This lesson gives a simple way to calculate the necessary rainwater storage requirement (assuming sufficient rainfall and catchment area)
Sizing a domestic RWH system: Demand-side approach
Usually, the main calculation when designing a domestic RWH system will be to size the water tank to give adequate storage capacity. The storage requirement will be determined by a number of interrelated factors.
- Local rainfall data and weather patterns
- Roof (or other) catchment area
- Runoff coefficient (this varies between 0.5 and 0.9 depending on roof material and slope)
- User numbers and consumption rates
A very simple method is to calculate the largest storage requirement based on the consumption rates and occupancy of the building.
As a simple example we can use the following typical data:
Consumption per capita per day, C = 20 litres
Number of people per household, n = 6
Longest average dry period = 25 days
Storage requirement T = C * n * 25 = 3,000 litres
This simple method assumes sufficient rainfall and catchment area which is adequate, and is therefore only applicable in areas where this is the situation. It is a method for acquiring rough estimates of tank size.