Water Tank Size
Choosing your water tank capacity
When working out how big your tank should be, first you need to know your water usage. The optimum tank for you will hold enough water to supply this demand during the driest periods in your area.
As a guide, rural houses typically have multiple large tanks 24,000 litres and above. Melbourne metropolitan areas more often have special purpose tanks specifically for high water volume tasks like gardening, toilets and laundry. We step you through how to work out what is needed below. Remember that you can never hold too much water. If you have the space, it is always better to go bigger.
In metropolitan Melbourne locations where mains water can back up your system during the dry periods (there are automatic devices for this) it is not as critical to have your full demand on standby. A modestly sized tank will still make a huge difference in reducing your mains water consumption throughout the year.
Working out your demand
For whole of house applications, a typical estimate is to 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. Alternatively, if you are converting from mains water you can check your actual usage from past water bills.
If your tank is for a specific purpose, you can readily calculate the demand for that task only. For example:
- 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 mains water can become quite unreliable due to overwhelming demand elsewhere. It is vitally important to be aware of your own resources, and to ensure your capacity well exceeds the requirements of your plan.
- Other uses: Topping up the pool (typical water loss rates with cover on/off), washing pets (size of the tub, rinsing), hosing down wet areas (pump and hose nozzle flow rates), and so on.
You should now have a pretty good idea of what your tank will need to be supplying every month.
How effective is your catchment
The formula for water catchment goes like this: 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 the dimensions of that section of roof is 12m x 6m. This gives you 72 metres of effective catchment area. For every 1mm of rainfall, you will be harvesting 72 litres of water.
From a water collection perspective, this hard roof surface calculation is very efficient in that you are collecting from the very first drop of rain. It is interesting that ground water calculations for filling dams actually requires quite a large volume of rainfall to firstly soak all ground surfaces before water can begin to drain into the collection area.
Determine the rainfall patterns in your area
Rainfall data is readily available on the internet. In Victoria a good place to start is the Water Data link for Rainfall Levels on the Melbourne Water website. Monthly data should be sufficient for this exercise, but if you're curious then weekly can be a revealing exercise too.
| 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 each monthly (or weekly) rainfall in mm by your roof catchment calculation from Step 2.
| 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
The answer will start jumping out at you now. If your catchment area is large enough, it is quite possible to catch more than you are needing to use from Step 1. In this case your tank would only need to be as large as your monthly usage.
More commonly though, and especially with Australian rainfall patterns, your monthly usage will exceed what you are collecting in the dryer months. A higher volume tank will be required to retain water from previous months that had better rainfall.
A simple tank capacity calculation in this case is to multiply your monthly demand from Step 1 by the number of months you wish to hold as a buffer. One good month of rain will then replenish your water ready for the next dry period.