How do I find the right storage tank size for my rainwater harvesting system?
Firstly is necessary to get an idea about the rainwater yield per year and the water demand.
Storage tank size can be selected using the following criteria:
- If the yield < requirement, then a storage volume of 3% of the average annual rainwater yield
- If the yield = requirement, then a storage volume of 5% of the average annual rainwater yield
- If the yield > requirement, then a storage volume of 5% of the average annual rainwater yield
Rainwater yield
For finding the Rainwater yield, the following calculation can be used:
| Rainwater yield = Roof area x Amount of precipitation x Runoff coefficient x Filter efficiency |
- Roof area: The total roof area in m².
- Amount of precipitation: The average annual amount of precipitation in mm/year can be found online as well as from the respective weather office.
- Runoff coefficient: The runoff coefficient determines the amount of precipitation in the total precipitation that runs off the roof.
The runoff coefficient of various roof coverings according to DIN 1989 Part 1 is shown in the table below:
| Roof Covering |
Discharge coefficient |
| Pitched hard roof |
0.8 |
| Flat roof / not gravelled |
0.6 / 0.8 |
| Green roof intensive / extensive |
0.3 / 0/5 |
Roof Covering Discharge Coefficient
Pitched hard roof 0.8
Flat roof/ not gravelled 0.6 / 0.8
Green roof intensive/ extensive 0.3 / 0.5
Table-1
• Filter efficiency: Let’s take 0.98 as a standard value.
Water Demand
Sample calculation of the daily non-potable water requirement for a 4 people household
Toilet flushing 30L per person x 4 120 L
Washing machine 10L/ person x 4 40 L
Garden irrigation 34 L
Total water requirement/ day 194 L
Annual Requirement 194 x 365 days 71 m3
Table-2
Sample Calculation
Step 1 - Find the Rainwater Yield
* Detached house with 120 m² roof area
* Annual precipitation = 750 mm/year
* Discharge coefficient = 0.8
* Filter efficiency = 0.98
Rainwater yield = 120 x 750 x 0.8 x 0.98 x 10-3 = 71 m³/year
Step 2 - Find the Water Demand
From the sample calculation shown in table 2, we know that the water demand is also 71 m³/year
As the rainwater yield = water demand, a storage tank of 5% of the average annual rainwater yield is required.
I.e., 5% of 71m3 / 71,000 L is 3,550L.
Step-3 As a precaution an additional 5% can be added to value we got in step 2
I.e., 3550 + 5%
A storage tank available with a capacity of about 3728L or more can be selected.