What Size Rainwater Tank Do I Need?

If you want to install a rainwater tank on your property, you need to determine the right size. And that will depend on your water requirements. Of course, bigger tanks store more water, but not everyone needs a large tank. Here are some things to keep in mind when deciding the size of your rainwater tank.

The Purpose of the Tank

Are you buying your tank for building compliance? If yes, you need to determine the minimum capacity stipulated by the council. This varies in different places, but councils require you to have at least a 1000-litre water tank. Once you know the minimum capacity, you can then choose how much additional capacity you want to store. This would be best done by working out how much water you use and deciding which application types you would like the tank water to supply.

Remember that rainwater may not be appropriate for all application types when calculating. Most people use it for toilet flushing, clothes washing and outdoor needs. Ideally, you should aim to cover or significantly contribute to your brown-water usage if connected to town water. If not connected to town water, ALL water usage will need to be covered by rainwater harvesting and you can use your existing water bill to etermine how much water you use.

Here’s a handy table to assist in your calculation of water uses by application type:

Application Water Used
Brown Water Uses
Toilet Flush (Half) 3 litres per flush
Toilet Flush (Full) 6 litres per flush
Clothes Washing (Front Loader) 80 litres per load
Clothes Washing (Top Loader) 100 litres per load
Sprinkler, Hosing, Filling Pool 15 litres per minute
Drip Watering 6 litres per hour
Car Wash 200 litres per wash
Drinking Water Uses
Shower 15 litres per minute
Bath 150 litres per bath
Dishwasher 30 litres per load
Brushing Teeth while tap running) 5 litres per minute
Kitchen tap 5 litres per minute

Once you have the number of these uses per week worked out, you can then calculate the amount of water required per year by multiplying by 52 weeks.

The Amount of Water You Can Harvest

The general rule of thumb says that each roof space can harvest about 1 litre of water for every 1
millimetre of rainfall. So you can use this formula to determine the amount of water you can
collect. Take (x) square metres of the available roof space where you connect your rainwater tank and multiply by the amount of rainfall received to help you make an accurate decision.

For instance, if your roof space measures 100 square metres and you receive 450mm of rainfall per year, using the above formula, you will multiply 100 square metres of roof space with 450mm of
rainfall to get 45000 litres.

Assuming that your tank fills thrice a year (this is entirely dependent on the rainfall and your usage patterns), the minimum size of your water tank should be 15000L. You can confirm the average
rainfall per annum here.

The Amount of Space You Have

How big your water tank should be, depends on the available space. If your space is limited, you’ll have no option but to opt for a smaller tank. Some Tetris-like configurations are possible such as installing several slimline tanks down the side of a building where a single round tank would be too wide. Alternative configurations are to put smaller round tanks at either end of a building or using underground tanks.

Whether You Use a “First Flush” Device

This device is used to divert the first (x) litres of water after every rainwater harvesting. The water is diverted to stormwater, reducing the chances of leaves, debris and other contaminants from the collection source entering the tank and polluting the water. While this method keeps the water clean, it can cause a slight reduction in your water harvesting ability.

Whether you Are Using Multiple Tanks or One Tank

If you have multiple catchment areas (such as the house, plus other constructions such as sheds), you could install a tank at each building. In this circumstance, your considerations would be the same as for choosing one tank per building. However, if you are using multiple tanks – allowing you to further increase capacity by adding more tanks at a later date – you might decide to choose slightly smaller tanks to reduce your costs for each outlay.

In Conclusion

The above tips will help you decide what size of tank you need. But if you’re still unsure about your decision, it helps to talk to an expert. Our staff can help point you in the right direction. So call us today.

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