What Size Electric Boiler Do I Need? A UK Home Sizing Guide (6kW to 27kW Explained)

One of the most common questions we get at ECA is: "What size electric boiler do I actually need?" It's a fair question — and getting the answer wrong in either direction costs you money. Too small and your home never gets properly warm. Too large and you're overpaying upfront for capacity you'll never use.

This guide walks you through everything you need to know to choose the right output for your home, from a small flat on single-phase power to a large family house with multiple bathrooms.

Why Electric Boiler Sizing Is Different from Gas

With gas boilers, sizing is mostly about your home's heat loss — how quickly it bleeds warmth through walls, windows, and the roof. Electric boilers follow the same principle, but there's an extra factor that gas homeowners never have to think about: your electrical supply.

Some homes are on single-phase power (standard domestic supply). Others, particularly larger or newer properties, have three-phase supply. This matters enormously because larger electric boilers draw more current than a single-phase supply can safely deliver.

We'll cover both below.

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The ECA Arceus Range: 6kW to 27kW

The ECA Arceus electric boiler is available in seven output ratings:

A quick note on model types: MT is the combi version (heating and instant hot water), CH is heat-only, and ST is a system boiler. If you're not sure which type suits your home, read our companion guide: Combi, System, or Heat Only: Which Electric Boiler Is Right for Your Home?

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Step One: Check Your Electrical Supply

Before anything else, confirm what power supply your property has.

Single-phase (standard domestic): Most UK homes have this. You can run the Arceus on single phase up to 15kW. That covers a significant portion of UK homes comfortably.

Three-phase: Required for 18kW, 24kW, and 27kW models. If you're not sure whether your property has three-phase, check with your electricity network operator or have a qualified electrician assess your supply. Upgrading from single to three-phase is possible but adds cost and lead time, so it's worth factoring in from the start.

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Step Two: Understand What the kW Number Actually Means

The kilowatt (kW) figure is the boiler's output — the rate at which it delivers heat into your home. Higher output means more rooms, larger rooms, or colder properties can be heated effectively.

A rough rule of thumb used across the industry:

• Well-insulated home: approximately 1–1.5 kW per room
• Average insulation: approximately 1.5–2 kW per room
• Older or poorly insulated property: 2 kW+ per room

These are starting points only. A proper heat loss calculation by a qualified engineer will give you the precise figure for your specific property.

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Step Three: Match Output to Home Size

Here's a practical overview of how the Arceus range maps to typical UK properties:

6 kW — Small flats and studio apartments

Best suited to a studio or one-bedroom flat with good insulation. Also works well as a dedicated central heating boiler in a small property that uses a separate immersion heater or electric shower for hot water. Note that 6kW and 9kW models are only available as CH (heat only) or ST (system) variants — they do not support instant domestic hot water.

9 kW — One to two-bedroom flats and small terraced houses

A step up from the 6kW, the 9kW handles a small terraced house or a two-bedroom flat with average insulation. Again, this output is available in CH and ST configurations only.

12 kW — Two to three-bedroom homes

The 12kW is the entry point for the combi (MT) variant, meaning it can deliver both central heating and instant domestic hot water from a single unit. It suits a two or three-bedroom home with reasonable insulation and one bathroom. Maximum hot water flow at this output is around 7 litres per minute.

15 kW — Three-bedroom homes, one to two bathrooms

The most popular output for a typical UK three-bedroom semi or terrace. It's the highest model available on single-phase supply, making it accessible to the widest range of homes without electrical upgrade work. Hot water flow increases to around 8 litres per minute.

18 kW — Three to four-bedroom homes, two bathrooms

Requires three-phase supply. Well suited to a larger family home where hot water demand is higher, particularly if two showers might be used in relatively quick succession. Delivers up to approximately 9 litres per minute of hot water.

24 kW — Four to five-bedroom homes, multiple bathrooms

A serious output for a larger property. At this level you get up to 10 litres per minute of hot water, and the boiler can cope comfortably with higher heat loss in older or larger homes.

27 kW — Large homes, light commercial, or high demand properties

The flagship of the Arceus range. Also delivers 10 litres per minute of hot water but with greater thermal capacity for central heating — useful in properties with many radiators, high ceilings, or where the heat distribution system is more extensive.

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The Hot Water Factor

If you're choosing a combi model (MT), the hot water output matters as much as the heating output. The Arceus delivers domestic hot water at a temperature range of 30–65°C, and the flow rate you can expect increases with output:

For reference, a standard shower uses roughly 8–12 litres per minute. If you have a high-flow shower head or frequently run hot water in multiple locations, size up.

It's also worth noting that for stable hot water performance, flow rates below 2.5 litres per minute aren't recommended — so a combi electric boiler works best with modern taps and showerheads that maintain a consistent flow.

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Underfloor Heating? Size Differently

If your property has underfloor heating (UFH) rather than radiators, the sizing calculation changes. Underfloor systems operate at much lower flow temperatures — the Arceus supports UFH at 30–45°C, compared to up to 80°C for radiators. Lower flow temperatures mean lower heat output per unit of time, which in turn means UFH systems often need a slightly different approach to sizing.

ECA's technical team can advise on UFH-specific sizing. Don't assume the same boiler that would heat your radiators will automatically be the right choice for underfloor.

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Insulation: The Factor That Changes Everything

Two homes of identical size can need very different boiler outputs depending on how well they're insulated. The Arceus manual highlights this directly: a home with double-glazed windows and insulated walls will retain heat far more effectively than a single-glazed Victorian terrace, dramatically reducing the output you need.

Before investing in a larger boiler, it's worth considering whether improving insulation could let you achieve the same comfort level with a lower-output unit — reducing both upfront cost and ongoing running costs.

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Getting It Right: What to Do Before You Buy

1. Check your electrical supply — confirm single or three-phase with an electrician if unsure.
2. Count your radiators and bathrooms — this gives a useful initial size indicator.
3. Consider your insulation level — honestly assess how heat-retentive your home is.
4. Ask for a heat loss calculation — a qualified heating engineer can calculate this precisely for your property.
5. Factor in hot water demand — if you're choosing a combi, think about peak morning demand.

If in doubt, the ECA technical team is available on 0800 640 9988 and can help you work through the right choice for your specific situation.

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Summary

The right size electric boiler makes your home comfortable, protects you from unnecessary running costs, and keeps your warranty valid — since the Arceus warranty requires the system to be correctly sized and properly installed. Take the time to get this right, and your boiler should serve you reliably for years to come.

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ECA Arceus electric boilers are available from ecaonline.co.uk. For technical support and sizing advice, call 0800 640 9988.