# Economically calculate the savings potential of a cistern

In order to calculate the most advantageous size of a cistern, some external influences have to be combined with individual consumption values ​​and local conditions. Regional average rainfall, roof type and shape as well as the size of the land and the intended use of water are included.

### 1. Determine ongoing water costs

To decide for or against a cistern, often the previous calculation is crucial. In addition to the ecological aspect, the economic factor can be decisively optimized by the most precise possible procedure.

Who plans a cistern, has first and foremost the saving of tap water and sewage charges in mind. Therefore, a survey on cubic meter prices and the distribution key on consumption is the first step. Furthermore, according to the local regulations, the split sewage charges must be calculated, which also requires the plan of all sealed areas on the property.

### 2. Determine consumption quantities

After collecting the current water costs, the next step is to determine the quantities of fuel that will be replaced by the cistern in the future. For this, the type of cistern and the connection options are defined. The following quantities of water can be deducted from the total consumption calculated:

• Garden irrigation taking into account the area and watering intensity
• Rinse water consumption when supplying the toilet flush to people transferred
• Washing water consumption for the washing machine according to consumption data

### 3. Check water yield

The result of the total consumption of water consumption shows both the cost saving potential and the necessary amount of rainwater and suitable cistern size. The next step is to determine if this amount of water is available to make the cistern worthwhile.

Based on the meteorological data, the average annual rainfall must be known. Added to this are the roof area and other areas where the rain is directed into the cistern. In addition to the roof pitch, the cover also plays a role in the calculation. In addition, any losses due to filters or diverters must be taken into account.

### 4. Make-up with drinking or well water

If the average rainfall in the dry summer months is insufficient, a drinking water supply can be installed, which must be deducted pro rata from the savings potential determined first. The feed-in can also take place at no cost through a groundwater well, but the investment costs for the well have to be included in the total bill.

### 5. Vorhaltemenge and infiltration

As an individual "soft" factor, a preferred set amount can be included in the calculation. If regionally highly divergent rainfall is known, consideration should be given to the connection and the additional cost of a percolation system as support for the cistern overflow.

### 6. Offsetting investment costs and savings

With the determined values, the capacity of the cistern can now be determined. Experience shows that a volume increase of between ten and twenty percent is advisable. Based on the size of the cistern, the initial costs are calculated, which are offset against the savings potential determined first.

### Tips & Tricks

There are not really too big cisterns. A problem can arise for you from the pollution by low water level. This problem is technically solvable.