Energy Smart Homes - Hot Water

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Image of lady enjoying an energy efficient hot water shower

Hot water can account for one third of an average Perth household's energy consumption.

Carefully considering your hot water needs and choosing the most appropriate system for your household can significantly lower your energy bills and environmental impact.

Whether you are replacing a hot water system or purchasing for a new house, your decision will depend on how much hot water you need, now and for the life of the system, as well as your personal priorities.

How much hot water do you need?
To determine which hot water system best suits your needs, you will need to calculate your daily hot water consumption. Hot water use varies widely depending upon a number of factors including how many people live in your home and the age of those people. Information is provided below to help you estimate your household hot water consumption. As an approximation, 50 litres per day per person is about average.

Remember to consider your future needs when estimating your hot water consumption. A hot water system will last you a number of years so you will need to consider any possible changes to your household in the future. Younger children grow up to become teenagers who have a tendency to take longer showers.

Bathroom
Typically, most hot water use occurs in the bathroom. The amount of hot water you consume will depend on whether you prefer to take showers or baths.

If you take showers, you can calculate the amount of total water you use by multiplying the running time of your shower by the flow rate of the showerhead.

Total water use = running time x flow rate of showerhead

Showerhead flow rates can vary between 6 and 25 litres per minute. A standard showerhead can deliver up to 25 litres per minute (250 litres for a 10 minute shower) while a low-flow showerhead typically averages a delivery rate of 6 to 15 litres per minute (60-150 litres for a 10 minute shower). Generally, the older the showerhead, the higher the flow rate. The best way to determine your showerhead flow rate is to time how long it takes to fill a container of known volume, such as a 5 litre bucket. The following equation will help you to calculate the flow rate (litres per minute):

Flow rate = (60 seconds ÷ seconds taken to fill container) x container volume in litres

For example, if a 5 litre bucket takes 40 seconds to fill, then the flow rate is equal to 7.5 litres per minute (60 seconds ÷ 40 seconds x 5 litres). If a 10 litre bucket takes 1 minute and 10 seconds to fill, then the flow rate is 8.6 litres per minute (60 seconds ÷ 70 seconds x 10 litres).

Once you've multiplied the flow rate of the showerhead by the shower running time, you need to multiply the result by the ratio of hot water to cold water used to determine your total hot water use. Generally, hot water is mixed with around 30%-40% cold water in the shower.

Total hot water use = total water used x hot water mix (60%-70%)

Using a low-flow showerhead with a flow rate of 9 litres per minute during a 10 minute shower will use 90 litres of water. With a hot water mix of 70%, the total hot water use is 63 litres (90 x 0.7).

If you take baths, you can calculate your hot water use by multiplying the total amount of water required to fill the bath by the hot water mix. If your bath takes 100 litres of water to fill (which is about average), it will require about 60 to 70 litres of hot water (based on a cold water mix of 30-40%). Spa baths typically take between 250 to 350 litres of water to fill and will therefore require larger amounts of hot water. Some spa baths and larger spa pools are connected to a cold water tap and heat their own water. If this is the case, their hot water consumption does not need to be taken into account when sizing your hot water system as they do not place a demand on your hot water system.

Laundry
The laundry is another room that places a demand on your hot water system. A standard warm wash cycle typically uses between 30 to 50 litres of hot water per wash. You can save on hot water use by washing clothes in cold water or using a front loading machine if you still need to wash clothes in hot water. Front loading machines generally use much less water than top loading ones.

Kitchen
The amount of hot water you use in the kitchen will vary depending upon whether you have a dishwasher and if so, how it is set up. Most dishwashers are connected to the cold water tap and use an electric element inside the dishwasher to heat the water. If this is the case, they will place a demand on your electricity bills and not on your hot water system.

If your dishwasher does have a hot water connection it will consume between 12 and 90 litres of hot water per wash and needs to be taken into consideration when sizing your hot water system. To find out which type of dishwasher you have and how much water it uses, simply consult the operating instructions.

If you wash up by hand, you will typically use around 10 to 25 litres of hot water per wash.

Choosing your hot water system
Once you've estimated your daily hot water consumption you will be in a better position to purchase a hot water system suitable to your needs. For a low demand (around 100 litres per day), an instantaneous gas hot water system may be the best buy. For a medium sized household with more than one bathroom (around 200 litres per day), a storage hot water system or a larger instantaneous system may be suitable. Solar and heat pump hot water systems return their investment cost quickest for households with a high demand.

Further information
For further information, see the attached file (PDF 257 KB), Choosing a Hot Water System brochure.

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