Domestic Rainwater Harvesting can save 50% to 80% of your mains water cost and typical payback is around year 7 – 10. However this totally depends on factors such as:
- Size of your roof area
- Annual rainfall
- The size of your system
- Your water demand
- Metered water price increase
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
Calculate your Savings
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.
Funding for Rainwater Harvesting Systems
There is currently no funding or grants available for domestic Rainwater Harvesting Arvesting systems. There are no financial incentives similar to the FiT or RHI for Rainwater Harvesting Systems. The savings are purely from reduced water bills.
Farmers may find funding for agricultural schemes through their local Regional Development Agency (RDA).
For businesses however, there is an Enhanced Capital Allowance (ECA) scheme which allows businesses to claim 100% of their first year capital allowances on investments in technologies included the ECA list of water efficient technologies and products.
Enhanced Capital Allowances enable companies to claim tax relief for investments in energy-efficient products that meet published energy efficient criteria. This helps to significantly reduce the payback period for energy-saving and carbon reducing projects and is designed to incentivise companies to buy energy efficient products. This scheme helps alleviate the initial capital outlay by allowing companies to claim back 100% of the cost in the first year, thereby giving the company a cash flow boost.
How Much Does it Cost
Domestic Rainwater Harvesting Systems, including full installation, typically cost from £1,000 for an above ground system to £3,000 for a large domestic sub-surface system, to £20,000 for a commercial sub-surface system.
Costs do vary and are dependent on the size of the system you choose to install and whether you choose to have your tanks sub-surface or above ground.
Commercial systems can cost a lot more depending on size and requirement, but have a much quicker ‘pay back’ period.
Commercial system prices can range from as little as £3,000 for an above ground garden system to £20,000.Costs