Speccing the right option for a job comes down to three questions: how many outlets need hot water, how often do they run together, and what electrical supply is actually available. An instantaneous unit rated for a decent flow rate can pull a serious amount of current, and not every property has the spare capacity on the consumer unit to support it. A cylinder spreads that load differently, drawing steadily rather than in a single high spike, which is often the more practical answer on an older supply.

Water pressure and flow available at the shower's cold feed matter just as much as the electrics. An electric shower can only heat the water that reaches it; a poor incoming flow rate limits performance regardless of how high the kW rating goes, since there is simply less water for the unit to raise in temperature.

Neither option is universally better. A trade buyer fitting out a small annexe with one shower and one basin is usually better served by instantaneous. A buyer working on a house with a family's worth of daily hot water demand is usually better served by a cylinder sized correctly for the job. Getting the sizing wrong in either direction shows up fast, as lukewarm water, long waits, or an immersion element working harder than it should.

This is a common issue on properties that have had a cylinder swapped out at some point without the heater or immersion being reassessed against the new capacity, or plumbers merchants where a replacement part has been fitted on availability rather than on matching the original specification. The parts most often involved in correcting it are the immersion element itself, sized correctly in kW for the cylinder's litre capacity, the thermostat controlling it, and occasionally the cylinder's internal coil where a heater draws on indirect heating rather than a direct element.

A storage or cylinder-based electric water heater works differently. It heats a fixed volume ahead of demand and holds it at temperature, ready to deliver decent flow to more than one outlet at a time. The trade-off is recovery time: once the stored volume is drawn down, the immersion element needs time to bring the next tankful back up to temperature. For a family bathroom, a kitchen and an en suite all wanting hot water within the same hour, a cylinder with enough capacity usually outperforms an instantaneous unit trying to keep pace.

Approval marks are the shorthand for all of the above. A hose carrying recognised approval marks has been tested against defined pressure, temperature and material standards, which takes the guesswork out of matching a part to a job. For a trade buyer working through multiple jobs a week, checking for the mark is faster than re-deriving the spec from first principles every time, and it is the difference between a part that is fit for purpose and one that merely fits.