Water heaters · Guide

Storage vs Instant Water Heater: Electrical Requirements

Wall-mounted storage water heater with isolator

The key difference: a storage heater draws a lower, steady current and needs a lighter dedicated circuit; an instant heater draws a high current on demand and needs a heavier one. Both need an isolator and proper earthing, and in an older flat, a loading check before the instant type goes in.

Why the load differs so much

A storage heater warms a tank gradually, so it sips power steadily. An instant heater has no tank, it heats water the moment you open the tap, which means a large current in short bursts. That is the whole electrical story: the instant heater’s convenience comes from a much heavier demand, and the wiring has to be built for it.

Dedicated circuit, isolator, earthing

Either type belongs on a dedicated circuit sized to the heater, with a local isolator so its power can be safely cut, and proper earthing. A heater is a wet-location appliance, so the earthing and isolation are not optional-nice-to-haves, they are the safety basis of the installation. A licensed electrician sizes all of it to your specific unit.

The older-flat loading check

Instant heaters are one of the heavier loads a home adds, so in a flat wired for lighter demand they can press against capacity. This is exactly the kind of addition our HDB electrical loading guide covers, the electrician checks your distribution board first and flags an upgrade if the load needs it, rather than discovering the limit through tripping breakers.

Plumber or electrician?

Usually both, split cleanly: the plumber does the water; the electrician does the dedicated circuit, isolator and earthing. A like-for-like storage swap may need little electrical work; a new circuit or an instant heater is firmly licensed electrical territory. As a guide, a heater connection starts around S$50, more with a new circuit, see the price guide.

Installing a water heater?

Get the wiring sorted

Water heater electrical questions

Do instant water heaters need a dedicated circuit?

Yes. Instant heaters draw a high current because they heat water on demand, so they need a dedicated circuit with correctly sized cable, a suitable isolator and proper earthing, not a shared socket. The exact rating depends on the heater’s power, which is why a licensed electrician sizes the circuit to the specific unit.

What is the difference between storage and instant heater wiring?

A storage heater warms a tank gradually and draws a lower, steadier current, so its circuit is generally lighter. An instant heater heats water the moment you turn on the tap, drawing a much higher current in short bursts, so it needs a heavier dedicated circuit. Both require an isolator and earthing; the instant type asks more of your electrical loading.

Does a water heater need an isolator switch?

Yes. A fixed water heater should have a dedicated isolator so its power can be fully and safely cut for servicing or in an emergency, independent of other circuits. This is standard practice and part of a compliant installation, the same principle as an aircon isolator.

Can my flat’s wiring handle an instant water heater?

Sometimes it needs checking first. Instant heaters are among the heavier loads in a home, so in an older flat wired for lighter demand, adding one may push against the flat’s capacity. A licensed electrician checks your loading and the distribution board before installing, and flags an upgrade if the load requires it.

Do I need a plumber or an electrician for a water heater?

Usually both, on different parts. The plumber handles the water connections; the electrician handles the dedicated circuit, isolator and earthing, the licensed electrical work. For a straight like-for-like storage-heater swap the electrical side may be minimal, but any new circuit or an instant heater is firmly electrician territory.

How much does water heater electrical work cost?

As a market guide, a water heater electrical connection starts from around S$50, with a dedicated circuit and isolator adding more depending on rating and cable run, a 20A isolator is around S$160. An instant heater needing a new heavy circuit costs more than a simple storage swap. Our price guide has the ranges.