What size water heater do I need?

Size it for the busiest hour rather than the whole day. Three back to back showers usually needs a tank around 100 litres, allowing that you never get the full tank as usable hot water.

How many litres of hot water does a shower use?

An eight minute shower at nine litres a minute is about seventy litres in total, of which roughly forty is hot in normal conditions.

Why does the hot water run out before the tank is empty?

Cold water entering the bottom mixes with the hot leaving the top. After about seventy per cent of the tank, what comes out is no longer hot enough to use.

Can I switch my water heater off in summer?

Many homes here do. Water sitting in a roof tank gets warm on its own for several months, so the heater has almost nothing to do and switching it off saves real electricity.

Is an instant water heater better than a tank?

It depends on the home. Instant units waste nothing standing by and never run out, but they need a large electrical supply and struggle when several outlets run at once.

Should the heater be near the bathroom?

Yes, where possible. Every metre of pipe between the heater and the tap is water you run off and waste while waiting for hot to arrive.

Water Heater Size Calculator

Showers in the busiest hour

How many people shower back to back in the morning.

Minutes per shower

min

Shower flow

Hot water in the mix

Other hot water in that hour

Basins, kitchen sink, washing up.

Usable share of the tank

Cold coming in mixes with hot going out, so you never get the full tank.

Your answer

Heater size to fit

250 litres

You need about 150 litres of usable hot water in that hour.

Water used by showers: 216 L
Hot water in those showers: 130 L
Other hot water: 20 L
Usable hot water needed: 150 L
Tank size before rounding: 214 L

Sizing is for the busiest hour. Over a full day a smaller heater usually copes fine, because it has time to reheat between uses.

Water Heater Size Calculator for Dubai and the UAE

This tool works out what size water heater a home needs. It sizes for the busiest hour of the day, not the whole day, because that is what actually decides whether somebody gets a cold shower.

A heater refills and reheats between uses. What matters is whether it can cover the morning rush before it runs out.

The answer comes in litres, and it tells you which standard tank size to buy.

Why the busiest hour is the number that matters

A water heater does not need to hold a whole day of hot water. It needs to get through the worst hour.

In most homes that is the morning, when several people shower one after another. If the heater covers that, the rest of the day looks after itself, because the tank reheats while nobody is using it.

So sizing from daily use gives you a tank far bigger than you need. Sizing from the busy hour gives you the right one.

The other thing that matters is how fast it reheats. A small tank that recovers quickly can outperform a bigger one that recovers slowly, if the showers are spread out rather than back to back.

You never get the whole tank

This is the part that surprises people. A hundred litre heater does not give you a hundred litres of hot water.

As hot water leaves the top of the tank, cold water comes in at the bottom to replace it. The two mix at the boundary, and the water coming out gets steadily cooler.

By the time you have drawn about seventy per cent of the tank, what is coming out is no longer hot enough to shower in.

So a hundred litre tank gives roughly seventy litres of genuinely usable hot water. That is what the usable share setting in the tool is for.

A well designed tank with a good baffle mixes less and gives you more. A cheap one mixes more and gives you less.

The UAE summer changes everything

For much of the year, the sizing above matters far less than it does elsewhere, and it is worth being clear about why.

Cold water here is not cold in summer. It sits in a tank on a roof, in full sun, and it arrives at the heater already warm.

That means two things. The heater has very little work to do, so it recovers almost instantly. And the hot share in a shower drops a long way, because you need much less hot water to reach a comfortable temperature.

A lot of households switch the heater off completely from around May to September and never notice. That is a real saving on electricity for months at a time.

So size the heater for winter, when it is doing real work, and then remember you probably will not need it for half the year.

Storage or instant

Both are common here and they suit different situations.

Storage tank
A tank that heats water and holds it. Handles several outlets at once, works with any pressure, and takes up space. The standard choice in villas and most flats.
Instant or tankless
Heats water as it flows. No standing loss and no waiting, but it needs a large electrical supply and it limits how many outlets can run at once.
Small under sink unit
A few litres, fitted right at a basin. Good where a single tap is a long way from the main heater.
Central for the whole home
One larger tank feeding everything. Tidy, but hot water has to travel, so distant taps run cold for a while first.
Solar
Makes obvious sense with this much sun. Usually paired with an electric element for the days it cannot keep up.

Getting sizing wrong, both ways

The symptoms are different and they both cost you something.

Too small: the last person gets cold
The classic sign. The tank empties partway through the third shower and the temperature drops away.
Too small: long waits between uses
If a bath cannot be run without waiting an hour afterwards, the tank is undersized for the way the home is used.
Too big: standing losses
A large tank keeps a large volume hot all day whether anybody uses it or not. That is electricity spent on nothing.
Too big: wasted space
Water heaters are usually squeezed into a ceiling void or a cupboard. A bigger one is harder to fit and harder to service.
Wrong place
A heater far from the bathrooms means a long wait and a lot of cold water run off before hot arrives, every single time.

Questions people ask

What size water heater do I need?: Size it for the busiest hour rather than the whole day. Three back to back showers usually needs a tank around 100 litres, allowing that you never get the full tank as usable hot water.
How many litres of hot water does a shower use?: An eight minute shower at nine litres a minute is about seventy litres in total, of which roughly forty is hot in normal conditions.
Why does the hot water run out before the tank is empty?: Cold water entering the bottom mixes with the hot leaving the top. After about seventy per cent of the tank, what comes out is no longer hot enough to use.
Can I switch my water heater off in summer?: Many homes here do. Water sitting in a roof tank gets warm on its own for several months, so the heater has almost nothing to do and switching it off saves real electricity.
Is an instant water heater better than a tank?: It depends on the home. Instant units waste nothing standing by and never run out, but they need a large electrical supply and struggle when several outlets run at once.
Should the heater be near the bathroom?: Yes, where possible. Every metre of pipe between the heater and the tap is water you run off and waste while waiting for hot to arrive.

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