Guide · Water heater
Electric water heater: which one to choose and how many litres
To choose an electric water heater, start from the showers you take one after another: for one or two people 50-80 litres is usually enough, for three or four go for 80-100. Then check the space on the wall, your meter power and the energy label. An instant electric heater is fine for a basin, almost never for a shower.
30 minutes Difficulty: EasyUpdated 25 October 2026
You do not need to touch anything to measure and take photos. Disconnecting the old water heater and fitting the new one is done by a technician: you are working with pressurised water and with electricity, and the safety valve must be fitted.
What you need
- Tape measure
- Latest electricity bill
- Phone for photos
Step by step
Tap a step to mark it as done.
How many litres of water heater do you need: 30, 50, 80 or 100
Capacity is chosen according to your habits. On retailers' data sheets a 30 litre unit is suggested for one person, a 50 litre for two and an 80 litre for three. With four people and showers close together you go up to 100 litres. A 30 litre unit also makes sense for a basin or a utility bathroom.
Temperature matters too. The water in the tank is hotter than the water you use and it mixes with cold water, so an 80 litre unit gives more than 80 litres of shower. Once emptied, though, it takes hours to be ready again: at 1,200-1,500 W recovery is slow, and two long showers in a row can be enough to finish it.
A water heater that is too big keeps litres hot that you do not use and loses heat all day. If you are undecided between two sizes, choose the larger one only if back-to-back showers are really frequent.
Instant electric water heater: when it makes sense
An instant electric heater has no tank and heats the water as it passes. That takes many kilowatts: an under-sink model of 3.5 kW gives, according to the manufacturer's data sheet, about 2 litres a minute with a 25 °C rise. That is enough to wash your hands, not for a shower.
Instant heaters designed for showers go beyond 20 kW and usually need a three-phase supply, which homes with a 3 kW meter do not have. With 3 kW of contracted power even a 3.5 kW appliance, on its own, goes over the limit.
It makes sense in a utility bathroom far from the main water heater, at the basin of a shop or an office, or at the sink of a technical room. For the shower at home, storage is almost always more practical.
Heat pump water heater: uses less, asks for more
A heat pump water heater takes heat from the air in the room and passes it to the water using a circuit similar to that of an air conditioner. It uses less electricity than a resistance boiler, but costs more and needs the right conditions.
The Ariston NUOS manual gives an idea of the requirements: at least 20 m³ of room if the air is not ducted outside, air between 10 and 37 °C, a condensate drain with a trap and no gas boiler or gas water heater in the same room. That is why it often goes in a large store cupboard, a laundry room or a technical room.
The GSE's Conto Termico 3.0, in force since the end of 2025, provides an incentive for replacing an electric or gas water heater with a heat pump one. Requirements and amounts change over time: check them on the GSE website before buying.
What it costs to replace one and what to do with the old one
In the Naples area, labour alone for replacing a water heater is around 120 €, appliance not included. That is an indicative market figure. It goes up if the connections have to be moved or sections of pipe redone.
If you switch from gas to electric, the gas point has to be closed off and made safe by a company authorised for gas systems, and you need an electrical line suited to the power of the new appliance. The old electric water heater is WEEE, electrical waste: it goes to the recycling centre, or the shop takes it back when you buy the new one.
When to stop and call a technician
- The new water heater is going on a plasterboard wall or one whose strength you do not know
- There is no dedicated socket or switch, or the meter already trips today with the oven and washing machine on
- You are replacing a gas water heater: the gas point has to be closed off by an authorised company
- The connections on the new model are in a different position and the pipes have to be modified
- You want a heat pump and you do not know whether the room is suitable
Frequently asked questions
Is a 50 or 80 litre electric water heater better?
A 50 litre unit is enough for one or two people who do not shower in the same few minutes. If showers are close together or there are three of you, an 80 litre unit saves you from running out of hot water.
Does an instant electric water heater use less?
It does not keep hot water waiting, so it does not lose heat, but while it works it draws a lot of power. For a shower you need power that a 3 kW meter cannot supply.
How much electricity does an electric water heater use?
The annual consumption in kWh is printed on the energy label and is calculated on the use given by the load profile. Real consumption depends on how much water you use and the temperature you set.
Is a heat pump water heater worth it?
It uses less electricity, but it costs more and needs a suitable room. It is worth it if you use a lot of hot water at home, if you have the space and, when you qualify, with the Conto Termico incentive.
Can I fit an electric water heater myself?
Better not. It has to be fixed to a wall that can carry the weight when full, connected to the water with the mandatory safety valve and powered by a suitable line. In Campania we replace it for you, with the price given before we start.
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