Is Rainwater Harvesting it right for me?
You should expect savings of up to 50% of your mains treated drinking water in houses and up to 80% of water in a business or commercial building by using a Rainwater Harvesting system.
The average house can harvest 64,000 litres of rain water a year from their roof…that’s 213 water butts!
The average person uses around 18,000 litres of water per annum just to flush toilets and wash clothes,
Rainwater harvesting is a logical choice.
How much do you need?
Rainwater harvesting systems can provide water for:
- Flushing Toilets
- Washing Machines
- Dish Washers
- Watering Gardens
- Car Washing
Please note that only Direct Fed Rainwater Harvesting systems can provide water for your washing machine and dishwasher.
Sizing a Rainwater Harvesting System
The volume of collectable rainwater (RWHsize) is equal to:
- your roof area size multiplied
- by the “run off” coefficient multiplied
- by the filter efficiency multiplied
- by the annual rainfall in your area.
The roof area is calculated by the length times the width.
The “run-off” coefficient is the amount of rainwater that hits the roof that can actually be collected.
A coefficient of “0” means no rainwater can be collected; whilst a coefficient of 1 means 100% can be collected.
Typical coefficients are:
- Pitched roof = 0.85
- Flat Roof (smooth surface) = 0.55
- Flat Roof (non-smooth surface) = 0.45
Filter efficiencies are nearly always supplied by the manufacturers. In the absence of a filter efficiency a figure of 0.8 (80%) should be used.
Monthly and Annual rainfall data can be obtained for free from the Met Office. www.metoffice.org.uk
The pitched roof area is 20m x 15m = 300 meters squared
The annual rainfall for the area is 600mm.
Therefore amount of rainfall we can collect is:
300 x 0.85 x 0.80 x 600 = 122,400 litres
122,400 / 1000 = 122.4 meters cubed of storage space as a maximum should be installed