How to size your rainwater tank
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Installing a rainwater harvesting system can reduce mains water use and save you money, but how big does the tank need to be?
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How much rainwater you will collect depends on (in order of importance):
In sydney, typcally just half of a rainwater tank's volume provides 90% of the rainwater supply! Ecological Design can conduct a tailored rainwater analysis for any location but the rule of thumb size guide presented here was developed for Sydney rainfall pattens.
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 LUMI – Illuminated rainwater storage by Logan Studio
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Estimate the average daily rainwater runoff for the chosen roof collection area.
Rainwater roof runoff is calculated as = roof area (sq. meters) x rainfall (mm) Av daily runoff in litres = av. daily rainfall (mm) x roof area (m²) |
Next step is to estimate how much rainwater will be used on average per day in litres/day. eg.
20l/person/day - toilet 3 star WELS 150 l/wash - front loading w/machine 20 l/minute - garden hosing If the whole house will be supplied with rainwater, use the average daily usage from water bills. |
The following rule of thumb guide was developed for Sydney but can be used for other regions where rainfall is on fairly evenly distributed throughout the year.
Note also that doubling the tank size from that target will result in about a 10% higher yield. |
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= 30 times daily usage |
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= 15 times daily runoff |
 screenshot of ATA Tankulator
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For other regions of Australia probably the best online tool for assessing rainwater harvesting is the Alternative Technology Association's Tankulator.
To use the Tankulator, you will need to sign in with an email address, and then fill out a questionnaire page (which you can always return to and modify). The input needed is similar to above. A default breakdown between all the household water uses will be applied or you can customise them. Once complete, you can run the Tankulator for different tank sizes, uses and rainfall data. |
To find out how we can help reduce your energy bills call John 0418 262706 or email [email protected]
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