A mixer shower works the opposite way: it blends hot and cold water drawn from the property's existing supply, whether that is a combination boiler, a vented cylinder, or an unvented cylinder, and its performance rides entirely on what that system can provide. On a property with an unvented cylinder or a combi boiler delivering strong, consistent mains pressure to both hot and cold, a mixer shower can perform very well, often better than an electric shower, because it is not limited by the kW-versus-flow trade-off that constrains instantaneous heating. On a property with a low-pressure vented cylinder fed from a loft tank, the same mixer shower may deliver disappointing pressure unless a pump is added to the system.
For a trade buyer speccing a replacement or a new shower, matching kW rating to both the property's electrical capacity and its incoming flow is the real decision, not simply picking the highest number on the shelf.
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 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.
Plumb2U carries the parts and spares for both instantaneous and storage electric water heaters, from elements and thermostats to the fittings that connect them into a system, at trade prices with next-day despatch across the UK. For merchants and installers working from a plumbers merchants account rather than a shop counter, that means the right spare is one order away, not a special one.
A higher kW-rated unit, 10.5kW against 8.5kW for example, has more headroom to maintain flow rate even as it works harder to raise the temperature, which is why higher-rated units tend to feel stronger and more consistent through the colder months. That headroom depends entirely on the property's electrical supply being able to support it: a higher kW shower draws more current, and the circuit and consumer unit need the capacity to match, which is not automatic on every property, particularly older ones.
That is the practical filter for a trade plumbing supplies buyer: check what is actually feeding the property first, cylinder type, boiler type and measured pressure, before recommending either option. An electric shower sidesteps a weak or inconsistent hot water system by generating its own supply on demand. A mixer shower only performs as well as the system behind it allows, which makes it the stronger choice specifically where that system is already strong.