Electric Boiler Sizing Guide
Choosing the right size electric boiler matters more than with gas: too small and you run short; too large and you hit the limits of your electrical supply. This guide sizes your boiler by home, hot-water demand and supply, then matches you to the right ECA Arceus model.
Talk to an expertShop electric boilers
The 4 questions that decide your size
Around 1.5 kW per radiator. Count every radiator and heated towel rail.
The biggest driver of hot-water demand. Two or more points to a system boiler.
More occupants means more simultaneous hot-water use, so stored water helps.
Most homes have an 80–100 A single-phase fuse. This can cap your max size.
Combi or system?
✓ Best for flats & one-bathroom homes
✓ Compact, fits where space is tight
✓ Best for two or more bathrooms
✓ Strong hot water to several outlets at once
Sizing at a glance
Indicative only, confirm with a heat-loss calculation and a check of your electrical supply.
The ECA Arceus range
| 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 wanting stronger flow |
| Arceus System 12 kW | 12 kW | System | Stored hot water with moderate demand |
| Arceus System 15 kW | 15 kW | System | Two or more bathrooms, higher demand |
Frequently asked questions
Typically around 12 kW, assuming roughly 7–10 radiators and one bathroom. With a second bathroom, consider a system boiler with a hot-water cylinder.
Yes, for heating and hot water, provided it is correctly sized and your electrical supply can support the output. Larger homes with high simultaneous demand are best served by a system boiler and cylinder.
Per unit, electricity costs more than gas, but electric boilers are nearly 100% efficient at the point of use, need no flue or annual gas safety check, and have lower installation and servicing costs.
Most UK homes' single-phase supply (80–100 A) runs both the 12 kW and 15 kW Arceus models. The 15 kW draws about 65 A, so a 100 A main fuse and correctly rated cabling are recommended.
