Water demand garden/year --> Is a cistern worthwhile?

Hello everyone,

I am currently trying to estimate whether installing a cistern as part of our new build is worthwhile. We will have about 100 sqm of planted garden/lawn. According to various online calculators for cisterns, I need about 8000 - 11000 liters of water per year for this in our region (550 mm/sqm rainfall). For water and wastewater, you pay about 2 euros per cubic meter of water. According to this calculation, the costs for garden irrigation without a cistern would then be about 22 euros/year. If I now install a cistern with a capacity of 1000 liters, I am calculating costs for the cistern, pump, and roof drainage connection of between 1000 and 1500 euros. Assuming you would calculate with 1000 euros, it would theoretically take over 45 years for the cistern to pay off (without increasing prices and repairs). Am I making a mistake here?
Could you maybe share your water consumption for the garden? Perhaps the calculators online do not correspond to practice.

Thank you

I claim that you won’t get the cistern for 1500. With pump and excavation, it will be more. But regarding the payback period, you are right.

Civil engineering would be involved in filling the basement. Since it is an acquaintance who does the civil engineering for us, the costs for this can be neglected. A cistern over 1000 liters is available at almost every hardware store for 500 to 700 euros.

That rather depends on what you want to do with it. For a little plant watering, I would also use tap water instead of a cistern. With us, fresh water alone costs €1.58/m³, plus wastewater at €2.35/m³, plus basic fees/charges (which would incur anyway, regardless of garden watering). Costs can be reduced by using garden water (if such an offer is available in your region). We currently have about 5m³ stored in 1000l barrels in the yard. These can be quite well filled through the house roof, garage, and shed.

Are you really talking about wastewater charges at 2,xx€/m³? Well, I have moved around Germany quite a bit, but everywhere the wastewater charges were around 4,xx, nowadays rather 5,xx€. Nevertheless, we will not build a cistern but a garden water meter, only paying for the fresh water with it.

A cistern under 5000 liters is completely pointless or would never be less than 8000 liters. Ours was already empty in April this year! and currently I'm struggling to prevent it from running dry again in May. All this with a 250 m² garden.

Economically, a cistern is not viable. But I find it useful for other reasons.