How can a hot water tank deliver more warm water than its capacity?

Whether it's electric storage, gas storage, or heat pump hot water systems, most tanks heat water to around 60°C. But when you shower or run the washing machine or dishwasher, the hot water is mixed with cold before coming out of the tap at around 38-40°C.

Because of this mixing process, the amount of hot water you actually get at a comfortable temperature is much more than the tank’s rated volume. For example, a 300L hot water heater at 60°C can deliver 500L or more at 40°C when it mixes with cold water of about 15-20°C. That’s why it’s useful to look at both storage and delivered volume. Generally, you can assume that you’d be able to get approximately 1.5 to 1.7 times the tank’s capacity of shower temperature water.

You can’t directly compare storage systems to instant hot water systems (also known as continuous flow or tankless systems). Instant systems heat water only as needed and are rated according to a flow rate of litres per minute, not total litres. Many run on natural gas or LPG and are popular due to their compact size and energy-efficient operation.

However, a storage hot water system can usually provide a higher water flow to multiple taps at once, making it a good choice for bigger homes with more people and more bathrooms. Gas hot water systems, especially storage models, tend to recover faster than electric ones, but often at higher running costs unless paired with off-peak tariffs.

Choosing the right size hot water system depends on your household size, the number of people who shower at the same time of day, and the water temperature you prefer to shower at. It’s not just about tank capacity - it’s also about how the system performs during peak times.

There are fewer gas storage tank systems available than electric ones, but due to the faster speed at which they heat water, a smaller capacity gas system can have a similar deliverable volume to an electric tank with a larger capacity.

Here’s a quick size guide:

• Small households (1-2 people): 125-160L electric or 90-130L gas storage

• Medium households (3-4 people): 250–315L electric or 130-170L gas

• Larger households (5+ people): 315-400L+ storage or high-capacity continuous flow

It’s worth considering solar hot water systems or heat pump models if one of your objectives is to achieve lower energy costs and potential rebates, especially in warmer parts of Australia like WA, NSW, SA, QLD and the NT. Pairing them with an off-peak schedule can help reduce your running costs even further.

We can deliver to 95% of Australians, so whether you’re ready to order a new storage tank, heat pump or continuous flow hot water system, odds are that we can get it to you by tomorrow (Mon-Fri), with Appliances Online's legendary FREE delivery. Plus, we offer paid Same Day Delivery in NSW, QLD, and VIC metro areas if you order before 12pm Mon-Fri.

You can check out the full range of water heaters here from brands including Rinnai, Rheem, Bosch and Stiebel Eltron. If you need some pointers on choosing the best type or size for you, call our friendly team 24/7 on 1300 000 500 or message us via the chat icon at the bottom of your screen.