Electric vs Mixer vs Power Shower: What Your Boiler Allows

The electric vs mixer shower decision usually gets made in a showroom, by pointing at whichever unit looks best. That is the wrong order. Your hot water system decides which types can physically go in, and only then does style come into it. Get it the wrong way round and you end up with a dribble, a shower that runs cold whenever someone opens a tap downstairs, or a boiler or cylinder being asked to do something it was never designed to do.

Table of Contents

Find out what heats your water first

Almost every UK home falls into one of three groups, and you can identify yours in about thirty seconds.

Combi boiler. No hot water cylinder anywhere, no tanks in the loft. The boiler heats water on demand as you draw it.
Unvented (sealed) cylinder. A large insulated cylinder in a cupboard, usually with a small expansion vessel and safety valves plumbed alongside it, and no tank in the loft. Hot water arrives at close to mains pressure.
Vented (gravity) system. A hot cylinder in the airing cupboard and a cold water storage tank in the loft. Hot water pressure comes from the height difference between that tank and the shower head.

That single distinction answers most of the question before you have looked at a single product.

Electric showers: independent of everything else

An electric shower has its own heating element. It takes cold mains water only, heats what passes through it, and ignores the boiler entirely. That independence is the point: it still works when the boiler is broken, when the cylinder has been drained, or when someone else is running a bath.

The number that matters is the kW rating, because it sets how fast the unit can heat water. Ratings run from 8.5 kW at the entry end through 9.5 kW to 10.5 and 10.8 kW. A higher rating does not raise water pressure, it raises how much water the unit can bring up to temperature per minute.

That is also why electric showers feel worse in January. Incoming mains water is several degrees colder in winter, so the same unit slows the flow to hit the same outlet temperature. Nothing has broken; it is the physics of a fixed element.

Electric showers do need a reasonable cold supply to work at all. Triton, one of the larger UK manufacturers, specifies a minimum of 1 bar running pressure with 8 litres per minute for its 7 kW to 8.5 kW units, and 1 bar with 9 litres per minute for 9 kW and 9.5 kW units. If your mains cannot deliver that, the unit will trip out or run cool rather than simply running slowly.

The electrical side is not a DIY job. An electric shower needs its own dedicated circuit from the consumer unit, protected by a 30 mA RCD as BS 7671 requires for any circuit serving a room with a bath or shower, and switched by a double pole pull cord or a switch outside the room. Installing a new circuit in a bathroom is notifiable work under Part P of the Building Regulations in England and Wales, so it goes to an electrician registered with a competent person scheme or through building control. The NICEIC guidance on bathroom electrics covers what householders should expect.

Best for: second bathrooms and en suites, homes with a small combi that cannot serve two outlets at once, and anyone who wants hot water that survives a boiler breakdown.

Mixer showers: the default for a main bathroom

A mixer blends existing hot and cold supplies through a valve. No heating element, no pump, so what comes out is determined entirely by what your system can deliver. Specify a thermostatic valve rather than a manual one: it holds the outlet temperature when pressure on either supply changes, which is what stops the scald or cold shock when a tap runs elsewhere in the house.

How well a mixer performs depends on the system:

On a combi: the shower can never flow faster than the boiler’s hot water output. That figure is printed on the boiler’s data badge as litres per minute at a stated temperature rise. Check it before buying a wide rainfall head, because a large head just spreads a limited flow more thinly.
On an unvented cylinder: usually the best shower in the house. Both supplies arrive at mains pressure and the cylinder has stored volume, so flow stays high until the hot water runs out.
On a gravity system: performance depends on the head, meaning the vertical distance between the bottom of the loft tank and the shower head. A bathroom directly under the loft often gives a disappointing trickle. A ground floor shower in the same house can be perfectly good.

Power showers: one system only

A power shower is a mixer with a pump built in. The pump is why people want one, and why it will not suit most homes. It is designed for a vented gravity system: a loft tank feeding a vented hot cylinder. It should not be fitted to a combi or an unvented cylinder. On a combi there is no stored hot water to draw from, and pumping the boiler’s outlet can damage the heat exchanger and its seals. On an unvented cylinder, pulling water out faster than the incoming main can replace it risks a partial vacuum in a sealed pressure vessel, which is a safety matter rather than a performance one.

On a gravity system, a separate twin impeller pump feeding a standard thermostatic mixer usually beats an all-in-one power shower unit. It gives a far wider choice of valve and head, and the pump can be sited somewhere accessible.

