How much water can a rainwater harvesting system save?

In a domestic house system, you can save up to 50% of your water consumption, although we feel it is more likely to be 40%. This will depend on peoples’ habits and usages but you can break it down as follows:

  • Toilet flushing = 25%
  • Garden and car washing = 5%
  • Clothes washing = 10%
  • Total = 40%

It will also depend on how efficient the system is, which is determined by the volume of water collected, the frequency of rainfall and the usage in the property. The tank size will also impact this. Let’s look at 2 examples to see how it can vary from property to property.

Example 1: standard house in low rainfall area

If we said a standard house has a roof area of 100m2 and is situated in an area of low rainfall (600mm per year) we can estimate how much water can be collected from the roof in one year with a simple calculation:

100 (m2) x 600 = 60000 litres of water. We deduct 20% to take into account that not all water will get in the tank, some will spill over the gutters in a heavy storm and some will evaporate in low rainfall. Therefore in one year, in this example we can say we would receive 48000L of water.

If the house had 4 people in it, using a total of 400L a day, this would be a demand of 146000 (of which we can safely use 40% with rainwater (58400L)

As you can see here, the rainwater from the roof, isn’t quite enough to meet the demands of the house and therefore some additional mains water will be needed. However, there is still a potential to save 48000L of water a year.

Example 2: Bungalow in similar area:

As opposed to the above example, a bungalow would have a larger roof area (say: 150m2), and possibly less bedrooms, let’s use 2 bedrooms for this example, with 2 people living there.

Water consumption may not be half of what the family of 4 use, but it will be close, so we will assume 250L/day for the household.

Again, let’s look at how much can be obtained from the roof: 150m2 x 600mm = 90000L, taking 20% off again we reach a total of 72000L Rainwater collected in the year.

With a total water consumption for the property of: 91250L (250L per day x 365 days), 40% of which (what we can replace with rainwater) would be 36500L.

In this example we are getting a lot more water than we need, we collect 72000L and only need 36500L.  This system would be very effective, very rarely needing to be topped up with mains water (only in long droughts).


As you can see from the above 2 examples, what can be saved from a rainwater harvesting system can vary dramatically depending on the individual property. However we can say that significant savings can be made in both examples.

Take home message:

With water becoming a more valued commodity, and long term forecasts starting to show the UK (especially the south east) as being a high water stress area, it’s the right time to think about re-using this precious resource, especially if you are building a new house.

What’s next?

Rainwater harvesting systems have the potential to save you a lot of money and make your home more sustainable. However, there are several different kinds of system available and it’s important that you find the right one for your requirements.


Last Update: September 5, 2019  

February 2, 2018   4315    Rainwater Harvesting FAQ  
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