Too small and the last shower runs cold; too big and the wiring cannot take it. How to choose the right size.
Instant water heaters are sized by power in kW; storage heaters by tank size in litres. ClickBina's water heater types guide puts a typical instant heater at 3.5–5.5 kW per bathroom and storage tanks at 15–80 L, with 30–50 L as the usual choice for a family terrace or semi-D. A multipoint instant unit of 11–16 kW can serve two or three bathrooms but needs a heavier electrical supply. Size for the busiest hour of the day, not the average, and check the wiring before choosing a bigger unit. ClickBina's guides put a basic instant heater at RM280 – RM530 installed and a 30–50 L storage heater at RM950 – RM1,800 (indicative 2026, Klang Valley). A site visit is RM150, waived if you go ahead.
An instant heater has no tank. It heats water as it flows, so its size is its power: more kW means it can heat more water per minute, or heat the same flow to a higher temperature. A storage heater keeps a tank of hot water ready, so its size is the tank: more litres means more people can shower before the water runs cold. The water heater types guide compares the types; this guide is about choosing the size within a type.
Getting the size wrong is common in both directions. Too small, and the second shower of the morning runs lukewarm. Too big, and you pay for heat you never use, or the wiring cannot carry the load.
The types guide gives a typical power of 3.5–5.5 kW per unit for single-bathroom instant heaters. Within that range, the trade-off is simple: for the same heater, a stronger flow comes out cooler and a gentler flow comes out hotter, because the same power is spread over more or less water. A rain shower head that pours a lot of water needs more heating power than a small hand shower to feel equally hot.
Where the problem is weak water pressure rather than heat, a bigger heater does not help. The instant heater with pump guide explains when a built-in pump is the better answer, and when the whole house needs a booster pump instead.
Every instant heater needs its own circuit. The water heater installation cost guide quotes the Suruhanjaya Tenaga water heater guideline's table: 4 mm² copper live, neutral and earth for heaters from 4,462 W to 5,711 W, protected at 32 A, rising to 6 mm² above 5,711 W, with the heater on its own dedicated circuit and an RCD under 10 mA. The storage heater parts guide sets out the same rule for storage heaters: a dedicated circuit, an RCD of not more than 10 mA and a double-pole switch or isolator.
The practical point: before you pick a more powerful heater, check what the existing heater point was wired for. Swapping a small heater for a bigger one on the old cable is how heater circuits end up running warm. Older homes often need a new dedicated circuit; the installation guide prices it separately.
A multipoint instant heater is one high-capacity unit piped to several outlets. The types guide puts these at typically 11–16 kW, serving two or three bathrooms and the kitchen, and notes they need a heavier single-phase or a three-phase electrical supply and careful pipe planning. That electrical demand is the deciding factor in most homes; the three-phase upgrade guide explains when a home actually needs it. Where the supply cannot take a multipoint unit, a storage heater or one instant heater per bathroom is the usual alternative.
Storage heaters are where litres matter. The types guide gives a range of 15–80 L for home tanks; the installation cost guide describes storage heaters as holding 15–50 L. In the storage heater parts guide's price table, a 15 L heater is listed as suiting one bathroom and a 30–50 L heater as suiting larger households with several outlets. A storage tank also needs space, a wall or ceiling that can carry its full weight, and a drain for the pressure relief valve.
A tank holds a fixed amount of hot water and then needs time to reheat. Two or three people showering one after another is the test: if the last person regularly gets cold water, the tank is too small, the thermostat is set too low, or the element is failing.
These starting points come from ClickBina's water heater guides. They are a basis for the conversation with an installer, not a rule:
| Home and use | Common choice | Size quoted on ClickBina guides |
|---|---|---|
| Studio, condo, one bathroom at a time | Instant heater in each bathroom | 3.5–5.5 kW per unit (types guide) |
| Condo with weak shower pressure | Instant heater with built-in pump | See the instant heater with pump guide |
| One bathroom, prefers a tank | Small storage heater | 15 L, suits one bathroom (parts guide) |
| Terrace or semi-D family, morning rush | Storage tank | 30–50 L (types guide) |
| Several bathrooms from one unit | Multipoint instant or larger tank | 11–16 kW, or a 30–80 L tank (types guide) |
The water heating article notes that hot water of about 50 °C is usual for showering, and that the competing risks of scalding and bacteria such as Legionella are commonly balanced by setting the thermostat to 55 °C. European guidance it cites recommends storing hot water at 60 °C. It also notes that skin burns in less than 5 seconds at 60 °C, which is why older people and children are most at risk.
This matters for sizing. Water stored hotter is mixed with more cold water at the shower, so the same tank delivers more usable warm water. Water stored cooler feels like a smaller tank. A thermostatic shower mixer lets you store water hot and still shower safely; the shower mixer guide explains how they work.
The central hot water system guide puts the rule simply: size the heater for the busiest time of day, not the average. A family with three bathrooms in use every morning needs more stored hot water, or faster recovery, than a couple who shower at different times. Count how many showers run at the same time in the morning rush, and whether a bath is ever filled, before choosing the size.
A heat pump or a solar system changes the sizing conversation because both store water in a tank and heat it more slowly. The heat pump water heater guide and the solar water heater guide cover tank and roof sizing for those systems; the gas water heater guide covers LPG units and their venting. All three cost far more to install than an electric instant heater, so they suit large households with high hot-water use.
ClickBina's published water heater guides give these figures (indicative 2026, Klang Valley):
| Heater | Installed price | Source guide |
|---|---|---|
| Basic instant heater | RM280 – RM530 | Instant heater with pump guide |
| Instant heater with built-in pump | RM500 – RM950 | Instant heater with pump guide |
| Storage heater, 15 L | RM650 – RM1,250 | Storage heater parts guide |
| Storage heater, 30–50 L | RM950 – RM1,800 | Storage heater parts guide |
| New dedicated circuit, where needed | RM150 – RM350 | Installation cost guide |
| Heat pump water heater | RM3,500 – RM6,000 | Heat pump guide |
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💬 WhatsApp for a quoteWhatsApp us your job details for a quote: how many bathrooms, how many people, and whether the bathroom is being renovated. ClickBina is an experienced KL & Selangor contractor for whole-house refurbishment and trade coordination, so the heater, its circuit, the pipework and the tiling can be planned together. We reply within the hour, and the price agreed is a flat price with no hidden charges. A site visit is RM150, waived if you go ahead.
Methodology: sizes and prices are quoted from the ClickBina guide named beside each, without merging their ranges; temperatures are from the encyclopaedia article named. Sizes are starting points, not a design calculation.
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