The supply of mains water to Adelaide is the most energy intensive of any capital city in Australia and as a result the most expensive. We have changed the way water runs through our landscapes and consequently the cost of water has gone through the roof. The price of water and the cost to the environment are ever increasing. Local waterways which were once filled with fish, birds, aquatic plants and native animals and ran for much of the year (thanks to ground water flow) are now a shadow of their former selves. The River Torrens, Adelaide’s premier river, currently looks like a rather pathetic, algae clogged stream rather than the impressive, healthy waterway in which the national swimming championships were held in 1912. Anyhow!

If you’re like me you can appreciate that by capturing and recycling water you take control of your own water supply, reduce your exposure to water price hikes and help reduce the burden on the environment. So, the question is “what size water tank do I need?” Here’s a bit of help sorting that out.

How much water do you use? The first way you can do this is by looking at previous water bills (which break down your use generally over quarterly periods). Alternatively, you can use a tool such the water calculators provided by most state water regulators such as this one here (

Your water use will vary with the seasons so look at water use over the year and take note of periods of highest demand (i.e. in summer you will water your garden more and therefore need more water storage). Using a calculator is a useful exercise which can highlight ways in which to save water. Take into account your aspirations regarding landscaping and growing food and where you can reduce water usage if that is not already included in your calculations.


How much rain can you capture? Calculate how much roof area you have and the amount of average rainfall per year that falls where you live. Don’t forget your carport and sheds. You can use Google Earth to calculate areas and the Bureau of Meteorology to locate the weather station closest to you which will give you average monthly and yearly rainfall totals. Multiply your annual rainfall by the roof area by 0.9 for metal roofs which is the runoff coefficient (which tells you that 90% of rain hitting your roof will be collected by your gutters) and you will have your total runoff from this area estimated for a year. One mm of rain on 1m2 of roof surface = 1 Litre of water.

Note that steeper pitch roofs will have a greater surface area than a flat pitched roof. For an average pitched roof multiply your total area calculated from Google Earth or your floor plan by 1.1.

Where will your rainwater be used? According to SA Water, the average suburban South Australian home uses water accordingly:

Area Use (%)
Garden and outdoor 40
Bath and shower 20
Laundry 16
Kitchen 11
Toilet 11
Other 2


Your usage may vary according to your situation so again, the water calculator will help you work this out.

Generally, the best place to use tank harvested water for small properties first is internally as water demand inside is year round. If you’re a, avid gardener maybe a tank plumbed for the outdoors makes more sense. Use the calculator to give you a better idea and remember that in South Australia the variability of rainfall (heavy falls in winter and dry summers) means you will need a larger tank if you tank water throughout summer. This also means your tank may not fill frequently if water is constantly being drawn for the tank for inside use so a larger tank (say 20,000L) may never fully fill.

To calculate the right size tank for your circumstance use the “TANKULATOR” ( and get harvesting. After all, there’s not much free these days but with some good planning rainfall can be one of them!


Not enough roof surface? Chat to your neighbor about their roof run-off. Perhaps you could give them something in exchange? Either way it may be an easy way of increasing the size of your roof catchment.

Don’t under estimate the importance of water and wait for a crisis before you act, and take care of your local water ways.