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What Water Tank Size Should You Buy?

What Water Tank Size Should You Buy?

When looking for a rainwater tank you may not know what tank size is appropriate for your needs. How much water does your household consume? How much rainwater can you harvest you’re your roof? What tank size is reasonable for an average suburban household? These are some question this article sets out to answer.

Household Water Usage

One way to get a feel for how much water gets used in your home is to look over your water utility bills. Such will contain how much water you consume, which is obviously valuable to knowing how much water you need to be self-sufficient.

If you don’t want to dig out your water bill, I have written an article to help people calculate their household’s water usage. Tank Shop also have an online water usage calculator that you might find helpful.  Want to take a shortcut? Then just assume one person in your household will on average consume about 250 litres per day. Of course, if you are trying to adhere to water restrictions, you own water efficient appliances and taps, then this might be less. Otherwise, 250L/day per person is a reasonable figure.

When you have a good idea of water usage in your home, what then? How do you take this information to get to what water tank size you need? Such requires investigating home much rainwater falls in your area and can be harvested from roofs.

Rainwater Harvestable From Rooftops

According the Australian Bureau of Meteorology (BOM) the average mean for summer is 210mm. Many suburban areas around Australia often exceed 500mm per year.  If you want to view more precise rainfall amounts that will fall upon rooftops on your property, then you can look up figures on the Bureau of Meteorology website.

To calculate how much rain is harvestable requires knowing the area of your rooftops that will be harvesting rainwater into your tank. As a rule, 1 millimetre of rainwater falling on 1m2 of roof yields 1 litre of water.

Many double story and small homes will have a roof size of about 100m2. If we assume you will receive 200mm of rain falling on your house over the summer period, then that works out to 20,000 litres of rainwater (almost 5,000 gallons) you could potentially harvest. This is a conservative estimate.

When deciding what size tank to get it is important to factor in usage. You might be able to harvest a lot of rainwater, but if it isn’t going to be used, then you should invest in a tank according to your usage rather than amount harvestable. If you are an individual or couple, then a practical size (in our opinion) would be 10,000 to 15,000 litres.

Finally, you also should consider that that rain doesn’t fall evenly over a month. When it falls, it normally pours down over the course of a week, and then you may not receive any rain for a couple of weeks. That means hundreds of millimetres of rain could literally fall in just one week.

Council Water Tank Size Requirements

Other factors that can be overlooked when considering an appropriate rainwater tank size for your needs are requirements by your local council. Some councils may require you:

  • to retain rainwater for fight bushfires (if you live in a bushfire prone area)
  • detain rainwater to assist with stormwater drainage during heavy storms
  • minimum rainwater tank size capacities for new homes (often 5,000 litres).

Rainwater tank and new home requirements can often be found on the website of your local council. You should consult your local council before proceeding with a new rainwater tank purchase, especially if building a new home, to ensure you comply.

Recommended Tank Sizes For Households

It can be a complicated task working out what tank size to go with, but it doesn’t need to be. If you are looking to be largely self-sufficient, then the Australian Government (YourHome) recommends up to 20,000 litres for a household with 2 adults and 2 children.

Based upon this Tank Shop recommend a 5,000 gallon (22,700 litre) water tank for a family, and this tank size generally provides the best dollar value per litre.

If you live alone or with one other person, then about half that amount (10,000 litres to 15,000 litres) would be a good size to have your water supply needs met.

Or, if you are looking to just do your bit for the environment, help Australia’s waterways, or comply with new home requirements, then a 5000 litre tank is a popular choice and minimal requirement. This is also the highest capacity poly slimline tanks are found in, making them a popular choice for new homes.

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