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What Size Rainwater Tank Do I Need?

rainwater tank

The size of your rainwater tank is determined by a number of factors; some of which you can control and some of which you cannot. For instance, the dimensions and slope of your roof play a large part in deciding what size of rainwater tank that you need. However, you also need to take the average yearly rainfall for your location into account.

Then, when you have factored in your roof dimensions and average rainfall, you will need to decide how to best channel the rain that your tank gathers. Northern Territory sees consistently heavy tropical rains. So, when deciding on the size of your rainwater tank, you will need to account for the months when rainfall is much lower.

Roof Dimensions – a Rough Guide

In order to collect one litre of water, your rainwater tank requires a surface runoff of 1m­­2 based on one millimetre of rainfall. So let’s say you live in a three bedroom house with a roof area that is approximately 200m2,  with average rainfall of 1727.3, you can expect 345,400 litres for the year. Now, let’s assume that you live in Darwin where the wettest month of the year is traditionally January. For the same size of roof, you are looking at around 84,000 litres for that month.

However, having a rough estimate for your year average and monthly averages is only part of a bigger picture. The size of your home and how you plan on connecting your water tank will affect your yield (The amount of water that you use, and how quickly you use it). For instance, if you plan on using your rainwater tank to supply intern applications, you will have a much higher yield. Your yield is further increased if you have a large family living at home.

Rainwater Tank by Size and Design

Modern rainwater tanks are designed to take both monthly and annual average rainfall into account. As such you will find that tanks come in all shapes and sizes. However, you can also categorise rainwater tanks by capacity. The following is a simple guide which is used to categorise rainwater tanks according to how many litres of water the tank can hold:

• Small – Capacity lower than 2,000L

• Medium – Can hold 2,000L to 10,000L

• Large – Capacity greater than 10,000L

As mentioned, rainwater tanks come in various designs to account for average rainfall levels. However, when choosing a design, you should also consider the type of gutters and downpipes connecting to your roof. If you want to determine if you’re guttering and downpipes will provide the optimum level of catchment for your rainwater tank, you can consult with a plumber or qualified builder.

Purchasing your Rainwater Tank

It may seem like there is a lot to consider whenever installing a rainwater tank. However, you shouldn’t have to do all the work yourself. The dimensions of your roof, average rainfall, intended uses, and number of occupants will normally provide sufficient information for the supplier. If you have concerns that a supplier is trying to sell you a larger tank than you actually need, speak to your plumber before completing the purchase.

 It is also a good idea to have details of your monthly and yearly water use, so you can offset the cost of installing the rainwater tank against any potential savings on your water tariffs. Obviously the more information you have at your disposal, the more informed purchase you can make. In that respect, take advantage of any advice you can get from skilled tradesmen, such as plumbers, builders, roofers and building inspectors.