Calculate with your own values
Defaults are illustrative scenarios, not current tariffs or measured savings. Use one currency for every amount; no exchange-rate conversion. Units are litres, °C and kWh.
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Method and example
Heat = litres × temperature rise × 0.001163 kWh; input energy = heat ÷ system factor. Add variable water and wastewater cost. Example: 12 litres/minute × 8 minutes = 96 litres; 10 to 38 °C requires 3.126144 kWh of heat. With factor 1, energy at 0.35/kWh and water at 5/m³, cost is about 1.57 per shower; at 8 litres/minute, about 1.05.
Measure what comes out
If safe, collect water at your normal shower setting for a timed interval. Litres collected divided by seconds, multiplied by 60, gives litres per minute. Avoid burns and slippery floors. Repeat the measurement: supply pressure and settings affect it. A nominal product flow is not necessarily the flow in your bathroom.
Use the right energy factor
Heat demand equals litres × temperature rise × 0.001163 kWh. Divide by useful heat delivered per unit of input energy. A factor of 1 is an illustrative direct-heating case, not a measured system efficiency. Use an appropriate factor for the actual equipment; hot-water heat-pump performance is not necessarily its space-heating rating. Distribution and storage losses are excluded.
Check compatibility before reducing flow
Some instantaneous water heaters require minimum flow, and low-pressure arrangements may restrict which fittings can be used. Read both showerhead and heater instructions. Do not lower stored-water temperature or alter hygiene cycles because this calculator uses a lower mixed shower temperature; those are different settings.
Compare variable costs honestly
Enter only the variable water, wastewater and energy rates in one currency. Standing charges remain outside the calculation. Equal duration and mixed-water temperature are assumed. If a lower flow makes you shower longer, update the minutes too. Payback does not include installation, maintenance or financing.
Common questions
Does mixed shower temperature set the storage tank?
No. Mixed outlet temperature and safe hot-water storage are different. Follow system hygiene requirements.
Can a water-saving head fit any heater?
No. Minimum-flow and fitting requirements must be checked for the actual system.
Sources and limits
Technical sources below are in German, checked for the original guide on 4 October 2026. These adaptations do not establish local rental law, installation rules or tariffs. Follow local requirements and the instructions for your actual system.