightly higher rate and lets you run a smaller tank. If you have solar, there is a case for skipping off-peak hot water altogether and putting the tank on a timer through the middle of the day so you heat with your own panels.
Continuous flow hot water systems: it is all about litres per minute
An instant hot water system (the same thing as an instantaneous or continuous flow unit) is rated in litres per minute at a 25°C temperature rise. That matters in a Sydney winter. Mains water in Prestons in July arrives at around 12 to 15°C and you want about 40°C at the shower, so the unit works much harder than in February and the real flow rate drops.
My rule: 16 litres a minute for a one bathroom home, 20 for a small family with one and a half bathrooms, 26 for two bathrooms and four or five people, and 32 for three bathrooms or a house where two showers and the washing machine run together. On natural gas the unit also needs a gas line sized for it. A 26 litre unit draws far more gas than the old storage tank it replaces, and undersized gas pipe is the most common reason a new instantaneous hot water system disappoints. That is licensed gasfitting work, not a job for the handyman.
When a 50L hot water system makes sense (and when it doesn't)
A 50L hot water system is a genuine option for a studio, a granny flat with one occupant, an office kitchen or a workshop sink. It is small, light and fits under a bench, which is why I fit a fair few in the granny flats going up around Austral and Leppington. It is also the tank people buy because it was cheapest at the hardware shop, then ring me from Green Valley six months later because two adults can't both shower before work. For two people, 80 litres is my floor on a continuous tariff and 125 to 160 litres on off-peak.
Bigger isn't always better
Every storage tank loses heat through its walls, so a 400 litre tank for two people pays to keep water hot that nobody uses. Bigger tanks are heavier, need more slab space, and a big electric unit on a continuous tariff can need a larger element and circuit. Heat pumps are different again: a 270 litre heat pump reheats slowly, so it relies on the big tank rather than fast recovery. I've covered that in my post on how heat pump hot water systems work. The right size is the smallest one that never runs out on your worst day. For most houses I fit in Liverpool, Casula and out through Oran Park, that is a 250 or 315 litre tank on off-peak, a 270 litre heat pump, or a 26 litre continuous flow unit.
What getting the size right costs
Sydney 2026 price guides put an electric storage system supplied and installed at $920 to $2,875, gas storage at $1,375 to $4,025 and a heat pump at $3,450 to $6,900 before rebates (see this Sydney plumber cost guide). Inside those ranges, size is only one factor. Stepping from 160 to 250 litres is a modest jump; what moves the price is relocating the unit, upgrading the gas line or electrical circuit, a new tempering valve, and after-hours work. Heat pumps attract federal STCs and the NSW hot water upgrade incentive, which together take roughly $900 to $1,350 off. Anything else is quoted on inspection, with the price agreed before I start. For the full breakdown, read what a hot water system costs to install in Sydney.
When to call a plumber
If you are still researching, the table and the FAQ below will land you close. Ring me when:
- Your tank runs out every day. That is the wrong size or a failed element or thermostat; see no hot water: what to check first.
- You are adding a bathroom, a granny flat or a second storey and the existing unit will not keep up. I do a lot of these as part of renovations and new builds.
- You are changing fuel, say gas storage to a heat pump. That changes the plumbing, the electrical and the tempering valve.
- The tank itself is leaking. That is a today job, and my emergency plumber service runs 24/7.
Sizing, supplying and installing a hot water system is licensed plumbing work in NSW, and a gas unit is licensed gasfitting on top. I do all of it under my own licence (NSW Lic 488158C) and will tell you straight if a smaller, cheaper unit will do. Done right. Done once.
Monarch Civil & Plumbing covers South West Sydney and the Macarthur region from our base in Austral. If you need a hot water system installed or replaced by a plumber in Liverpool, Edmondson Park, Camden or anywhere in between, ring 0455 222 292 and I will size it properly the first time.
Related guides: hot water system installation cost.
Need it sorted? Ring 0455 222 292 any hour or book online. Monarch Civil & Plumbing is a hot water plumber, no call-out fee: repairs where they are worth it, same-day replacement where we can, and the NSW rebate sorted.
No hot water in Campbelltown, Liverpool or Camden?
Campbelltown has its own page: hot water service Campbelltown. For Liverpool, Camden and the rest of South West Sydney start with hot water systems, then the suburb pages plumber Liverpool and plumber Camden. Same-day replacement for common electric and gas sizes where stock allows, no call-out fee, NSW rebate sorted for heat pumps.
Frequently asked
What size hot water system do I need for a family of 4?
On a continuous electric tariff, 160 litres. On off-peak, 250 litres, because the tank only heats overnight. In gas storage, 130 to 135 litres. For continuous flow, a 20 litre a minute unit for one bathroom or 26 litres a minute for two. A 270 litre heat pump covers a family of four comfortably. If your four includes teenagers who take long showers, move up one size.
Is a 50L hot water system big enough for two people?
Not really, unless you shower at different times of day. A 50 litre tank suits one person in a studio or granny flat, or a kitchen sink in an office. For two adults I fit 80 litres as a minimum on a continuous tariff, and 125 to 160 litres if the tank is on off-peak, because it has to hold a full day's hot water.
What size continuous flow hot water system do I need for two bathrooms?
A 26 litre a minute unit is the safe pick for two bathrooms and four or five people. A 20 litre unit will run two showers in summer but struggles in a Sydney winter, when the incoming water is colder and the unit has to work harder to reach shower temperature. Go to 32 litres a minute for three bathrooms or a house where showers and the washing machine run together.
Do I need a bigger tank on off-peak hot water?
Yes. Controlled load 1 heats the tank for a block of hours overnight and then leaves it alone, so the tank has to carry the household all day. Go up at least one size compared with a continuous tariff: 250 litres for three or four people, 315 litres for five or six. Controlled load 2 adds a daytime boost and lets you run a slightly smaller tank.
What size heat pump hot water system do I need?
Most brands make a 180 litre and a 270 litre unit, and a few make a 315 litre. The 180 litre suits one or two people, the 270 litre suits two to five, and the 315 litre suits five or six. Heat pumps reheat slowly, so they rely on the tank size rather than fast recovery, which is why I rarely fit the small one to a family home.
Can I replace my 250L tank with an instantaneous hot water system?
Often, yes, but it is not a straight swap. An instant unit needs a gas line big enough to feed it, or a heavy electrical circuit for the electric type, plus a compliant tempering arrangement and flue clearances. If the gas line is undersized you will get poor flow and temperature swings. I check the gas run, the pressure and the bathroom count before I quote it.
Licensed plumber and drainer based in Austral, 13+ years in the trade, working across Liverpool, Fairfield and the Macarthur region. About Paul