Electric shower or combi shower: which should you choose?

You are planning a second bathroom and need to choose between an electric shower or combi shower. A mixer uses hot water made by the combi boiler. An electric shower takes cold mains water and heats it at the point of use. Choose the mixer for stronger flow when the boiler and mains can support it. Choose electric when a shower independent of the boiler matters more.

In short

A thermostatic mixer is usually the natural partner for a capable combi, because it can deliver a more generous flow without a new high-load shower circuit. An electric shower is the more resilient option: if the boiler alone fails, you can still wash, although it still depends on electricity and the cold mains. A conventional pumped power shower is a different product and generally does not belong on a combi.

How the two shower types work

A combi boiler heats mains water on demand, without a hot-water cylinder. A mixer blends that boiler-heated supply with cold water at the valve. A thermostatic mixer also reacts to supply changes, which helps keep the outlet temperature steadier when someone opens another tap.

An electric shower has only a cold-water connection. Water passes through a heating element inside the unit. Asking for hotter water usually means a slower flow, especially when the incoming water is colder in winter. An 8.5 kW, 9.5 kW or 10.5 kW rating is heating input, not pressure. A higher rating can warm more litres per minute by the same temperature rise, but it cannot create mains pressure. The difference is set out in the electric shower water pressure guide.

A mixer depends on the boiler and its fuel. The electric unit bypasses the boiler and loads the electrical installation. Neither option is fully independent of household services.

Choose a combi-fed mixer for flow and comfort

Choose a mixer for the showering experience. A suitably sized combi can heat water at a higher flow than many electric showers, so a well-matched mixer often feels fuller and less seasonal. You also avoid a second appliance heating water with electricity at the shower.

The limit is the whole system. A quoted domestic-hot-water flow assumes particular test conditions. The real result depends on incoming mains flow, pressure and temperature. A large shower head cannot deliver more hot water than the boiler can heat, and a more powerful boiler cannot overcome a restricted main.

Simultaneous use is the test. If one person showers while another runs a hot tap, both outlets share the combi's hot-water capacity. Two mixer showers can work, but flow may fall if the mains or boiler is already near its limit. Measure cold-mains flow and check the boiler specification before assuming a second outlet will match the first. A thermostatic mixer is preferable for most family bathrooms, yet it cannot hold a strong shower if the available water collapses. If walls and floors must be opened, compare the route and making good, not only the valve price.

Choose an electric shower for independence

An electric shower makes sense when continuity matters. A boiler breakdown does not automatically remove the shower, provided the cold main, the electricity and the unit itself still work. That helps in a second bathroom, a home with an older combi, or a household that cannot go several days without washing. The electric shower does not use the combi's hot-water output, though both users still share the incoming cold supply.

The trade-off is flow. Fixed heating power sets the limit. Colder incoming water needs a larger temperature rise, so the unit normally passes less water. A 10.5 kW model has more headroom than an 8.5 kW model and demands more from the circuit. At a nominal 230 V, 10.5 kW is roughly 46 A, far beyond a 13 A socket. Cable, protection, RCD, isolation and routing have to be designed together. See the cable and kilowatt guide. A lower-rated circuit is not automatically suitable for a higher-powered replacement. Listed units are on the 11 kW electric shower comparison.

Running cost has no universal winner. The electric shower uses electricity only while it runs. The mixer uses the boiler fuel plus water, and tariffs and flow rates vary. Compare a realistic duration and measured flow. A worked electric example is in what an electric shower costs to run.

Do not confuse an electric shower with a power shower

Power shower is often used for any vigorous shower. A conventional power shower is a mixer with a pump, designed to draw hot and cold water from stored supplies. The pump increases flow. It does not heat cold water the way an electric shower does. That arrangement is generally unsuitable for a combi, which already supplies mains-pressure hot water. Mira and Bristan advise against pairing a combi with the conventional pumped type. Pumping a supply that was not designed for it can run poorly and may breach the appliance instructions.

A 10.5 kW electric shower is not a 10.5 kW power shower. The higher rating is heating capacity, not an integral pump and not guaranteed pressure. For a stronger combi-fed shower, start with measured mains flow, boiler output, pipe size and any restriction in the valve or head. If those are adequate, a thermostatic mixer may be enough. If they are not, a product labelled powerful does not remove the limit.

Check the services before you choose

Begin at the stop tap. Ask an installer to measure dynamic flow and pressure while water is running. A static pressure figure does not show what happens in a shower. Repeat the check with another outlet open if two people will use water at once.

For a mixer, record the combi make, model and domestic-hot-water output, and check which shower arrangements the manufacturer allows. Long or narrow pipe runs can undermine a capable boiler. For an electric shower, survey the electrics before you settle on 9.5 kW or 10.5 kW: consumer unit, protective devices, bonding where relevant, cable route and isolation. A unit that fits the old screw holes can still need a circuit upgrade. Bathroom electrical work has to meet current UK requirements. What to settle before you order is in the 11 kW electric shower buying guide.

Feed an electric shower from the cold mains, not from a combi hot outlet, and use the model's own pressure range. Price the whole job: valve or unit, cable or pipework, consumer-unit work, isolation, making good and tiling. Do not buy the appliance first if either service is uncertain.

The best choice for a second bathroom

If you want a more comfortable everyday shower and the combi and mains have spare capacity, a thermostatic mixer is usually the stronger choice. It uses the existing hot-water plant and often delivers a fuller flow than an instantaneous electric unit.

If you want resilience, choose an electric shower. It heats washing water without the boiler. That is most useful when the existing bathroom already has a combi-fed mixer, because a second mixer repeats the same point of failure. Two combi-fed mixers can exceed the boiler's hot-water output. An electric shower plus a mixer avoids sharing that output, but both still share the incoming main. Measure under realistic demand.

A short hot-and-cold pipe route can make the mixer the cheaper complete job. A suitable existing shower circuit can favour electric. If neither service is there, quote both installations. Fit a thermostatic mixer for daily comfort, or an electric shower for boiler backup. Reject a conventional pumped power shower on a combi unless both manufacturers explicitly allow it. This comparison uses public UK installation practice and manufacturer guidance. It is not a laboratory flow test.