Instant electric water heaters: the hidden cost of power capacity
An analysis shows that instant electric water heaters need up to 21 kW, forcing a higher contracted power capacity and a higher fixed bill without saving on consumption.
English · Original discussion in Spanish · Published
Instant electric water heaters: the hidden cost of power capacity
More than 16 years ago, a tenant with a butane water heater wondered about replacing it with an instant electric one. Manufacturers promised savings and efficiency. The reality, laid out in a technical analysis, shattered the myth: tankless heaters need an enormous amount of power that most Spanish homes simply cannot handle.
The power needed for a shower
The first sticking point was the thermodynamic calculation. To raise the water temperature by 30 °C at a flow rate of 10 litres per minute — normal for a shower — you need 21 kW of instantaneous power. That equals 91 amps on a 230-volt single-phase installation. Commercial low-power models rarely go below 3 kW, barely enough for a lukewarm trickle. Instant heaters that actually work (13-15 kW) require a three-phase supply and a connection few flats have. They are common in northern Europe, but homes there are wired with industrial-grade cabling.
The fixed bill nobody calculates
The second blow came with the fixed cost. A real case illustrates the problem: a tenant with a Junkers ED24-1S instant heater (400V, 24 kW, 35A) found himself forced into a contracted power capacity of 10.392 kW, paying 57.17 euros every two months in fixed charges alone. With a conventional electric storage water heater, the contracted capacity could be reduced to 3.3 kW, saving tens of euros a year on the fixed charge alone. Variable consumption, however, is practically identical: heating a litre of water takes the same energy, whether done instantly or slowly.
Safety and durability
When it comes to safety, storage water heaters win hands down. High-power instant heaters require an installation with impeccable earthing, a circuit breaker of at least 20 A and a working residual current device. Without earthing — common in older buildings — the risk of electrocution is real. Experience shows that a well-maintained conventional storage heater (with a replaceable sacrificial anode) can last more than six years, although limescale in hard-water areas shortens its life. Instant heaters, moreover, face wear from constant thermal stress on their heating elements.
What about butane? An overlooked option
For many, the most sensible solution remains butane: it requires no change in contracted capacity, the cost per kWh is lower and, in mild climates, a single bottle can last up to two years. The downside is having to haul bottles around and fine-tuning the temperature to avoid alternating between scalding and cold water.
Given the data, the conclusion is clear: an instant electric water heater is only viable if the home already has a three-phase supply and a very high contracted capacity. For most households, a storage heater or butane works out cheaper and safer.
Related discussions in the forum
More than 16 years ago, a tenant with a butane water heater wondered about replacing it with an instant electric one. Manufacturers promised savings and efficiency. The reality, laid out in a technical analysis, shattered the myth: tankless heaters need an enormous amount of power that most Spanish homes simply cannot handle.
The power needed for a shower
The first sticking point was the thermodynamic calculation. To raise the water temperature by 30 °C at a flow rate of 10 litres per minute — normal for a shower — you need 21 kW of instantaneous power. That equals 91 amps on a 230-volt single-phase installation. Commercial low-power models rarely go below 3 kW, barely enough for a lukewarm trickle. Instant heaters that actually work (13-15 kW) require a three-phase supply and a connection few flats have. They are common in northern Europe, but homes there are wired with industrial-grade cabling.
The fixed bill nobody calculates
The second blow came with the fixed cost. A real case illustrates the problem: a tenant with a Junkers ED24-1S instant heater (400V, 24 kW, 35A) found himself forced into a contracted power capacity of 10.392 kW, paying 57.17 euros every two months in fixed charges alone. With a conventional electric storage water heater, the contracted capacity could be reduced to 3.3 kW, saving tens of euros a year on the fixed charge alone. Variable consumption, however, is practically identical: heating a litre of water takes the same energy, whether done instantly or slowly.
Safety and durability
When it comes to safety, storage water heaters win hands down. High-power instant heaters require an installation with impeccable earthing, a circuit breaker of at least 20 A and a working residual current device. Without earthing — common in older buildings — the risk of electrocution is real. Experience shows that a well-maintained conventional storage heater (with a replaceable sacrificial anode) can last more than six years, although limescale in hard-water areas shortens its life. Instant heaters, moreover, face wear from constant thermal stress on their heating elements.
What about butane? An overlooked option
For many, the most sensible solution remains butane: it requires no change in contracted capacity, the cost per kWh is lower and, in mild climates, a single bottle can last up to two years. The downside is having to haul bottles around and fine-tuning the temperature to avoid alternating between scalding and cold water.
Given the data, the conclusion is clear: an instant electric water heater is only viable if the home already has a three-phase supply and a very high contracted capacity. For most households, a storage heater or butane works out cheaper and safer.
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Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (43 replies).
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