Electric Boiler Sizing: A Practical Guide for Installers

Electric boiler sizing is not simply a matter of matching boiler power to floor area.

A 200 m² house may need a very different boiler output from another 200 m² house. Insulation, outdoor design temperature, heat loss, heating system, domestic hot water demand, and electrical supply all affect the result.

At JNOD, we work with installers, distributors, contractors, and project buyers in different markets. In our experience, the most common sizing mistake is using one simple rule for every building.

So, how do you size an electric boiler correctly?

Quick Answer: How Do You Size an Electric Boiler?

Start with the building’s design heat loss.

A practical process is:

Heat Loss → Required Heating Output → Boiler Selection → Electrical Check

For a preliminary estimate, floor area can be useful. The final selection should also consider:

• Insulation
• Outdoor design temperature
• Windows and doors
• Ventilation and air leakage
• Radiators or underfloor heating
• Domestic hot water
• Voltage and phase

The basic rule is simple:

Choose the boiler for the building’s real heat demand, not floor area alone.

What Factors Affect Electric Boiler Size?

Several factors can change the required output.

Factor	Why It Matters
Heating area	Larger buildings usually have higher heat demand
Insulation	Poor insulation increases heat loss
Outdoor temperature	Colder design conditions require more output
Windows and doors	Openings can create significant heat loss
Ventilation	Fresh-air exchange adds heat loss
Heating system	Radiators and floor heating may use different water temperatures
DHW demand	Hot water requirements can affect system capacity and configuration
Electrical supply	Boiler output must match the available electrical capacity

This is why two buildings with the same floor area can need different boiler sizes.

How to Calculate Electric Boiler Output

The most reliable starting point is the building’s design heat loss.

Required Boiler Output should be selected based on the calculated design heat loss, with any appropriate design allowance determined by the project requirements and local design practice.

The design heat loss normally includes heat lost through the building envelope and heat lost through ventilation or air leakage.

The calculation should consider:

• Indoor design temperature
• Outdoor design temperature
• Walls, roof, floor, windows, and doors
• Building insulation
• Ventilation
• Building construction

For a real project, understanding how an electric boiler works can also help installers evaluate the system correctly.

Example: 180 m² House

Suppose an 180 m² house has a calculated design heat loss of 15.2 kW.

The starting point is therefore 15.2 kW of heating demand.

It would be too simple to calculate:

180 m² ÷ 10 = 18 kW

That 1 kW per 10 m² rule can be used for rough planning, but it is not a universal sizing method.

After checking the design conditions, heating system, electrical supply, and any appropriate design margin, the installer may consider a boiler in the relevant output range.

The important sequence is:

Calculate the load first. Select the boiler second.

Can You Size an Electric Boiler by Square Meter?

You can use floor area for a rough estimate, but it should not be the final sizing method. Rules such as 1 kW per 10 m² do not account for insulation, climate, windows, ventilation, or building construction.

A 100 m² well-insulated apartment in a mild climate may need much less output than a poorly insulated 100 m² house in a cold climate. Use floor area to start the estimate, then use heat loss to select the final boiler output.

How Climate and Insulation Affect Boiler Size

Climate is especially important in cold markets.

The key value is the outdoor design temperature used for the heating calculation.

A building in a mild climate may have a lower heating load than a similar building in a colder region.

Insulation has the same effect. Heat can be lost through:

• External walls
• Roof and ceiling
• Windows
• Doors
• Floors
• Ventilation and air leakage

For this reason, installers should use local climate data and the actual building specification when sizing the boiler.

Electric Boiler Sizing for Radiators

Electric boilers can work well with hydronic radiator systems, but boiler capacity is only part of the design.

Radiator output depends on:

• Water temperature
• Room temperature
• Radiator size
• Flow rate
• Return temperature

A boiler may have enough heating capacity, but a room can still stay cold if the radiator is too small or the hydraulic system is poorly balanced.

So installers should check both:

Boiler output + Heat emitter output

They must work together.

Electric Boiler Sizing for Underfloor Heating

Underfloor heating normally works at a lower water temperature than many traditional radiator systems.

This changes the system design, but it does not remove the need for a heat-loss calculation.

Important factors include:

• Floor area
• Pipe spacing
• Floor construction
• Insulation
• Required room temperature
• Flow temperature
• Heat output per m²

JNOD’s BO series electric system boilers can be used with underfloor heating and are offered in multiple output and electrical configurations.

Does Domestic Hot Water Change Boiler Sizing?

Yes.

A heating-only system and a system that also provides domestic hot water may need different configurations.

System Boiler + Hot Water Cylinder

This arrangement uses an external cylinder for domestic hot water. It can be useful when a project needs more stored hot water. JNOD electric system boilers can work with an external hot water cylinder for domestic hot water applications.