One regulatory point worth knowing: under the Water Supply (Water Fittings) Regulations 1999, a pump or booster drawing more than 12 litres per minute that is connected directly or indirectly to the supply pipe must be notified to your water company before installation. Consent is deemed granted if they do not respond within ten working days, but the notice still has to be given.

Flow rate is what you feel, not pressure

People say pressure and mean flow. Pressure is measured in bar and describes force; flow is in litres per minute and describes how much water arrives. A shower can be on a high pressure main and still feel weak if something upstream restricts flow. Measuring yours takes a minute: hold a bucket under the shower on full hot for exactly sixty seconds and measure what you collected. Roughly, an electric shower gives 6 to 9 litres per minute, a mixer on a combi is capped by the boiler’s rated output, and a mixer on a good unvented cylinder exceeds both.

Running costs: the arithmetic, not a headline

A 9.5 kW electric shower running for eight minutes uses 9.5 x 8/60, or about 1.27 kWh of electricity. A mixer flowing 12 litres a minute for the same eight minutes heats 96 litres of water. Raising 96 litres by 35 degrees takes roughly 3.9 kWh of heat, or about 4.3 kWh of gas at a realistic boiler efficiency.

So the mixer uses more than three times the energy, but that energy is gas, which costs considerably less per kWh than electricity. Multiply each figure by your own unit rates and the two land far closer together than most people assume. The electric shower also uses roughly half the water, which matters on a meter.

What this usually means for a Kent home

Older stock across Kent, particularly interwar and 1950s semis that have never had a heating overhaul, tends to still be on gravity. Those are the houses where a power shower or pumped mixer makes sense and where a mixer alone disappoints. Flats and newer houses on a combi want a thermostatic mixer, with the boiler’s rated hot water flow checked before anyone orders a large head or twin outlet.

If the heating is due for work anyway, converting to an unvented cylinder gives the best showers long term, but it is regulated work. Unvented hot water storage systems fall under Part G3 of the Building Regulations and must be installed by someone holding a current G3 qualification, who then self-certifies through a competent person scheme or notifies building control.

The shower is one line in a bigger budget, so it is worth reading alongside our bathroom renovation guide and the figures in what a new bathroom costs in 2026. For more on planning and layouts across Kent, see the rest of Contemporary Structures.

Frequently asked questions

Can I fit a power shower to a combi boiler?

No. A power shower’s integral pump needs stored hot water to draw from, and a combi has none. Pumping directly on a combi’s hot outlet can damage the heat exchanger and its seals. If you have a combi and want a stronger shower, the routes are a thermostatic mixer matched to the boiler’s rated flow, a larger boiler, or an electric shower.

Is an electric shower cheaper to run than a mixer?

Often, yes, though not by as much as the water saving suggests. An electric shower uses roughly a third of the energy of a mixer for the same length of shower, because it heats far less water, but that energy is electricity rather than gas. Work it out on your own tariff. On a water meter, the lower flow adds a second saving.

Why does my shower go cold when someone runs a tap?

Because a manual mixer cannot compensate when one supply loses pressure. A thermostatic valve fixes it in most cases. If the shower is on a combi and it happens whenever a second hot outlet is opened, the boiler is being asked for more hot water than it can produce, and a valve will not help.

What kW electric shower should I buy?

Higher is better if the supply allows it, because the rating governs how much water the unit heats per minute. Around 8.5 kW is entry level, 9.5 kW suits most households, and 10.5 or 10.8 kW gives the best winter performance. The limit is your consumer unit and incoming supply, so have an electrician confirm what the property takes before ordering.

Do I need an electrician to fit an electric shower?

Yes. An electric shower needs its own dedicated circuit, 30 mA RCD protection and correct isolation, and installing a new circuit in a room containing a bath or shower is notifiable under Part P of the Building Regulations in England and Wales. Use an electrician registered with a competent person scheme so the work is certified without a separate building control application.

Electric vs Mixer vs Power Shower: What Your Boiler Allows

Find out what heats your water first

Electric showers: independent of everything else

Mixer showers: the default for a main bathroom

Power showers: one system only

Flow rate is what you feel, not pressure

Running costs: the arithmetic, not a headline

What this usually means for a Kent home

Frequently asked questions

Can I fit a power shower to a combi boiler?

Is an electric shower cheaper to run than a mixer?

Why does my shower go cold when someone runs a tap?

What kW electric shower should I buy?

Do I need an electrician to fit an electric shower?

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