The selection of the capacity of rainwater tanks depends on:

• annual average rainfall for a given location,
• surface drained from rainwater,
• runoff ratio,
• type of building,
• type and number of water points. 

Below, we will try to discuss each aspect of estimating the rainwater profit.

Average annual rainfall

The precipitation value for a given region can be read from the precipitation map on the website of the Institute of Meteorology and Water Management. It is a list of precipitation counted from the decade in the 1990s. Unfortunately, currently the climate is tending to unpredictable changes and they are quite irregular in rainfall, most often causing heavy rains. Therefore, when designing the capacity of the tank in this regard, it is good to add about 20% of additional security in order to take over the excess rainfall.

Area of rainwater catchment

It is the size of the roof area, terrace, pavement and other surface from which we will collect rainwater into the tank. Due to the fact that the rain falls from above, it is calculated vertically on the basis of a simple formula:

F = a∙b  [m²]

Where:
a - building length, m
b - building width, m

When determining the drained area for the purposes of calculating the capacity of the rainwater tank, we do not take into account, among others slopes, angles or roof slopes. We count the flat area.

The coefficient of runoff of the catchment material

When determining the drained surface, standardized coefficients of runoff were adopted, defining the differences between the amount of rainfall and the actual amount of rainwater runoff, taking into account the slope and roofing.

Type of building development

Due to the type of buildings, different values of water demand for individual purposes are assumed.

There are 4 types of objects:

• residential building
• tourist accommodation building (e.g. hotel)
• commercial and service building (e.g. office)
• public utility building (e.g. school)

Type and number of water points and estimated consumption

Examples of standards for the demand for individual purposes, which, as can be seen in each of the buildings, has a different nature, which results in the amount of water consumption [l / year]:

Other demand data:
Car washing: [700l / year]
Watering the garden with a garden hose: [100-300l / year / m²]

An example of calculating the capacity of a rainwater tank

          Assumptions for the project:

• The geographical location of the facility: Wrocław
• Building length: 18 m
• Building width: 14 m
• Roofing material: ceramic roofing tiles
• Roof pitch: inclined roof
• Number of users: 4 people
• Type of building: dwelling building
• The points of use: toilet, urinal, washing machine, surface cleaning, car washing, garden watering
  

1) Determination of the average annual rainfall for the location of the object

On the precipitation map, we check the average value of precipitation for the city of Wrocław, which is 550 mm.

2) Calculations of the roof catchment area

Using the formula for the surface area, we calculate the area of the roof catchment from which we will obtain rainwater.

3) Determination of the runoff coefficient

Taking into account the slope and the roof material, we read the appropriate value of the runoff coefficient from the table. For a pitched roof covered with ceramic tile, the value is 0.9.

4) Rainwater yield

Q_m = F ∙ ϕ ∙ q [l/year]

Where:
F – roof area
ϕ - runoff coefficient
q - value of the average annual precipitation (for the city of Wrocław)

Q_m = 252 ∙ 0,9 ∙ 551 = 124 966,8 l/year

5) Calculating the water requirement (residential building)

 The calculations were made according to the formula:

Water demand for specific purposes ∙ number of people = demand for domestic water [l / year]

In order to calculate the domestic water demand for watering the garden, it is necessary to calculate the area of the watered area beforehand.

Domestic water demand calculations are presented in the table.

124 966,8 l / year > 71 660 l / year

The condition that the annual rainwater yield is greater than the rainwater demand has been met.

In this case, we obtain the result of the annual estimate of rainwater yield and its consumption. To obtain the appropriate capacity of the tank, the result should be divided by 365 days and multiplied by the water storage factor, which is normally 21 days.

In our example, the water yield indicates that the tank capacity should be: 124 966.8 l / year: 365 days ∙ 21 = 7189.77 l

However, our consumption indicates the volume that we will use: 71 660.00l / year: 365 days ∙ 21 = 7189.77 l = 4122.93 l

When interpreting our result and increasing it by 20% (as an additional safe excess), the higher result should always be interpolated. It is wise to choose a rainwater tank that will have a capacity of at least 5000l.