Smart Reservoir · Practical economics

Start with the easy buildings.

Not every property is suitable, and each installation must be assessed individually. But we don't need every house. By starting where tanks and drainage are already easy to connect, we can learn, demonstrate value, and expand gradually.

Where would we start?

Three common starting points, ordered by likely effort—not a guarantee of cost.

Already has a rainwater tank

Add monitoring and a controlled outlet, where the existing plumbing permits it. Usually the most interesting software-first retrofit: the storage has already been paid for.

One accessible roof drain

Intercept the roof downpipe, add a tank, and reconnect through an approved outlet. Less new piping can make the installation economical—if there's space.

Larger roofs, shared systems

More connected roof area may justify a larger, shared installation. Fixed control and connection costs can be spread over more storage—but roof size alone doesn't guarantee savings.

And the less convenient sites?

No tank space, several distant downpipes, hard excavation, difficult sewer connections, roof-access constraints, permits, and unexpected work can make a project costly. It is reasonable to leave those for later.

A real infrastructure benchmark

Vienna's Gelbe Haide reservoir

10,000 m³Physical stormwater storage capacity
~€1,000/m³€10 million announced project investment ÷ 10,000 m³

Vienna announced an approximately €10 million investment in 2019, and the reservoir entered service in 2020. This is a historical project-budget benchmark, not a verified final invoice or an equal-hydraulic-performance comparison. It includes major construction and site-specific infrastructure. Original announcement ↗

Editable estimate · not a quote

What might one building cost?

Choose the existing infrastructure, tank size, and roof-cooling option. All prices are tentative gross EUR estimates for simple illustrative cases in Austria.

Does the building already have a suitable tank?

New tank capacity is part of the estimated investment.

150 m²
50 m²500 m²

Used to estimate collectible rainwater and the extent of roof-wetting equipment—not the cost of underground storage itself.

10 m³
2 m³30 m³

For an existing tank, set the volume that could realistically be controlled.

Connection / site complexity

Represents site work, access and connections. The difficult category can quickly defeat the low-cost hypothesis.

Add evaporative roof cooling?

A tentative extra pump or valve, filter, roof-distribution lines, fittings and labor. This is not a designed or approved roof installation.

Estimated storage + controls — New storage installation
Cost per m³ new capacity — Up-front cost / nominal tank volume
Selected storage scenario—
Gelbe Haide · 2019 budget ratio~€1,000/m³

Nominal-capacity ratios only. Different scope, year, operating conditions and sewer-system effects.

These are scenario assumptions, not a contractor quotation. A low cost per m³ is not proof of equivalent flood protection.
Optional roof cooling
Extra cooling installation€0Not included
Storage + cooling—Installed CAPEX estimate

The cooling option is separate from stormwater storage; no cooling benefit has yet been experimentally calibrated.

How much water could this roof collect?Assuming an empty tank
50 mm
Potential captured water—

80% collection efficiency assumed. This estimates roof runoff captured in available storage, not CSO overflow prevented, tank refill across events, or evaporated water.

How the numbers are calculated

These assumptions are editable in the page's JavaScript; they are not installation quotes.

  • New plastic tank: €360 per m³ plus €100 fixed hardware allowance, inspired by example Austrian retail listings of approximately €1,899 for 5 m³ and €3,622 for 10 m³. Actual ground type, tank system, transport and connections vary.
  • Storage control kit: €620 baseline including sensing, controlled outlet, basic electronics and commissioning allowance. Assumption, not an integrated product price.
  • Installation/site works: new tank €790 / €1,550 / €3,800 by complexity. Existing tank retrofit €260 / €620 / €1,400. All are rough placeholders until quotes arrive.
  • Rainwater roof cooling: €160 pump, €110 filtration/protection, €140 control and fittings, €2.50/m² distribution plus €120 manifold, and €380–€1,200 labor. Assumes a simple accessible roof and existing tank/pump compatibility.
  • Tap-water roof cooling: €170 valve and appropriate protection allowance, €80 filter allowance, €150 controls, €2.50/m² distribution plus €120 manifold, and €350–€1,100 labor. Additional water charges apply; a plumber must assess the actual backflow separation and legality.
  • Contingency: 15% on each storage/cooling subtotal. Gross EUR scenarios are illustrative; VAT applicability, design, permits, supervision, unexpected earthworks and long-term maintenance need individual treatment.
  • Water captured: min(tank m³, roof m² × rainfall mm × 0.80 ÷ 1000). Assumes storage fully empty before rainfall.

Real values must be established by technical inspection, hydraulic design and installer quotes. To estimate flood benefit, catchment and sewer-capacity data are still missing.

Public source examples and cost caveats

Installed storage is not automatically equivalent to the engineering performance of a municipal storm basin. Existing-tank retrofits enable managed storage rather than new capacity. Tap water consumes potable supply and may cause limescale, especially with hard water. Rooftop wetting requires roof-material compatibility, drainage, access and electrical safety assessments.

Start small. Measure. Improve.

We don't need to retrofit an entire city immediately. Start with favorable buildings, learn the true installation costs, confirm safe operating conditions, and expand from evidence—not assumptions.