What size water tank do I need for my household?

Most homeowners who are considering the option of installing a water tank system either harvest rainwater or to supplement the often-interrupted municipal water supply, battle with the question as to what size tank they will need.

This would depend on a variety of factors such as:

  • How many people are using water in the home daily?
  • What aspects of the running of the home can/will the water be used for?
  • How often would the tank be called into use?
  • How many days/weeks would the household usually need to rely on the alternate water supply from the tank?
  • How easy is it to replenish the tank if all the water has been used?

How to replenish the tank

Your water tank can be replenished from the existing municipal supply by connecting your tank to the existing municipal system, with a cut off valve to stop the flow to the tank once it is full. It the system is relying on rainwater harvested from a suitable rooftop, then calculations and predictions become a bit more complicated and unpredictable.

How to estimate tank size – is there any sort of calculator?

If you did connect the tank to your roof gutter system to capture rain runoff, a rule of thumb calculation for the quantity you roof would yield is:

1 square meter receiving 1mm of rain will yield 1 litre of water. So, a 100 square metre roof, receiving 10mm of rain, will yield 1000 litres of water, and so on.

This calculation can be done more accurately by using this research-backed calculator designed by Australia’s Clark Tanks (*although not a South African calculator, our countries face similar water shortage challenges, and for the most part the estimations gained from using such a calculator are useful regardless of country-of-origin). Click on the link at the end of this post to access the Clark Tanks calculator.

You could also work out what size tank would suit you by applying the questions above to the various aspects you would use the water for in any given day. Then, multiply this per-day total usage by the number of days you anticipate relying on your tank as an alternate water supply. This would give you some idea of the tank size that would suit your needs as a household.

It is also a rather cool calculator just to see how much water you are using and where the bulk of the usage is. This may even assist in saving water if you target the areas where the usage may appear excessive.




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