ᐅ Annual Garden Water Demand – Is a Rainwater Harvesting System Worth It?

Hello everyone,

I am currently trying to estimate whether installing a rainwater cistern is worthwhile for our new build. We will have about 100 sqm (1,076 sq ft) of garden/lawn to plant. According to various online cistern calculators, I need roughly 8,000 to 11,000 liters (2,113 to 2,906 gallons) of water per year for this area in our region, which receives about 550 mm/sqm (22 inches/sq ft) of rainfall. The price for water and wastewater is approximately 2 euros per cubic meter. Based on this calculation, the cost for garden irrigation without a cistern would be around 22 euros per year.

If I now install a cistern with a capacity of 1,000 liters (264 gallons), I expect costs for the cistern, pump, and roof drainage connection to be between 1,000 and 1,500 euros. Assuming a budget of 1,000 euros, it would theoretically take more than 45 years for the cistern to pay for itself (without considering rising prices or repairs).

Am I missing something here? Could you possibly share your garden water consumption? Maybe the online calculators don’t match real-life usage.

Thanks

I would say you won’t get the cistern for 1500. With the pump and excavation work, it will definitely cost more. However, you are right about the payback period.

Earthworks would be included when backfilling the basement. Since it is a friend who is handling the earthworks for us, the costs for this can be considered negligible. A cistern with a capacity of over 1000 liters (265 gallons) is available at almost every home improvement store for 500 to 700 euros.

That really depends on what you want to use it for.
For just watering a few plants, I would also use tap water instead of a rainwater harvesting system. Here, fresh water alone costs €1.58 per m³ (about 0.35 per cubic foot), plus wastewater charges of €2.35 per m³ (about 0.67 per cubic foot), in addition to fixed fees that would apply regardless of garden irrigation.
Costs can be reduced by using garden water (if such an option is available in your area).

We currently have about 5 m³ (177 cubic feet) stored in 1000-liter (264-gallon) barrels on the property. They can be filled quite well using rainwater from the house roof, garage, and shed.

Are you really talking about wastewater fees at €2.xx per cubic meter (m³)? I have moved around Germany quite a bit, and everywhere the wastewater fees have been around €4.xx, nowadays more like €5.xx.

Nevertheless, we will not install a rainwater tank but a garden water meter, so we only pay for the fresh water.

A cistern under 5000 liters (1320 gallons) is completely pointless and would never be less than 8000 liters (2113 gallons). Ours was already empty in April this year! And currently, I'm struggling to prevent it from running dry again in May. This is all with a 250 m² (2690 sq ft) garden.

A cistern is not economical. But I find it useful for other reasons.