What size of tank do you need?
The size of a rainwater holding tank must match the demand for water with its availability as closely as possible. The tank size chosen must be a balance between budget, storage capacity and the need to enable an overflow at least twice a year to flush out floating debris. The Environment Agency publication ‘Harvesting rainwater for domestic uses: an information guide’ provides an authoritative and independent means of calculating the optimum size of tank for household water use and suggests that 5% of the annual rainwater supply, or of the annual rainwater demand, is used to determine the optimum size of tank, using the lower figure of the two.
What is your annual rainwater supply?
TANK = AREA x DRAIN x 0.9 x RAIN x 5
where:
TANK = Optimal tank size (litres) for your application
AREA = Catchment area
Is the width and length of your roof in m. Remember you do not have to collect water from the entire roof. This can help to reduce installation costs.
DRAIN = Drainage coefficient
It is difficult to collect every drop of rain that falls on your roof. Light rainfall will only wet your roof and then evaporate and heavy rainfall can overflow gutters. A drainage coefficient is used to adjust the tank size calculation to allow for this.
ROOF = Roof type drainage coefficient
Pitched roof tiles 0.75-0.9
Flat roof smooth tiles 0.5
Flat roof with gravel layer 0.4 -0.5
0.9 = Filter efficiency
During initial rainfall some rainwater will skim over the filter, clearing away any debris. During heavy flow some rainfall will also bypass the filter to overflow ensuring that the filter is not overloaded. 90% of rainfall is caught, therefore the Balmoral filter coefficient is 0.9.
RAIN = Annual rainfall
This can vary dramatically over a relatively small area, so a reading (in mm) within 10 miles of the property is preferable. The local Environment Agency or the Met Office can supply rainfall data or you can use the regional guide figures we have provided below. In the ‘annual rainwater supply example’ we have used the average figure for the UK.
Region average rainfall (mm)
UK - 1125
S England - 781
N England - 944
Wales - 1433
Scotland - 1520
NI - 1111
EIRE E - 875 (source: Irish Met service)
EIRE W - 1125
5 = Constant percentage factor
What is the right tank for you?
Balmoral produces four tanks for domestic use so all you need to do is select the tank that best suits your needs. To help you we have calculated the correct size of tank for you based on your annual rainwater supply and your annual rainwater demand. Select the tank size from either table and if they indicate different tank sizes then you should choose the smaller of the two. These tables have been produced using the average annual rainfall for the region and assumes a pitched roof with a conservative drainage coefficient of 0.8. If your roof area is beyond the capacity of our largest tank or if the number of people indicates a smaller tank than your roof area will collect, then this is acceptable as it will mean that the tank will overflow more often, flushing out any floating debris.
