Do electric combi boilers exist?
Yes. Several UK manufacturers sell electric boilers that provide both central heating and hot water from a single unit, and they are marketed as electric combis. The name is borrowed from gas combi boilers, but the way they deliver hot water is often different. A gas combi at 30 kW or more heats cold mains water as it flows through. An electric combi has far less power to work with, so most models keep a quantity of hot water ready in a tank inside the casing.
If you only need heating, or you are happy to keep a separate cylinder, a standard electric flow boiler may suit better. Our guide to the electric boiler covers those, along with running costs and supply limits in more detail.
Why electric combis cannot match a gas combi's flow
It comes down to how much power your home can draw. Most UK homes have a single-phase supply, commonly protected by a 60, 80 or 100 amp main fuse. At 230 volts, 100 amps is about 23 kW for the entire house. An electric boiler has to share that with the cooker, kettle, electric shower and everything else, which is why single-phase electric boilers are usually rated at 12 to 15 kW at most.
Heating water instantly takes a lot of power. Raising water by 35°C, roughly from cold mains to shower temperature, needs about 2.4 kW for every litre a minute. Put the other way round, each kW heats only about 0.41 litres a minute. So:
| Power available for hot water | Instant flow at a 35°C rise | Compare with |
|---|---|---|
| 9 kW | About 3.7 litres a minute | A modest electric shower |
| 12 kW | About 4.9 litres a minute | A good electric shower |
| 30 kW (typical gas combi) | About 12 litres a minute | A strong mains pressure shower |
In winter, when mains water is colder, the flow at the same temperature falls further. That is why a purely instantaneous electric combi can only really supply one outlet at a time, and slowly. The stored-water designs below get around it.
The two types of electric combi boiler
Combis with a built-in hot water store
These heat water gradually into a tank inside the unit, then deliver it at a much higher flow than the elements alone could manage. Once the store is used, it takes time to recover. Two examples on the UK market:
- Heatrae Sadia Electromax. Available at 6 kW and 9 kW, with a 180 litre stainless steel unvented cylinder built in. Heatrae Sadia says it gives mains pressure showering, fast filling baths and a balanced supply to several taps. The cylinder carries a 10 year warranty and the boiler and controls 2 years. Its programmable thermostat is designed to work with a range of off-peak tariffs.
- Trianco Aztec Maxi combi. Sold at 8, 10 and 12 kW. Retailer specifications list a stored water capacity of 37 litres on the 12 kW model, in a unit about a metre tall.
The size of the store matters as much as the kW rating. A small store gives a strong flow for a few minutes and then drops back to what the elements can heat. A large unvented store behaves much more like a system boiler with a cylinder.
Instantaneous electric combis
Some compact models heat hot water on demand with no store, much like an electric shower that also runs the radiators. They save space, but they are limited to the flow rates in the table above. They suit a one bedroom flat with a single shower better than a family home.
How much hot water can an electric combi deliver?
For an instantaneous model, roughly 0.4 litres a minute for each kW available at a 35°C rise, so under 5 litres a minute even at 12 kW. For a stored model, the answer has two parts: the volume in the store and how quickly it reheats.
A 180 litre store heated to around 60°C and blended with cold water gives considerably more than 180 litres of water at shower temperature, enough for several showers or a bath in a row. Once it runs down, a 9 kW element takes time to bring it back to temperature. As a rough guide, heating 180 litres by 50°C needs about 10.5 kWh, which at 9 kW is a little over an hour of full power. A small 37 litre store gives one short shower at a good flow before relying on the elements.
What size electric combi boiler do I need?
Size it on two separate needs, and ask the installer to show their working for both:
- Heating. A room-by-room heat loss calculation shows the kW your home needs on a cold day. Well-insulated flats may need only 4 to 6 kW. Larger or older houses may need more than any single-phase electric boiler can give.
- Hot water. Count the people, bathrooms and baths. A household that showers one after another in the morning needs a bigger store than one person in a flat.
- Supply. An electrician should confirm your main fuse size and whether the boiler can run alongside your other big loads. Some installations use a load-management device that pauses the boiler while an electric shower runs.
- Space and weight. A stored model with a large cylinder is floor-standing or needs a strong wall and a cupboard to itself once full of water. Check the dimensions and the full weight in the installation manual before choosing where it goes.
Running costs on the current price cap
Every unit of heat from an electric combi comes from electricity at close to 100% efficiency. From 1 October to 31 December 2026, the Ofgem price cap average direct debit rates are 26.32p per kWh for electricity and 7.97p per kWh for gas. After allowing for a gas boiler's losses, electric heat costs roughly three times as much as gas heat on standard rates.
Stored models can take advantage of time-of-use tariffs, heating the store when electricity is cheaper. Whether that saves money depends on the tariff's peak and off-peak rates and how much heat you use outside the cheap hours. Check your supplier's current rates before relying on it.
Are electric combi boilers good?
They are good at some things and poor at others:
| Strengths | Weaknesses |
|---|---|
| No gas supply, flue or condensate pipe needed | Much higher running costs than gas on standard rates |
| No annual gas safety check | Limited heating output on a single-phase supply |
| Quiet, compact and can go almost anywhere with a suitable circuit | Instant models give low hot water flow |
| No carbon monoxide risk from the boiler | Stored models are bigger and heavier than a gas combi |
For a flat where gas is not available or a flue is not allowed, an electric combi with a decent store is a sensible, simple option. For a family house with mains gas, a gas combi boiler will almost always give stronger hot water for less. Off the gas grid, compare it with an oil boiler and an air source heat pump before deciding.
Installation and safety
Installing an electric combi means a dedicated high-current circuit from the consumer unit. In England, a new circuit is notifiable under Part P of the Building Regulations, so use an electrician registered with a competent person scheme. Models with a built-in unvented cylinder also need an installer qualified to fit unvented hot water systems under Part G3 of the Building Regulations, because the cylinder has pressure and temperature safety devices that must be set up correctly.
Do not open the casing of an electric boiler while it is connected to the supply. If you are replacing a gas boiler, the old gas supply must be capped by a Gas Safe registered engineer. If you ever smell gas, call the National Gas Emergency Service on 0800 111 999.