How Many Litres
How much water do you need
Start by estimating your daily water usage. You need to secure this demand for the duration of the driest periods in your area - one month is a good starting point, so multiply your daily demand by about 30 days. If this is enough information to get you going then go straight to our handy tank chooser then enter this as your minimum litres.
As a guide, rural houses typically have multiple large tanks 24,000 litres and above. Melbourne metropolitan areas more often have smaller tanks for high demand tasks like gardening, toilets and laundry. Remember that you can never hold too much water. If you have the space it is always better to go bigger.
Metropolitan Melbourne locations with mains water available can use automatic mains switchover to supplement their stored water during extended dry periods. In this scenario, a modestly sized tank will still make a huge difference in reducing the bulk of your mains water consumption throughout the year.
Working out your daily demand
For whole of house applications, allow at least 200 litres per person per day. Most Victorians will be familiar with the much publicised Target 155 campaign (raising awareness to reduce daily usage to 155 litres) however when calculating water storage it is best to over estimate. If you are converting from mains water you can get your actual usage from past water bills.
If your tank will be used for a specific task, you can estimate demand from simple rules of thumb such as these:
- Gardens: For a smaller garden, 20 litres per minute from your hose (this is a very high flow) for 30 minutes each week works out to over 2400 litres per month. Remember that gardens prefer infrequent but thorough watering.
- Toilets: Usage depends on the number of people at your house and how frequently they are used. Some reasonable estimates can be made. Assuming 8 flushes a day from a 10 litre toilet cistern (older toilets will flush more, modern ones will flush less), this also adds to over 2400 litres per month.
- Laundry: Number of loads per month x litres per load. Top load washing machines can use around 120 litres per wash. Front load washing machines usually use about half of that.
- Car Washing: Intermittent spraying for up to 10 minutes. Anything up to 20 litres per minute of water depending on your hose and nozzle.
- Fire Fighting: Consider the flow rates of your fire fighting pump and hose with nozzle, and the length of time you need to have available to you to protect your property. Always follow your fire plan - in extreme conditions town water from the mains can become quite unreliable due to overwhelming demand from other properties. It is vitally important to be aware of your own resources, and to ensure your tank capacity well exceeds the requirements of your plan.
- Other uses: Topping up the pool (typical water loss rates with the cover on or off), washing pets (size of the tub, rinsing), hosing down wet areas (pump and hose nozzle flow rates), and so on.
Now let's make sure that your roof catchment will keep up with your daily demand.
How effective is your catchment
The water catchment formula is: 1 square metre of catchment x 1mm of rain = 1 litre of water
Let's say you collect from one side of your house and that section of roof is 12m x 6m. This will give you 72 metres of catchment area. So for every 1mm of rainfall, you will harvest 72 litres of water.
This hard roof surface calculation is simple and efficient because you collect from the very first drop of rain. Calculations for ground water collection into dams happens to be more complex because rain must first sufficiently soak all ground surfaces for water to start running off into the open collection area.
Now let's determine how much water will be available to collect in your area.
Rainfall in your area
In Victoria, look for 'Rainfall Levels' on the Melbourne Water website. Example data below is for illustrative purposes only.
| Month | Rainfall (mm) |
|---|---|
| Jan | 32 |
| Feb | 48 |
| Mar | 39 |
| Apr | 147 |
| May | 28 |
| Jun | 18 |
| Jul | 24 |
| Aug | 74 |
| Sep | 103 |
| Oct | 81 |
| Nov | 47 |
| Dec | 63 |
It starts to get really interesting now. Multiply the rainfall by your roof catchment area.
| Month | Rainfall (mm) | x 72 (Litres) |
|---|---|---|
| Jan | 32 | 2,304 |
| Feb | 48 | 3,456 |
| Mar | 39 | 2,808 |
| Apr | 147 | 10,584 |
| May | 28 | 2,016 |
| Jun | 18 | 1,296 |
| Jul | 24 | 1,728 |
| Aug | 74 | 5,328 |
| Sep | 103 | 7,416 |
| Oct | 81 | 5,832 |
| Nov | 47 | 3,384 |
| Dec | 63 | 4,536 |
Putting all of this together
If your catchment area is large enough, each month you will catch more than you are needing to use.
However it is possible that your monthly usage could exceed what can be collected in dryer months. This just means that a bigger tank is required to secure water from an earlier month with more rainfall. One good month of rain will then replenish your tank ready for the next extended dry period.
Now you can go ahead with entering your minimum litres into our handy tank chooser to select your tank.