What Size Electric Boiler Do I Need? A UK Sizing Guide

Choosing the right size electric boiler matters more than with gas. Pick one too small and you'll run short on heating or hot water; too large and you may hit the limits of your home's electrical supply — or pay for capacity you never use. This guide explains how electric boilers are sized in the UK and how to match that to the right model.

The short version: electric boiler size is measured in kilowatts (kW). Most UK homes need between 12 kW and 15 kW from the ECA Arceus range, decided by your radiators, hot-water demand, and electrical supply.

1. Electric boiler output is measured in kW

An electric boiler has a single kW output that does all the work. A rule of thumb for central heating is around 1.5 kW per radiator; ten radiators points to roughly a 12–15 kW heat demand. A proper heat-loss calculation is best.

2. The four questions that decide your size

a) How many radiators? More radiators means more heat output. Count every radiator and heated towel rail.

b) How many bedrooms and bathrooms? Bathrooms are the biggest driver of hot-water demand. One bathroom suits a smaller boiler; two or more points to a system boiler with a stored cylinder.

c) How many people live there? More occupants means more simultaneous hot-water use, where stored hot water helps.

d) What's your electrical supply? Most UK homes have single-phase with an 80 A or 100 A main fuse. At 230 V a 12 kW boiler draws about 52 A and a 15 kW about 65 A — both fine on single-phase, though 15 kW is best with a 100 A fuse. Very large or commercial systems may need three-phase. Check with a qualified electrician.

3. Electric combi vs electric system boiler

	Electric combi	Electric system
Hot water	Heated on demand, no cylinder	Stored in a cylinder
Best for	Flats & smaller homes, one bathroom	Larger homes, two or more bathrooms
Simultaneous use	Limited	Strong

Heating water instantly with electricity is power-hungry, so electric combis suit lower simultaneous demand — ideal for a flat or one-bathroom home. For several outlets at once, a system boiler with a cylinder performs much better.

4. A quick sizing guide by home size

Home	Radiators	Bathrooms	Output	Likely model
Studio / 1–2 bed flat	up to ~6	1	~12 kW	Arceus Combi 12 kW
2–3 bed house	~7–10	1	~12 kW	Arceus Combi 12 kW
3–5 bed, single bathroom	~10–13	1	~15 kW	Arceus Combi 15 kW
3–4 bed house	~10–13	2	~12–15 kW	Arceus System + cylinder
4+ bed / large home	13+	2+	15 kW	Arceus System 15 kW + cylinder

Spot the pattern: the number of bathrooms decides combi vs system, while the radiator count and size decide 12 kW vs 15 kW. A large 3–5 bedroom home with 10–13 radiators but one bathroom is an ideal match for the Arceus Combi 15 kW.

5. Matching this to the ECA Arceus range

Our ECA Arceus electric boilers come in four models — combi and system, each in 12 kW and 15 kW. They run near-silently with zero on-site emissions, ideal for homes off the gas grid.

Model	Output	Type	Best suited to
Arceus Combi 12 kW	12 kW	Combi	Flats & smaller one-bathroom homes
Arceus Combi 15 kW	15 kW	Combi	Larger one-bathroom homes
Arceus System 12 kW	12 kW	System	Stored hot water, moderate demand
Arceus System 15 kW	15 kW	System	Larger homes, two or more bathrooms

Not sure which fits? Get in touch with your radiators, bathrooms and main-fuse rating and we'll recommend the right output. In London we also offer supply and installation.

Frequently asked questions

What size electric boiler for a 3-bedroom house?

Typically around 12 kW, assuming roughly 7–10 radiators and one bathroom. With a second bathroom, consider a system boiler with a cylinder.

Can an electric boiler run a whole house?

Yes — for heating and hot water — if it's correctly sized and your supply can support the output. Larger homes with high simultaneous demand suit a system boiler and cylinder.

Are electric boilers more expensive to run than gas?

Per unit electricity costs more, but electric boilers are nearly 100% efficient at the point of use and need no flue or gas safety check. Running cost depends on your tariff and usage.

Do I need a special electrical supply?

Most UK single-phase supplies (80–100 A) run both the 12 kW and 15 kW models; 15 kW is best with a 100 A fuse. Only very large installs need three-phase. Confirm with a qualified electrician.

Do electric boilers need servicing?

No combustion means no annual gas safety inspection, but a periodic check keeps things in good order and helps protect any warranty.