What Size Rainwater Tanks Do You Need – 2

Author: Brian March 10, 2021

How Do You Plan To Use The Water?

The most common uses for rainwater are for toilet flushing, garden watering and car washing.

Toilet Flushing

Basically a new duel flush Australian toilet will use 6 L for it’s main flush and 3L for it’s half flush.

Older toilets will use 9 L and 4.5L respectively.

Assuming two main flushes and 2 half flushes per person per day gives a monthly average usage between 540 – 810L per person.

For a family of four that’s between 2,160L and 3,240L of water.

If you are reading this in other country your numbers may be a bit different!

Garden Watering

This depends on the area that you want to water, whether you are watering an area or spot watering, and how often you want to water.

A reasonable summer months watering of an area would be the equivalent of 10mm a week, which would be 10L/m² of garden watered so for each m² of garden you need to store 45L/month. (To water say 200m² of garden weekly would use 2000L which would add up to 9,000L per month)

You can see from this how much water can be used on the garden compared with indoors.

Approximatly 3 times the volume used in the toilet!

Car Washing

Washing a car using a pressure washer would probably only take about 30L so the monthly total per car for washing once a week should be less than 150L.

Rainwater is Sole Water Supply

Every family is different so its hard to predict your water use.

We are fairly careful, don’t water the garden, only have 3 minute showers, use a front loading washing machine and only run the dishwasher when full.

All this means we use around 75L/person/day say 28kL/year.

The average usage in Melbourne with drought restrictions over the past few years has been about double that. . . so there must have been lots of people taking 30 minute showers!

Perhaps the best thing to do is monitor your water meter where you live now.

If your only supply is rainwater its worth checking with your Council if they have a minimum requirement.

Its also quite common for councils to require a bush fire reserve storage capacity for rural properties which can range from 10,000 litres to 20,000 litres depending on your State and location.

How Long Do You Want The Water to Last

Once you know the intended amount of useage, you can then find the total volume to supply a months usage.

For example: If a family of four was only using the water for toilet flushing and car washing a storage volume 3,500L (3.5kL) will be required for each months storage.

If the same family also wanted to also water 200m² of garden 12,000L (12 kL) would be required for a summer month. (For Winter months there would be no garden watering so the storage would easily last for several dry winter months..

As you can see the less garden watering you do the smaller the tank needs to be. . . or the longer you can survive without rain.