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How to choose the right rainwater tank size

What Size Rainwater Tank Do I Need?

For anyone thinking about thinking about harvesting rainwater, it’s a common dilemma deciding on the right rainwater tank size! We explain the main considerations you need to account for in order to find the ideal tank for you. You’ll need to think about your annual rainfall, roof size, and likely usage, as well as any regulatory requirements.

Average annual rainfall

Wherever you live, your average annual rainfall will determine how much rainwater you can possibly harvest in a year. You clearly can’t catch more rain than falls and just because you install a 50,000 litre rainwater tank doesn’t mean it will fill with rain. It is therefore important to check the average rainfall in your area and consider how regularly a rainwater tank can be filled.

You can find out how much rainfall you receive by looking up records from your local weather station. This map, courtesy of the Bureau of Meteorology, shows the average annual rainfall in NSW based on the averages of a 30 year climatology from 1961-1990.

NSW Average Annual Rainfall

How big is your collection area?

The amount of rain your collection area, or roof, will yield depends on its surface area. This is another important indicator which helps to determine what size rainwater tank is right for you. You can convert your roof surface area into litres of rainwater using the following simple ratio.

Roof size (m²) x Rain (mm) = Rainwater yielded (L) I.e. 1mm x 1m² = 1L

In simple terms, every square metre of roof yields 1 litre for every millimetre of rain received.

How Much Rainwater Does Your Roof Collect?

Water usage

Matching your tank capacity to your usage reaps the best value from your free water supply. Your water usage depends largely on whether your tank will be connected to mains supply or only garden taps. Of course, it will also depend on the number of people in your household and the water efficiency of your fixtures. If you are connecting bathrooms or laundry, you will use your tank water much faster than general garden use. You will require even greater volume if you are also planning on washing your car with your rainwater supply every week! A quick way to determine your water usage is by examining your most recent water bill.

Council requirements

Building regulations like BASIX will also determine the rainwater tank size you will require. These requirements may vary depending on the location, size and type of your home and what you will be using your rainwater for. Council may require a minimum or maximum volume, and may impose certain height or width restrictions. In addition to planning requirements, there may also be plumbing/installation requirements and health requirements to consider. If you are unsure what your relevant regulatory requirements are, you should contact your local council.

Still not sure what size is best for you? Give us a call on 1800 218 000 and speak to one of our helpful experts!

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