Household size isn't everything: when hot water is used

As shown in the table above, in addition to the number of people, you need to consider how frequently hot water is used when choosing tank capacity; if showers are split between morning and evening, the unit has more time to reheat the water. That is why a four-person household where showers are staggered could manage with a smaller water heater than one where everyone showers back to back before leaving the house. In the latter case, having a larger reserve helps prevent the hot water from running out halfway through the last shower. Furthermore, your choice should also factor in shower duration, water flow rate (the amount coming out of the tap), temperature and the appliance's power rating.

Calculate how many litres your electric water heater needs

To choose an electric water heater, you can calculate how much hot water you need, bearing in mind that the water stored in the tank is mixed with cold water before reaching the shower, resulting in a larger volume. You can estimate this using the following formula:

Water heater capacity = (Number of people × Litres per shower × Simultaneity factor) ÷ Mixing factor

Here is a simple breakdown of each element so you can see how it works:

• Number of people: how many people live at home.
• Litres per shower: how much water each person uses during a shower at the desired temperature.
• Simultaneity factor: adjusts the estimate according to how showers are spaced out. It depends on how much time the water heater has to recover its temperature and how much water is used in each block.
• Mixing factor: indicates how much water at showering temperature can be obtained for each litre of stored hot water. For example, if your water heater is set to 60 °C, cold tap water comes in at 12 °C and you want to shower at 40 °C, the factor would be 1.71. In other words, each litre of water stored at 60 °C yields approximately 1.71 litres of water at 40 °C.

Practical example: what water heater does a family of three need?

Imagine three people live in your home and each person uses around 40 litres of water per shower at a temperature of 40 °C. Let's also assume that all three showers are taken back to back, so we use a simultaneity factor of 1. The formula would look like this:

Capacity = (3 × 40 × 1) ÷ 1.71

Capacity = 70.17 litres

This theoretical calculation shows that, in this scenario, you need a water heater with a capacity of around 70 litres of water stored at 60 °C to obtain the 120 litres of water at 40 °C needed to cover all three showers.

Remember that when choosing a real-world water heater, you also need to factor in heat loss, shower flow rate (which we will help you calculate below), the duration of each use, and so on. The formula therefore serves as a useful guide, rather than an absolute guarantee.

How long does an electric water heater take to reheat water?

Knowing how long your storage tank takes to bring the water back up to temperature is essential for organising household routines, especially once the water starts running cold. The time needed to heat the entire volume of the tank depends on the power rating of its heating element. For a temperature rise from 15 °C to 65 °C with a 1,500 W heating element, approximate waiting times range from 55 minutes for a 30-litre model to nearly 3 hours for a 100-litre model.

Litres and power rating: what is the difference?

It is very common to confuse the capacity of the storage tank with the power rating of the electric heating element, even though they measure completely different things:

• Capacity (litres): refers to the internal volume of the tank and indicates the total amount of hot water available.
• Power rating (watts or kW): represents the speed at which the heating element converts electrical energy into heat. A higher power rating heats the water in less time, but does not add a single extra litre to the tank's capacity.

In other words, an 80-litre water heater fitted with a 1,200 W heating element and another 80-litre model with a 2,500 W element provide exactly the same amount of hot water for the shower, but the second one will take less than half the time to recover its temperature after being emptied.

Why you may need greater capacity in winter or in a cold area

Outdoor temperature and climate directly affect the unit's performance, especially in cold regions or during the winter months.

In inland or mountain areas, mains water enters the home at lower temperatures, whereas in coastal areas or during the summer its temperature is milder.

Because the incoming water is colder, the tap draws a higher proportion of hot water from the heater to reach the desired temperature (around 40 °C for a shower). As a result, the tank empties faster. This should therefore be factored in if you live in a cold climate, where increasing the calculated capacity by around 20% is recommended.

If you use a bath, do you need a larger water heater?

The answer is a resounding yes. Whereas a standard shower requires around 30 to 35 litres of mixed water, filling a full bath takes between 150 and 200 litres of warm water all at once.

So, if you want to run a bath using an electric water heater, you will need a storage tank of at least 150 to 200 litres, as an 80- or 100-litre model would run out before the bath is even full.

Does the water heater temperature change the amount of usable water?

Yes, the temperature selected on the appliance's thermostat directly affects how many litres of mixed water you can get:

• Water heater set to 50 °C: the water leaves the tank less hot, meaning you mix it with less cold water at the tap to regulate your shower. This causes the storage tank's reserve to run out faster.
• Water heater set to 65 °C: because the water is hotter, you need to add more cold water to balance the temperature, which increases the final volume of usable warm water.

However, bear in mind that keeping the water heater constantly above 65 °C causes it to lose more heat while on standby and encourages limescale build-up on the heating element. To strike the best balance between appliance lifespan and energy savings, it is advisable to set it between 55 °C and 60 °C.

How to calculate how much water you actually use in a shower

Follow these steps to find out the actual flow rate of your system:

1. Use a bucket or measuring container.
2. Turn on the shower at the pressure and temperature you normally use.
3. Time how many seconds it takes for the water to fill the container.

Example: if a 10-litre bucket fills up in 60 seconds, your shower head has a flow rate of 10 litres per minute. A 6-minute shower will use 60 litres of mixed water (equivalent to around 42 litres drawn from the storage tank at 60 °C).

What happens if you choose a water heater that is too small?

A unit that cannot keep up with household demand can lead to several drawbacks:

• Running out of hot water: the last person to shower may be left with cold water halfway through.
• Accelerated wear and tear: the electric heating element will run in continuous cycles to try to recover the temperature, increasing mechanical wear on the appliance.
• Higher electricity costs: the appliance will draw power continuously, which can push up your electricity bill.

So, is a bigger water heater always better?

Not necessarily. Installing an oversized storage tank can also have drawbacks:

• Standby heat loss: a large-volume tank loses heat through its casing even when no water is being used.
• Space and weight requirements: a 200-litre model weighs over 220 kg when full and stands nearly 1.5 metres tall, requiring load-bearing walls or special mounting brackets.

How does the size of the water heater affect your electricity consumption?

An electric water heater uses energy for two purposes: to heat the water and to maintain its temperature while the unit is on standby.

Assuming the same daily water consumption, a larger-volume water heater uses slightly more electricity due to standing heat losses through the tank's insulation. However, the actual cost difference between an 80 L and a 100 L model with efficient thermal insulation is barely a few cents a month.

Are an electric water heater and an electric boiler the same thing?

Confusing a boiler with a hot water cylinder or electric water heater is very common. In reality, they are not the same, as they are two different appliances with distinct functions and installations:

• Electric water heater: an appliance designed exclusively to heat and store water for use in showers, washbasins and sinks.
• Boiler: a central heating system that heats the water circulating through radiators or underfloor heating across the entire home and, depending on the model or whether it includes an external cylinder, also produces domestic hot water.

Whether you have a boiler or a water heater at home, check out Endesa's maintenance services: professional technical support for complete peace of mind at home and optimal maintenance.