That's the joke of the century anyway. You install heat pumps that only pay off after 15 years... Photovoltaics that could never pay off at the current moon prices. And that stuff has a lifespan of... 15 years? Maybe 20.
But with a cistern, which saves increasingly precious drinking water, many cubic meters every year... suddenly it has to pay off, and preferably already after 3 years?
A concrete cistern lasts... 40, 50 years?
The 3-4,000 EUR is very well-invested money.
Anyone who doesn't have a cistern should better not mention the words ecology, environmental protection, and climate change.
It takes an incredible amount of energy to turn wastewater into drinking water... just for the lawn?