Electric Combi Boiler

A combi boiler combines heating and domestic hot water in one system. It can save installation space, but DHW demand and required water flow need to be checked. For compact residential applications, an electric combi boiler can combine space heating and domestic hot water in one unit.

JNOD offers both electric system boilers and electric combi boilers for different project requirements.

For projects placed on the EU market, electric resistance boilers are included within the Ecodesign scope for central heating boilers. Combination heaters are evaluated for both space heating and sanitary hot water performance. The European Commission’s Ecodesign framework for space heaters provides further regulatory information.

Does Voltage and Phase Affect Electric Boiler Sizing?

Absolutely.

An electric boiler is also a high electrical load.

For example:

12 kW ÷ 230 V ≈ 52 A

That is a substantial current for a single-phase supply.

For higher-output boilers, three-phase power may be more practical or necessary, depending on the product and local installation requirements.

Before selecting the final model, installers should check:

• Boiler output
• Supply voltage
• Single-phase or three-phase supply
• Main breaker capacity
• Cable requirements
• Other building loads
• Local electrical regulations

For retrofit projects, an electrical upgrade may also need to be considered when the existing service capacity is limited. JNOD’s current residential electric boiler range includes different power, phase, and voltage combinations, so the electrical configuration should be confirmed together with the required heating output.

For EU projects, electrical equipment within the applicable voltage range falls under the framework of the Low Voltage Directive 2014/35/EU.

Should You Oversize an Electric Boiler?

Not automatically.

A much larger boiler can increase:

• Equipment cost
• Electrical demand
• Installation requirements
• Infrastructure requirements

It also does not solve problems caused by poor insulation, undersized radiators, or incorrect hydraulic design.

A reasonable design margin may be appropriate, but oversizing should not replace proper heat-loss calculations.

More kW does not automatically mean better heating.

Common Electric Boiler Sizing Mistakes

Using Floor Area Alone

Floor area is useful for preliminary planning, but it is not enough for final selection.

Ignoring Outdoor Design Temperature

A boiler sized for a mild climate may not provide enough capacity in a colder design condition.

Forgetting Domestic Hot Water

DHW requirements can affect both capacity and boiler configuration.

Ignoring Heat Emitters

Radiator or floor-heating output must match the building heat demand.

Checking Electrical Supply Too Late

A high-power boiler can place a significant load on the building’s electrical system.

Oversizing Without a Clear Reason

A larger boiler does not fix an incorrectly designed heating system.

What Electric Boiler Size Should You Choose?

JNOD residential electric boilers currently cover common output levels from around 5.5 kW to 26 kW, depending on the product series and configuration.

These product ranges should not be treated as direct recommendations for specific floor areas.

For example, two 150 m² houses can require very different boiler outputs because of climate, insulation, and heat loss.

Use the following sequence:

1. Calculate heat loss

2. Check heating system requirements

3. Check DHW requirements

4. Check voltage and phase

5. Select the closest suitable boiler configuration

For larger projects, commercial electric boilers and multiple-unit systems may also be considered. JNOD’s commercial range includes higher-power three-phase models and supports multi-unit applications.

Electric Boiler Sizing Checklist

Before choosing the final model, check:

Building

• Heating area
• Insulation
• Windows and doors
• Ventilation
• Building type

Climate

• Outdoor design temperature
• Local winter conditions

Heating

• Radiators or underfloor heating
• Required water temperature
• Existing hydraulic system

Hot Water

• Instant DHW or storage
• Peak hot water demand

Electrical

• Voltage
• Phase
• Main supply
• Breaker capacity
• Cable requirements

Product

• Required kW
• Boiler configuration
• Certification
• Controls
• Service and spare parts

This checklist helps prevent the most common sizing problems before ordering.

How JNOD Supports Electric Boiler Selection

At JNOD, we recommend selecting the boiler from the complete application, not from floor area alone.

For a project inquiry, the most useful information is: Heating Area, Heat Load, Climate, Heating System, DHW, Voltage & Phase

Our electric boiler range covers system, combi, open-vented, and higher-power configurations for residential and commercial applications.

For distributors, installers, contractors, and OEM/ODM customers, this information helps us recommend a more suitable product configuration.

FAQ

conclusion

Good electric boiler sizing starts with the building’s heat demand.

Do not choose the boiler from floor area alone. Check heat loss, climate, heating emitters, domestic hot water, and electrical capacity before selecting the final model.

For installers, this approach reduces sizing mistakes.

For distributors, it also helps match the right boiler range to the local market.

The right electric boiler is not the one with the most kW. It is the one that matches the project.

Looking for the Right Electric Boiler?

For a project recommendation, send us:

• Heating area or calculated heat load
• Local design temperature
• Voltage and phase
• Radiator or underfloor heating
• DHW requirements
• Target market
• Estimated order quantity
• OEM/ODM requirements

Talk to JNOD about your electric boiler project.