How Much Hot Water Does an Electric Combi Boiler Really Give You?
Arda DaneIf you are thinking about switching to an electric combi boiler, the question that matters most day to day is simple: will the shower be good enough? The honest answer is that it depends on three things - the boiler's power, the temperature of the water coming into your home, and how hot you want the water coming out. This guide explains how to read electric combi hot water figures properly, what an E.C.A. Arceus 12 kW or 15 kW combi realistically delivers, and when a different setup makes more sense.
The short answer
Based on the published figures in the Arceus operating manual, normalised to a typical 10°C mains water temperature:
| Model | Flow at 40°C (shower temperature) | Flow at 44°C (hotter bath) | Time to run a 120 L bath at 40°C |
|---|---|---|---|
| Arceus 12 kW | approx. 5.7 L/min | approx. 5.1 L/min | approx. 21 minutes |
| Arceus 15 kW | approx. 7.2 L/min | approx. 6.3 L/min | approx. 17 minutes |
These are calculated nominal values with no allowance for pipe heat loss, and the manual states a flow tolerance of ±15%. They are a realistic guide, not a guarantee for every home.
Why electric combi flow rates need context
An electric combi heats water instantly as it flows through, so the amount of hot water it can produce is limited by its power. A fixed amount of energy can heat a small volume of water by a lot, or a larger volume of water by a little. That is why every honest hot water figure comes with a temperature rise, written as ΔT (delta T).
The temperature rise is the difference between the cold water coming in and the hot water coming out. It is easy to misread: a 30°C rise does not mean 30°C water. If your mains water arrives at 10°C and the boiler adds a 30°C rise, you get 40°C water at the tap, which is a comfortable shower temperature.
The Arceus manual publishes these figures:
- Arceus 12 kW: 7 L/min at a ΔT of 24.6°C
- Arceus 15 kW: 8 L/min at a ΔT of 26.9°C
Those numbers look higher than the table above because they are measured at a smaller temperature rise. To compare fairly, we convert them to the same conditions using a simple rule: flow multiplied by the original temperature rise, divided by the new temperature rise. For the 15 kW model at a 30°C rise, that is 8 × 26.9 ÷ 30 ≈ 7.2 L/min.
What that means in everyday use
Showers. At around 40°C, the Arceus 15 kW delivers roughly 7 litres per minute and the 12 kW roughly 5.7 litres per minute. Many households find this comfortable for a single shower, particularly with a good quality or water-efficient shower head. If you are used to a high-flow power shower fed from a stored cylinder, expect a gentler flow.
Baths. A 120 litre bath at 40°C takes around 17 minutes on the 15 kW model and around 21 minutes on the 12 kW. Raising the target to 44°C reduces flow by about 12% on both models, so a hotter bath takes a few minutes longer. It does not drop to a trickle.
One outlet at a time. Like most combi boilers, an electric combi is designed to supply one hot water outlet at a time. If someone turns on the kitchen hot tap while you are in the shower, the available hot water is shared between them. The Arceus 15 kW is not designed to supply two baths or two showers simultaneously.
Why hot water feels different in winter
Mains water is colder in winter, so the boiler has to work harder to reach the same outlet temperature. If incoming water drops to 5°C, reaching 40°C means a 35°C rise instead of 30°C. Using the same calculation, that brings the Arceus 15 kW to roughly 6.1 L/min and the 12 kW to roughly 4.9 L/min at 40°C. It is worth keeping this in mind when judging performance, as a combi that feels generous in August will feel slightly less so in January.
12 kW or 15 kW: which should you choose?
From overseeing Arceus sales, technical support and aftercare across the UK, E.C.A. UK's guidance is to start with the 12 kW combi for most homes with one to three bedrooms and one bathroom, and to move to 15 kW only when a proper heat loss calculation shows the home genuinely needs it. Around 80% of the Arceus combi boilers E.C.A. UK expects to sell are 12 kW models.
Two practical points shape that decision:
- Heating load comes first. Boiler size should be chosen from a room by room heat loss calculation, not radiator count alone. As a rough screening guide, around eight conventional radiators often points to a 12 kW conversation, while nine or more is worth a closer 15 kW check.
- Your electrical supply matters. On a single-phase 230 V supply, the Arceus 12 kW draws around 52 A and needs a minimum B63 breaker, while the 15 kW draws around 65 A and needs a minimum B80 breaker. Both models also run on 400 V three-phase supplies. A qualified electrician should confirm your incoming supply and maximum demand before you order, and your network operator may need to be consulted.
When a combi is not the right answer
An instantaneous combi is a great fit for smaller homes with one bathroom and one person using hot water at a time. It is not the best choice for every household. If you have multiple bathrooms, several people showering back to back at peak times, or you want to run two outlets at once, a system boiler with a hot water cylinder is usually the better engineering answer.
The Arceus range includes an ST system boiler in 12 kW and 15 kW versions, designed to work with a correctly sized indirect unvented cylinder. Stored hot water can then be delivered to several outlets at once, with the boiler reheating the cylinder in the background. There is also a CH heat only version for homes that need space heating only or already have a separate hot water arrangement.
How to compare electric combi boilers fairly
When you are looking at different brands, headline flow figures are often published at different temperature rises, which makes a lower-rise figure look more impressive than it is. Before comparing, ask:
- At what temperature rise (ΔT) was this flow rate measured?
- What would the flow be at a 30°C rise, which is roughly a 40°C shower in most of the UK?
- Can the boiler supply more than one outlet at a time?
- What does the electrical supply need to be for this model?
Converting every figure to the same conditions is the only way to compare like with like.
Keeping hot water performance at its best
Electric boilers use heating elements rather than a burner, and in hard water areas limescale is the main enemy of long-term performance. The Arceus is designed so that engineers can test each element individually, descale it, and replace a single element if needed without removing the main tank. Good water treatment, correct filtration and an annual service all help the boiler keep delivering the flow it was designed for.
Arceus boilers come with three years of parts and labour cover, plus ten years of cover on the heating elements. Registration, annual servicing and current written terms apply.
Frequently asked questions
Is an electric combi good enough for a shower?
For a single shower in a typical one bathroom home, yes. The Arceus 15 kW delivers around 7 L/min at 40°C and the 12 kW around 5.7 L/min, based on a 10°C mains temperature. Expect a steady, comfortable flow rather than a power shower.
Can I run a bath and a shower at the same time?
Not comfortably. An electric combi supplies one outlet at a time. If your household regularly needs hot water in two places at once, consider an Arceus ST system boiler with a cylinder.
Why is my hot water flow lower in winter?
Colder mains water means the boiler needs a bigger temperature rise to reach the same outlet temperature, so flow reduces. This is normal for any instantaneous water heater.
Will a 15 kW boiler give me much more hot water than a 12 kW?
At 40°C, the 15 kW delivers roughly 1.5 L/min more than the 12 kW. The larger model is best chosen when your heating load requires it and your electrical supply can support it, rather than for hot water alone.
Talk to E.C.A. UK
Not sure whether an electric combi or a system boiler is right for your home? Our UK team can help you think through heat loss, electrical supply and hot water demand before you buy. Call 0800 640 9988 or explore the Arceus electric boiler range.
Figures are calculated from the published values in the E.C.A. Arceus Operating Manual 2026, assuming a 10°C mains inlet with no distribution losses. Actual performance varies with water temperature, pressure, pipework and installation. Always have your system specified by a competent installer.
