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What Size Tank Should I Buy?

So you have decided you would like a rainwater tank? Or you think you might like to have a rainwater tank – but you are not sure where to start or what to do?

The best solution for you and your home will be determined by what you want to use the water for, the look/style of your home, your personal tastes and your budget. So let’s get started.

What do you want to use the rainwater for?

Rainwater can be used for many applications including:

  • Laundry (washing machine)
  • Toilet flushing
  • Outdoor use
  • Pool / pond / spa top up
  • Garden irrigation
  • Hot water use
  • Firefighting
  • Cooling towers
  • Drinking water (check with your local health authority first)

Please note that allowed rainwater use various between the states and territories.

The average distribution of water consumption in Australian households is:

Water Use                               Percentage of Water

  • Garden                                         35-50%
  • Bathroom                                       20%
  • Hot Water                                     15-25%
  • Toilet flushing                               10-20%
  • Laundry                                        10-20%
  • Kitchen                                            5%

Waterplex Rainwater Tank Information Graphic

It makes sense to use rainwater in the areas around the home where there are the greatest savings for the lowest cost. Inside the house it is usually easiest to use rainwater for the laundry if the tank is being retrofitted. If the tank is part of a new build, then it can also be connected to other areas of the house.

How much rainwater can I collect and store?

The golden rule for rainwater harvesting is 1mm of rain on 1m² of roof will capture 1 litre of water. Applying this golden rule for your home allows you to calculate how much rainwater you might collect during a typical rain event.

For example, if your home has a roof area of 200m², then a 10mm rainfall event would allow you to collect 2000 litres of rainwater. A 50mm rainfall event would allow you to collect 10,000 litres. These figures assume that all your downpipes are connected to the tank. If only half the downpipes are connected then divide the number by 2.

If you live in a capital city in Australia, the median annual rainfall is around 1200mm which means a 200m² home would potentially harvest 240,000 litres in a year. Of course, you will not need this much storage all at once. The balance that needs to be achieved is trying to ensure you have enough storage to collect the maximum possible amount when it rains and that will last as long as possible between rainfall events.

What Size Tank do I need?

The size of tank you need and can install depends on:

  • your available space (what size tank will fit within it)
  • the size of your roof (how much water you can capture)
  • what you are using the water for (how much you need)
  • how much you can afford; and
  • you need to be happy with how it looks!

Did you know that 1000 litres of water measures a cubic metre (1m³) and weighs 1 tonne? So 10,000 litres of water measures 10m³ and weighs 10 tonnes. You need to have a space large enough to be able to store 10,000 litres (= 10 cubic metres) of water. For example, 10,000 litres would fit into a tank measuring 5m long x 1m wide x 2m high. You would also need to prepare the ground (usually with a concrete slab) to be able to take the weight of the water and the tank.

A general “rule of thumb” is to assume it rains every two weeks. So take your maximum potential annual rainwater collection and divide it by 26 (the number of fortnights in a year). Assuming 240,000 litres of potential rainwater collection divided by 26 fortnights then a tank of around 9,000 litres would be required.

How much rainwater do I need to collect for my planned use?

Let’s assume you are going to use the rainwater for your garden and laundry. And that you want to water your garden 4 times a week for a ½ hour and you have 7 laundry wash cycles a week (for a family of four). And let’s assume you have a standard water pump (say 30 litres per minute) and your top load washing machine uses 100 litres per cycle. Then your weekly water use will be:

Application Calculation Water Use in Litres
Washing Machine

7 cycles x 100 litres x 52 weeks 36,400

Watering Garden

4 cycles x 30 mins x 30 lts x 52 weeks 187,200
Total Requirement 223,600

Divide this water use amount by 26 fortnights and it comes close to the ideal tank storage requirement for a 200m² home assuming a 1200mm p.a. rainfall of 8,600 litres (223,600 litres / 26 fortnights).

What Sort of Tank Should I buy?

In general terms, you should buy the largest tank you can that fits within your budget and your space. That’s because you’ll always be able to use the water. At a more detailed level, you need to make sure the tank fits, looks good and is suitable for your requirements. Have a look at some of our steel tanks here to get a better idea.

Taking the above assumption into account and working with an approximate tank capacity of 8-10,000 litres, here are some considerations in choosing the right tank for you and your home:

  • Aesthetics

This might be really important to you. Do you hide the tank so it doesn’t visually impact your home? Or do you want to make a feature of your tank? Or perhaps you don’t care. If you want to hide your tank then underground, bladder tanks or under deck tanks might be best for you. If you want to make a feature of your tank then you might consider corrugated steel water tanks – that classic Australia rural look but designed for Australian urban blocks.

  • Available Space

Remember that you need 9m³ of space for a 9000L tank. It may be that you need more than one tank to reach your required capacity. So you need to find the right tank footprint(s) that fit your space and provide the right capacity.

  • Access

If the tank is prefabricated then you need to be able to get it to its final location without damaging it. If it is in a tight spot or access is limited then you might consider modular or flat-pack tanks or bladder tanks

  • Budget

If your primary consideration is price, then the cheapest possible tank is a round plastic one. You need to have the space for it, be able to get it to the right location and you need to be happy looking at it. To give you an idea on the size or a 10,000 litre poly tank, the diameter is approximately 3m and the wall height is close to 2m.

So now you should have some idea about where you want to use your harvested rainwater and how much you need to capture and store. And hopefully you will also have an idea on the type of tank you think you might like or be able to fit somewhere on your property. if you are looking for some ideas on the type of tank, click here.

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Why Waterplex

The Waterplex Group are specialists in producing innovative solutions for storing & harvesting water without taking up all of your valuable space around the home. Waterplex are a leading provider of water tanks in Australia and we have been part of the Australian water industry for over 10 years. We have built up a reliable national network of resellers and highly experienced installers that deliver a "supply and install" solution that is second to none.

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