Hot water from the tank is not the same as mixed shower volume
A shower delivering 40 liters of warm water does not necessarily draw 40 liters from the hot supply. Cold water mixed at the valve contributes part of the volume.
Calculate from the mixed stream
In a fictional example, 40 L reaches the fixture at 38°C and cold inlet water is 13°C. Ignoring losses, delivered heat is 40 × (38 − 13) × 0.001163 = 1.163 kWh. At 100% conversion and USD 0.25/kWh, that is USD 0.29075, or USD 0.29. It is not the complete heater bill.
Check the same result from the hot fraction
If hot supply reaches the mixing valve at 63°C, its fraction is (38 − 13) ÷ (63 − 13) = 0.5. The shower uses 20 L hot and 20 L cold. Heating the hot share requires 20 × 50 × 0.001163 = 1.163 kWh, the same ideal balance. Using the full 40 L with the 50°C rise would double the result.
Choose one consistent calculator entry
Enter 40 L and a 25°C rise, or 20 L and a 50°C rise. Do not add both results. Apply efficiency or COP to obtain purchased energy. Water charges use all metered volume, so price the full 40 L separately if the energy calculation uses only the 20 L hot share.
Pipes lose heat and water may run before the fixture warms up. Add warm-up volume only if it is absent from your measured total. This balance assumes steady temperatures and similar water properties. Changing inlet temperature changes the hot fraction. The example temperatures illustrate arithmetic and do not recommend tank or tap settings.
Worked example: Price one daily shower at its mixed temperature
Related reading: What it costs to heat water
Reference: ASHRAE Handbook: service water heating.