Instantaneous Electric Water Heater vs Traditional Boiler: Which Is Right for Your Property?

Choosing between an instantaneous electric water heater and a traditional boiler is a decision that depends far more on your property type and hot water demand than most people realise. For some households and commercial premises, a conventional boiler is the obvious answer. For others – particularly smaller properties, rentals or buildings without a gas supply – an instantaneous electric heater is not just a compromise; it’s genuinely the better solution. This article sets out the key differences clearly, so you can make the right call for your specific situation.

The question comes up regularly – particularly among landlords, flat owners and small business operators who are either replacing an ageing system or fitting out a property from scratch. An instantaneous electric water heater heats water on demand as it flows through the unit, with no storage tank and no standby heat loss. A traditional boiler – whether combi, system or heat-only – heats water using gas, oil or electricity and either stores it or delivers it on demand via a pressurised system. Both work. The question is which one works better for your property.

How traditional boilers work

A combi boiler – the most common type in UK homes – heats water directly from the mains as it’s needed and also provides central heating. System and heat-only boilers work with a hot water cylinder, storing a volume of pre-heated water ready for use. Gas remains the dominant fuel for traditional boilers in the UK, though oil, LPG and electric variants exist. The advantages are well established: high flow rates, the ability to supply multiple outlets simultaneously and compatibility with existing radiator systems. The downsides are equally familiar: dependency on a gas supply, the need for a flue, regular servicing by a Gas Safe registered engineer and, for stored systems, the ongoing energy cost of keeping water hot even when it’s not being used.

How instantaneous electric water heaters work

An instantaneous electric water heater – sometimes called a tankless or on-demand water heater – heats water only when a tap or shower is opened. Cold water enters the unit, passes over a heating element and exits at the target temperature within seconds. There’s no storage tank, no standby heat loss and no gas connection required. The WARMTEC Opti-Shower, available from Culm Stores, is a good example: an 11kW single-phase unit that is compact enough to fit in a cupboard, a utility room or even under a sink, and straightforward enough to install without specialist gas qualifications.

Key differences at a glance

When a traditional boiler is the better choice

For larger households with high simultaneous hot water demand – multiple bathrooms, a busy kitchen and a central heating system – a combi or system boiler remains the most practical solution. The flow rates available from a modern combi boiler are difficult to match with a single instantaneous unit, and where gas infrastructure is already in place, the running cost per kWh of heat is typically lower than electricity. If you’re replacing a boiler in a well-connected property with a full radiator system, a like-for-like boiler replacement is usually the path of least resistance.

That said, even in gas-connected properties, an instantaneous electric unit can be a useful secondary solution – a point-of-use heater for a remote bathroom or a garden office where running a hot water supply from the main boiler would be expensive or impractical.

When an instantaneous electric water heater makes more sense

This is where the case for on-demand electric heating becomes compelling. Consider:

• Studio flats and small apartments where a full boiler installation is disproportionate to the hot water demand.
• Properties with no gas supply – rural locations, converted commercial buildings, upper-floor flats where a flue is impractical.
• Rental properties where simplicity and low maintenance are priorities for the landlord.
• Small commercial premises – workshops, salons, small offices – that need reliable hot water at one or two points without the complexity of a full boiler installation.
• Temporary or modular installations where a permanent gas connection is not available or not justified.

In each of these scenarios, an instantaneous electric water heater delivers exactly what’s needed: reliable, on-demand hot water from a compact unit that requires minimal ongoing attention.

Running costs and energy efficiency

This is the honest part of the comparison. Electricity costs more per kWh than gas in the UK – that’s simply the current reality of the energy market. A traditional gas combi boiler will generally cost less to run per litre of hot water produced than an electric instantaneous heater, assuming both are used at similar demand levels.

However, the comparison shifts when you factor in standing heat losses. A hot water storage cylinder loses heat continuously – even a well-insulated tank loses energy around the clock. An instantaneous heater has zero standing loss: it uses energy only when hot water is actually flowing. For low to moderate hot water demand – a single person, a small flat, a washroom in an office – the absence of standing losses can offset the higher electricity unit cost, making the overall running cost more competitive than it first appears.

Worth bearing in mind: for properties that cannot access gas, the comparison is moot. An instantaneous electric heater isn’t competing with a gas boiler in those cases – it’s simply the most practical available solution.

Installation considerations

Installing an instantaneous electric water heater requires a suitable electrical supply – typically a dedicated circuit from the consumer unit, rated for the unit’s power draw (11kW for the Opti-Shower). No flue, no gas pipework and no Gas Safe engineer are required. For landlords and property managers, that simplicity has real value: fewer regulatory requirements, no annual gas safety certificate for the water heater, and a smaller pool of qualified tradespeople needed for installation and maintenance.

Scale is worth considering in hard water areas. The WARMTEC Opti-Shower is available bundled with an ARAGO 1/2″ descaler, which reduces limescale build-up on the heating element and extends the unit’s service life – a practical addition in areas where scale is a known issue.

Common questions

Can an instantaneous electric water heater supply a shower and a tap at the same time?

A single 11kW unit is generally sized for one outlet at a time. For simultaneous supply to multiple outlets, multiple units or a higher-capacity system would be needed.

Does an instantaneous water heater work with low mains pressure?

The Opti-Shower is designed to work with limited water supply, making it suitable for premises where mains pressure is lower than typical – an advantage over some storage-based systems.

Is an instantaneous electric water heater suitable for a rental property?

Yes – the low maintenance requirement, absence of annual gas servicing and compact installation make it a practical choice for many rental properties, particularly smaller units.