Electric water heater: which volume and power to choose?

The quick answer

Here is the method in five points. Details and calculations follow.

• Allow about 20 to 25 litres of water at 60 °C per shower (assumption explained below), plus a small volume for the kitchen and basin.
• A bath needs far more: about 80 litres of water at 60 °C for a 150-litre bath mixed to 38 °C.
• Pick the available size just above your calculated need, not two sizes above.
• Power does not change the amount of hot water, only the recovery time: litres × temperature rise × 1.163 Wh ÷ power.
• Read the load profile and class on the label: only compare classes between units with the same profile.

Calculating volume with your own assumptions

Water stored at 60 °C is mixed with cold water at the tap. Calculation assumptions: cold water at 12 °C, a shower of 40 litres mixed to 38 °C. The hot share is (38 − 12) ÷ (60 − 12), about 54 %, so about 22 litres are drawn from the tank per shower. Add, for example, 10 litres per person per day for the basin and kitchen.

A 150-litre bath mixed to 38 °C likewise draws about 81 litres from the tank. These are assumptions: an economy shower head or a higher flow rate changes them considerably.

• Family of 4, one shower each: 4 × (22 + 10) ≈ 128 litres a day.
• Same family with a daily bath as well: about 128 + 81 ≈ 210 litres.
• Add a margin if several showers follow each other in the morning, because the tank does not recover instantly.

Power, recovery time and night tariff

The energy to supply is calculated from the heat capacity of water, about 1.163 Wh per litre per kelvin. Assumption: water from 12 °C to 60 °C (48 K), with no losses. That is about 5.6 kWh for 100 litres and 11.2 kWh for 200 litres. Recovery time is that energy divided by the element's power.

If your contract includes a night tariff or an exclusive night meter, the tank must be able to recharge fully within that window and hold enough for the whole day. Ask your supplier and your distribution network operator: the offer depends on the contract and the region. We give no tariffs here.

• 100 litres: about 2 h 45 at 2 kW, about 1 h 50 at 3 kW.
• 200 litres: about 5 h 35 at 2 kW, about 3 h 45 at 3 kW.
• In practice, add the tank's losses; check the power and the electrical circuit specified on the datasheet.

Wall-mounted or floor-standing, tank and element: what changes

The choice depends first on space and support, then on corrosion protection and ease of maintenance.

• <strong>Wall-mounted</strong>: saves floor space, but the fixing must carry the weight of the full tank; <strong>floor-standing</strong>: does not load the wall, useful for large volumes. Check the filled weight on the datasheet.
• <strong>Enamelled</strong> or <strong>stainless steel tank</strong>: the manufacturer states the protection provided (magnesium anode or not) and the warranty conditions; follow its manual for checking the anode.
• <strong>Immersed element</strong> (in contact with the water) or <strong>dry element</strong> (steatite, in a sleeve): the second is designed to be replaced without draining the tank; check this on the datasheet, especially if your water is hard.
• Leave the clearance stated in the manual to reach the element, the anode and the safety group.

Reading the label: load profile and ErP class

Water heaters fall under ecodesign (Regulation EU No 814/2013, appliances up to 400 kW and storage tanks up to 2,000 litres) and energy labelling (Delegated Regulation EU No 812/2013, appliances up to 70 kW and storage tanks up to 500 litres). Regulation 812/2013 provides load profiles from 3XS to XXL for the label, and an energy class from A to G, with classes A+, A++ and A+++ added at the top.

The declared profile corresponds to a 24-hour test draw-off cycle. The reference energies in Annex VII are 5.845 kWh for profile M and 11.655 kWh for profile L. The class is calculated per profile: so compare only units with the same profile.

• Choose a profile that matches your use: an L or XL profile assumes more draw-off than an M, bath included.
• Note the annual electricity consumption and the volume shown on the label.
• Also note the sound power level if the tank is installed near a living space.

Safety, maintenance and temperature setting

A closed water heater is connected to the mains under pressure: follow the manufacturer's manual strictly for the safety group, any pressure reducer, the discharge connection and anode maintenance. Have the installation done by a qualified professional, and switch off the power before any intervention.

On temperature, here is what Buildwise sources say. Legionella grows steadily between 20 and 50 °C, with a peak between 35 and 40 °C; stagnant water makes it worse. The Belgian best available techniques specify production at 60 °C, and trials showed that production at 45 °C is hard to control. Those texts concern sanitary installations; for your appliance, follow the manual and avoid any zone of lukewarm, stagnant water.

• Never plug the safety group's discharge, even if it drips: have the installation checked.
• Use all draw-off points regularly to avoid stagnation.
• Hot water can scald: a thermostatic mixing valve limits the temperature at the taps.
• Respect the applicable regional rules (for example on building energy performance) for a new build or renovation.

Common mistakes

• Choosing the volume from the number of occupants without counting baths.
• Taking two sizes up "just in case", then paying for the standing losses.
• Believing that higher power gives more hot water: it only speeds up recovery.
• Comparing energy classes of units with different profiles.
• Fixing a full tank to a support that has not been checked.
• Forgetting access to the anode, element and safety group.

What to check before buying

Take each model's datasheet and energy label and note the following points before comparing.

• Usable volume, empty and filled weight, dimensions and clearances needed.
• Power, supply voltage and the electrical protection required.
• Declared load profile, energy class and annual consumption.
• Standing losses and sound power level.
• Tank material, corrosion protection and element type (immersed or dry).
• Whether a safety group is supplied, and the maximum working pressure stated.
• Warranty conditions, notably the maintenance the manufacturer requires.

Frequently asked questions