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Buy a hot water system that is too small and you will run out mid-shower on a winter morning. Buy one that is too big and you are paying to heat water you never use. Getting the size right is the difference between a system that quietly does its job for a decade and one that annoys you every day. A 250L hot water system suits a lot of Sydney families, but the right answer depends on how many people live in your home and how you all use hot water.
Here is how to size it properly, whether you are choosing storage, continuous flow or a heat pump.
Most hot water complaints are not about a faulty unit. They are about a unit that was the wrong size for the household from day one. Sizing is about matching two things: your peak demand, which is the busiest hot water moment of your day, and your recovery rate, which is how fast the system reheats once it is drained. Get both right and you never think about your hot water. Get either wrong and you notice constantly.
This trips people up. A storage system is sized by how many litres it holds, because it heats a tank in advance and you draw from the reserve. A continuous flow system, sometimes called instantaneous, is sized by how many litres per minute it can heat on demand, because it never stores anything. So comparing a 250 litre tank to a continuous unit by litres alone is meaningless. One number is capacity, the other is flow rate.
As a rough guide for a storage system, the usable capacity you want climbs with the number of people and the type of fuel.
A one or two person household is usually well served by a smaller tank, often in the 125 to 170 litre range for electric, or less for gas because gas reheats faster. A three or four person household, the most common Sydney family, typically lands on a 250L hot water system for electric storage, or around 135 to 170 litres for gas. A five-plus person household often needs 315 litres or more in electric, or a larger gas unit, to avoid running dry during the morning rush.
The reason electric and gas numbers differ so much is recovery. An electric storage unit reheats slowly, so it needs a bigger reserve to cover back-to-back showers. A gas storage unit reheats much faster, so it can get away with a smaller tank for the same household.
The figure that actually decides whether you run out is your peak demand, not your daily total. Picture a winter weekday: three people showering inside an hour, perhaps while the dishwasher runs. That single window is what your system has to cover. A household that showers at staggered times all day can run a smaller unit than a household of the same size that all showers between seven and eight in the morning. When in doubt, size for the busiest hour, not the average day.
Electric storage needs the most capacity because of its slow recovery, and it is cheapest to buy but often dearest to run unless it is on a controlled off-peak tariff. Gas storage and gas continuous recover quickly, so they run smaller for the same demand, and continuous gas in particular gives effectively endless hot water as long as the flow rate suits the number of outlets running at once. A heat pump is an electric system that works like a reverse air conditioner and uses far less power, but it heats more slowly again, so heat pumps are generally sized generously, often 250 to 315 litres, to cover demand while they gently reheat.
Whatever size you choose, the tank itself is kept hot, at 60 degrees or above, to stop Legionella bacteria growing, and a tempering valve brings the delivered temperature down to a safe maximum of 50 degrees at your bathroom taps. That is a national requirement under AS/NZS 3500.4, not a setting you adjust to save space or money.
For most families of four it is a sensible electric storage size, particularly if showers are spread out rather than all crammed into one hour. A family of four that all showers in the same short window, or that has teenagers, may be better with 315 litres or a continuous gas unit.
As a rule of thumb a 250 litre electric storage tank comfortably covers three to four people's daily hot water, assuming normal shower lengths. Long showers, baths and a running dishwasher all draw down the reserve faster.
Continuous flow gives effectively unlimited hot water, which suits households with unpredictable or heavy demand. The trade-off is that flow rate is shared, so running two hot taps at once can reduce pressure. The right choice depends on how many outlets you use at the same time.
A bigger storage tank loses a little more heat standing idle, but the main running-cost driver is the energy source, not the size. Heat pumps and off-peak electric or gas are far cheaper to run than continuous peak-rate electric, regardless of capacity.
Not sure what size suits your household? Call Plumberoo on (02) 9191 8787. We are licensed (289252c), across Sydney, and we will size and quote your hot water system at a fixed price before any work begins.