How much energy can you recover?
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Let's put that into numbers.
At 8 litres per minute, a shower uses: 480 litres per hour.
With water entering at 10°C and wastewater at 38°C, the available thermal energy associated with that 28°C temperature difference is approximately: 15.6 kWh of thermal energy per hour.
At a heat recovery efficiency of 70.6%, approximately: 11.0 kWh of thermal energy per hour can be recovered under these conditions.
Of course, nobody normally showers for an hour. So let's look at typical shower durations.
| Shower duration | Approx. recovered thermal energy* |
| 5 minutes | 0.92 kWh |
| 7.5 minutes | 1.38 kWh |
| 10 minutes | 1.84 kWh |
| 15 minutes | 2.76 kWh |
| 20 minutes | 3.68 kWh |
*Calculated from the stated 70.6% heat recovery efficiency at 8 L/min, with 10°C incoming water and 38°C wastewater entering the heat exchanger. Actual energy savings depend on the complete installation and hot-water system.
These numbers illustrate why shower heat recovery can make such a difference.
A single 7.5-minute shower under these conditions represents approximately 1.38 kWh of recovered thermal energy.
If one person takes one such shower every day, that corresponds to roughly:
500 kWh of recovered thermal energy per year.
For a household of four people, the same example would represent approximately:
2,000 kWh of recovered thermal energy per year.
That is energy that would otherwise have been discharged with the wastewater.
