What you need to know
- Convert minutes to hours before multiplying by kW.
- Multiply by the number of showers and days.
- Water cost is separate from electricity cost.
Use your own measurements
Use the linked BricoDato resource to check these inputs in your own situation. An instantaneous electric shower can have a high rated power but is used for relatively few minutes.
The basic formula
Energy per shower is heater kW × shower minutes ÷ 60. Multiply that by the electricity price to get energy cost per shower.
A short time change matters
At 7.5 kW, one extra minute is about 0.125 kWh. Across several people and every day, small time differences add up.
Water flow also matters
Electrical power affects how much water can be heated to the desired temperature. Flow rate and incoming-water temperature influence comfort.
Electrical safety is the priority
These are high-power fixed appliances. Cable size, protection, earthing and local electrical requirements are not DIY guesswork.
What does a shorter shower change?
An instantaneous electric shower can have a high rated power but is used for relatively few minutes. Energy depends on its true operating power, minutes per shower and frequency.
Example to adapt
At an illustrative constant 7 kW draw, a 10-minute shower uses 7 × (10 ÷ 60) = 1.17 kWh. At €0.20/kWh the energy cost is about €0.23. Cutting the duration to 6 minutes uses 0.70 kWh, or €0.14. The comparison ignores water cost and any difference in power during temperature regulation.
| Duration at 7 kW | Electrical energy | Example energy cost |
|---|---|---|
| 6 min | 0.70 kWh | €0.14 |
| 10 min | 1.17 kWh | €0.23 |
| 15 min | 1.75 kWh | €0.35 |
Checks before acting
- Confirm whether the shower actually uses the maximum listed kilowatts.
- Use minutes of active heating, not total time in the bathroom.
- Include water use separately if comparing household total cost.
- Only a qualified electrician should assess fixed wiring, protective devices and safe shower installation.
Final check
Calculate from electrical input power and the minutes for which heating actually runs. A hypothetical 7 kW shower used for eight minutes needs 7 × 8 ÷ 60 ≈ 0.93 kWh if operating at full power; at €0.22/kWh that is about €0.21 in energy alone. Water and standing charges are separate. Never treat this arithmetic as evidence that the fixed circuit, RCD and bathroom installation are adequate.
- Use the rating-plate kW.
- Time a normal shower.
- Keep water cost separate.
- Have fixed high-power wiring checked by a qualified professional.
Keep a small job sheet
Record model, rated kW, typical shower duration, frequency and tariff period.
Turn the guide into a decision
Record the exact model, rated kW, use duration and tariff before using the calculator. High-power electrical work near water must be assessed by a qualified electrician; a lower estimated shower bill is not a reason to bypass safety protection.