Between an instantaneous water heater and a storage one, what decides is not the volume but the pairing of declared load profile and genuinely available electrical power. Regulation (EU) No 814/2013 defines no profile in litres: its Annex III sets a reference energy Qref, 2.100 kWh for the S profile, 5.845 kWh for M and 11.655 kWh for L, and it is on that sequence that the appliance's efficiency is measured. On the power side, an instantaneous electric unit holding 12 l/min at a 25 K rise calls for close to 21 kW, that is 91 A at 230 V single phase, where a domestic service fuse tops out at 100 A for the whole dwelling. The arbitration therefore happens on the product fiche and at the consumer unit, never in the sales pitch.
The two figures that decide, and the ones that do not
The declared load profile, not a volume in litres
Annex III of Regulation (EU) No 814/2013 defines ten profiles, from 3XS to 4XL. Each is a time-stamped sequence of draw-offs, with a useful energy, a flow rate and a minimum temperature for every draw. The profile the manufacturer declares sets the water heating energy efficiency threshold the appliance has to meet: the two figures are inseparable on the product fiche. No article of the regulation ties a profile to a storage volume, and any table that does is a commercial construction. The ten profiles and their maximum flow rates are covered in the DHW load profiles.
| Declared profile | Qref (kWh) | Minimum water heating efficiency |
|---|---|---|
| XXS | 2.100 | 32 % |
| XS | 2.100 | 32 % |
| S | 2.100 | 32 % |
| M | 5.845 | 36 % |
| L | 11.655 | 37 % |
| XL | 19.070 | 37 % |
These thresholds are minimum placing-on-the-market values from Annex II of the regulation, not site targets. A heat pump water heater sits well above them, and it is that gap, quantified on the product fiche, that holds up against a cheaper direct electric cylinder.
The connection capacity, which rules out options before any calculation
The flow of an instantaneous unit follows directly from its power. Raising one litre of water by one kelvin takes 1.163 Wh, so one litre per minute per kelvin takes 69.8 W. The table below applies that relation, with the current drawn at 230 V single phase.
| Power | Flow at ΔT = 25 K | Flow at ΔT = 35 K | Current at 230 V single phase |
|---|---|---|---|
| 3.5 kW | 2.0 l/min | 1.4 l/min | 15 A |
| 7 kW | 4.0 l/min | 2.9 l/min | 30 A |
| 12 kW | 6.9 l/min | 4.9 l/min | 52 A |
| 18 kW | 10.3 l/min | 7.4 l/min | 78 A |
| 21 kW | 12.0 l/min | 8.6 l/min | 91 A |
| 24 kW | 13.8 l/min | 9.8 l/min | 104 A |
France gives the sharpest illustration of the constraint: procedure Enedis-NMO-RAC_010E sets a ceiling of 12 kVA on an individual single-phase connection in a dwelling, with a 9 kVA step in collective supplies, and everything above goes three phase. The British picture is the same arithmetic against a 60, 80 or 100 A service fuse. An instantaneous electric shower heater is therefore a three-phase appliance, or one you do not fit. The same trade-off plays out on a combination appliance, where the declared profile rather than a litres-per-minute figure compares two products, as set out in the gas combi with micro-storage.
What the survey sheet has to carry
The survey decides the system, not the other way round. Four lines are enough, provided they are measured rather than declared.
- Subscribed power read at the meter, type of supply, and a photo of the consumer unit showing the free ways.
- Flow measured at the shower head and at the sink with a graduated bucket, not the flow printed on the box.
- Water volume between generation and the furthest outlet, to know whether the 3 litre threshold is crossed.
- Cold water temperature read at the tap nearest the meter, which sets the rise to be produced.
The instantaneous water heater: where it holds, where it does not
The calculation to run before it goes on a quote
Start from the most demanding outlet and from winter cold water, not from the annual average. On a main at 8 °C and a shower blended to 40 °C, the rise is 32 K, not the 25 K of the brochures. Run the table above with that figure, then set the resulting current against the actual space in the consumer unit. On a 400 V three-phase supply, a 21 kW unit draws 30 A per phase, which brings it back into range. That single line of arithmetic is what protects the quote.
The limits in multi-point use and in winter
An instantaneous unit stores nothing: the flow is shared between the open outlets and the temperature falls at constant flow. Two simultaneous draws on a unit sized for one do not give two comfortable half flows, they give two lukewarm trickles. Add the season: between summer cold water at 16 °C and winter cold water at 8 °C, the energy to supply rises by close to a third at equal flow. A unit that suits in October fails in February.
The cases where it remains the right answer
A single outlet a long way from the generator, a hand basin, a plant room sink, a let property in intermittent use: there the instantaneous unit avoids keeping a cylinder hot for nobody. On gas it also opens configurations a cylinder cannot fit, provided flue and combustion air are treated as a work item in their own right, with their own lines on the quote.
The storage water heater: what it buys, what it costs
Peak demand, which is the real reason to store
Storage does not buy comfort in general, it buys simultaneity. Two showers at once and a bath filled in twenty minutes are power demands only an already-hot volume absorbs. It is also the only arrangement compatible with slow generation, heat pump or solar, where the heating output is low and the volume is what holds the profile. The sizing method is set out in the DHW storage volume.
Standing losses, as they are actually declared
Do not repeat the orders of magnitude in circulation. For a hot water storage tank, Regulation (EU) No 814/2013 requires the standing loss to be declared in watts, and for a water heater the daily electricity or fuel consumption tied to the declared profile. Those values sit on the product fiche of the exact model, and only they are enforceable. A quote line promising a saving without citing the fiche of the model delivered binds nobody except the person who wrote it.
Reheat time, the forgotten sizing criterion
Adding litres costs in losses, in space and in price, while the real peak criterion is recharge speed. A 200 litre appliance that reheats in an hour covers a second peak that a slow-recharging 300 litre one will not. On a heat pump water heater, check the electric immersion backup and its switching logic before stepping up a profile: it is what carries the heavy days, not the compressor.
The duties and the evidence, whichever system you fit
The temperatures imposed by the French order of 23 June 1978
Article 36 of the French order of 23 June 1978 caps domestic hot water at 50 °C at outlets in rooms intended for washing and at 60 °C in other rooms. In parallel it requires at least 50 °C at every point of the distribution as soon as the volume between generation and the furthest outlet exceeds 3 litres. British practice lands in the same place through Approved Document G and the HSE guidance on legionella. Both are met together by a thermostatic mixing valve on the outlet, never by lowering the storage setpoint, which would move the subject onto the sanitary risk covered in the DHW sanitary safety devices.
What attracts support, and what does not
In France the energy savings certificate scheme funds the heat pump water heater through the BAR-TH-148 measure and the solar water heater through BAR-TH-101, while the direct electric cylinder and both instantaneous types, electric and gas, fall under no residential measure. In Great Britain the Boiler Upgrade Scheme covers space heating, so a stand-alone water heater draws nothing, and a cylinder only enters through an ECO4 package. That absence is not an oversight: presented as a whole-life cost gap rather than as red tape, it is a selling argument. The version that applies is the one in force at the commitment date, archived with the signed quote.
The lines that commit you, on the quote and at commissioning
Put the exact commercial reference on the quote, along with the declared load profile, the water heating energy efficiency and, for a cylinder, the storage volume. Those four figures come from the product fiche and carry across unchanged from a catalogue built on the verified technical characteristics of the ranges. At commissioning, record the setpoint read, the temperature at the least favourable outlet after blending, the thermostatic mixing valve setting and the pressure at the reducing valve, then date it and have it signed. Without that document, a dispute over comfort is settled against you, including under the structural cover on the pipework.
