The load is not bigger, it is different
People shop for a smart switch by wattage. That works for lamps and it fails completely for water heaters, because a heating element is not a lamp that happens to draw more current.
A lamp is a few tens of watts and it is switched on for seconds at a time. A storage water heater is 2 kW to 4.5 kW and it stays on for twenty minutes to an hour, every single day, in a sealed plastic box with no airflow.
Those are two completely different thermal problems, and the switch has to be built for the second one.
Where the number on the housing comes from
When a smart switch says "16 A" or "20 A", that figure is usually a resistive rating measured under test conditions, and it is often the peak the relay survived rather than the current it can carry for an hour in a wall box.
The rule electricians actually work to is the continuous load rule: a device sitting on for three hours or more should not be loaded past roughly 80% of its rating. So a "16 A" switch is realistically a 12-13 A continuous device, and it should be protecting against a load that itself should not exceed that.
Do the arithmetic on a real heater:
- 3 kW at 230 V = 13.0 A. On a 16 A circuit that is 81% — already at the edge.
- 4.5 kW at 240 V = 18.8 A. In North America a 4.5 kW element is the common tank size, and it belongs on a 30 A circuit with a two-pole breaker, not on 20 A wiring.
So the smart switch you need is not "a switch rated higher than the heater". It is a switch rated for the circuit the heater is on, and the circuit has to be sized for continuous duty first. If any of that is unknown, the answer is not a product recommendation, it is an electrician.
Relay, not triac
This is the one specification that actually separates a light switch from a power switch, and almost nobody reads it.
A triac-based switch scrapes a volt or so across the semiconductor for every amp that flows through it. At 2 A that heat is trivial. At 16 A it is a small heater mounted inside your wall box, next to the thing it is switching, with nowhere for the heat to go.
An electromechanical relay closes a contact instead and dissipates a fraction of that. For sustained high current, that is the right component, and it is worth asking which one is in the box you are buying. If the datasheet does not say, assume the cheaper one.
The no-neutral trap works in reverse here
A lot of smart switches sold for retrofits are single-line designs: they have no neutral, so they power themselves by bleeding a trickle of current through the load even when the load is off.
With a lamp, that trickle is the famous problem — it makes LED lamps glow faintly in the dark, because a few milliamps is enough to tickle an LED driver.
With a heating element, that same trickle does nothing. A few milliamps through a 30 ohm element produces millivolts and no heat. So the trick that causes complaints on lighting is harmless here.
The trap is that people learn the lesson in one direction. A switch that behaves perfectly on a bedroom lamp can behave badly on a 3 kW element for the opposite reason: it is not the trickle, it is eight hours at 90% of rating. And a switch that is specified for a 20 A resistive load with a neutral connection is a more robust product than one that has to steal its own power through the load.
What to check on the spec sheet
Before the price:
- Resistive rating, not "max" — you want the number for a resistive load, stated for continuous duty.
- Relay or triac — relay for sustained power.
- Neutral — if the wall box has one, use a switch that expects it.
- Temperature rating and enclosure — it will live in a box with the heat it makes.
- The circuit, not the appliance — 2.5 mm² cable, the right breaker, and in the UK a 20 A double-pole isolator next to the heater.
If you are putting a switch in front of a fixed appliance for the first time, the wiring already has to be correct for a 20 A circuit before the smart part matters at all.
Where to start
If the job is a storage water heater on a dedicated circuit, look at switches that are explicitly built and rated for that class of load rather than adapting a lighting product. The 20 A Wi-Fi water heater touch switch and its US 120 V counterpart are examples of the category — the rating, the relay and the panel size are all chosen for a high-current resistive load instead of a lamp.
The wider range, including the standard lighting and socket versions, is worth comparing side by side so you can see how different the internal ratings are: the smart switch category lists them with their loads.
Manufacturing-side detail on how these are built and rated is on XingLiao's factory site.
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