**Rainwater**

In some cases rainwater may be used as a means of reducing water consumption to achieve higher water efficiency performance levels.

Where rainwater is to be used, follow these instructions to calculate the contribution from rainwater (litres/person/day):

- Enter the collection area into column (a), the yield co-efficient and hydraulic filter efficiency into column (b) and the average annual rainfall into column (c). Press ‘calculate’ to calculate daily rainwater collection per person; this will populate column (d). This calculation is based on guidance from BS8515:2009.
- Enter the percentage of rainwater collected in column (e). This should be based upon the daily rainwater yield per person calculated in line with BS8515:2009 divided by the daily non-potable water demand per person. Daily non-potable water demand should be calculated using water use for WCs and washings machines only, as specified in the ‘Water Calculator’. Where the daily rainwater yield is greater than the daily non-potable water demand, enter 100% as the percentage collected (column (e)).
- Enter the number of occupants into column (f). This should be based on two occupants in the master bedroom and one occupant in each additional bedroom.
- Press ‘Calculate’ to calculate the volume of rainwater used.

Notes:

Where a communal rainwater system is to be provided supplying more than one home, this table can be used in the same way however the figures entered need to be apportioned down to an individual dwelling basis.

The above calculation is sufficient for evaluation the principles of the system in the proposed development however, for sizing of storage capacity and all other design and installation details, BS8515 should be followed.