Having an independent water supply on your property gives you the freedom to use water as you wish, without restriction.  It can also help you to save money on your water rates, and make the most of a free natural resource.

When designing your rainwater harvesting system, there are many factors to consider. This guide should help you to determine the size of the tank that you require.

Step 1 – Water Requirements

The first thing to do is to establish an estimate of your water consumption.  Work out how much water you are likely to use in your garden, filling a swimming pool, washing a car, flushing toilets and running a washing machine.

Make an estimate of this consumption as a weekly amount, measured in litres.

Step 2 – Rainfall Rates

Estimate the average weekly rainfall in your suburb.  Statistics are available from the BOM site. Keep in mind that the rainfall may vary greatly by seasons, and that you may have some very long dry periods in between rainfall depending on the local climate. 

Step 3 - Find a Place for Your Tank. 

Usually this will be against a wall of the house, but it can be away from the house as long as there is some gradient so the water can flow toward the tank.  Consider the location of the downpipes to make sure that there is a good portion of the roof area that will feed the downpipe.  Calculate this roof area in square meters.  A good rule of thumb is that at least 50m2 of roof area should be draining to the downpipe that feeds the tank. In some cases it may be possible to have 100m2 or even more directed to the water tank.

Tankworks unique made to measure water tanks can be built to fit in just about any space. Use the Size-a-lator to design your tank to get the best possible solution for your home.

Step 4 – How Much Water Will I Catch?

For every 1mm of rain that falls on 1m2 of roof area, you will capture 1L of water.  Generally, we assume that you will lose up to 20% in splash and first flush water diverter etc, so for 1mm of rain landing on 100m2 of roof, you will capture about 80L.

Thus if your average weekly rainfall is 20mm (as it is in Sydney), you would capture about 1600L per week from a 100m2 roof.  In Melbourne the average weekly rainfall is lower, about 12mm, so you would capture about 1000L per week from a 100m2 roof.

Step 5 - Sizing Your Tank

This step is a combination of the above information. You will generally want to store between two and four weeks of average water usage to allow for variation in the average weekly rainfall, however you will need more if your rainfall is particularly seasonal and you regularly experience more than four weeks without rain.

For example, if your estimate of water consumption is 1000L per week, and your average weekly capture rate is 1000L then we would recommend a tank of 2000L to 4000L, or more if your rainfall is highly seasonal.