What Size Water Heater Do I Need? Capacity Guide

“What size water heater do I need?” is the most common question we are asked, and the one most often answered badly. The usual reply points at household size, which is only half the story. Two families of four can need very different systems depending on when they use hot water, not how much they use in total.

This guide gives you the actual method, with figures you can work with, for homes and for commercial buildings.

The Rule That Governs Everything: Peak Hour

Taille sur peak-hour demand, not daily consumption. This single distinction explains most disappointing installations.

Hot water is not drawn evenly across a day. It concentrates into short windows, typically morning and evening. A household using 300 litres spread across twelve hours can be served by a modest tank. The same household using 200 of those litres between 6:45 and 7:30 needs considerably more stored volume, or much faster recovery.

So the question is not “how much hot water do we use?” It is “how much do we need within our busiest hour?”

Step 1: Count Your Peak Hour Draw

Work out what actually happens in your busiest sixty minutes. These are practical working figures for hot water content, not total water volume:

Add up only what occurs inside your peak hour. If three people shower in that window with standard heads, you are looking at roughly 120 to 180 litres before anything else is counted.

Step 2: Adjust for Recovery

Your tank is not frozen during that hour. It is reheating while being drawn, so you do not need stored volume equal to the entire peak.

How much reheating helps depends on the power rating, and this is the specification most buyers ignore. Capacity tells you how much hot water you have; power rating tells you how fast it comes back. A higher rating lets a smaller tank serve the same peak, which is why capacity and power must be chosen together rather than separately.

As a working approach: if your demand is spread across the hour, a tank somewhat smaller than your peak figure will cope. If your demand is compressed into twenty minutes, size closer to the full peak, because recovery has almost no time to contribute.

Residential Sizing Guide

Notice where the change happens. Up to two bathrooms used in sequence, a single storage heater works. Once two or more run at the same time, you move to a central system, because adding a second small tank costs more to buy, install and maintain than one correctly sized unit.

Small Capacities for Single Fixtures

Not every outlet needs a bathroom-scale tank. For a kitchen sink, guest washroom or staff area, an 8 to 26 litre compact unit is the correct answer, and installing a 50 litre tank there simply heats water nobody uses.

The same logic applies to outlets far from your main heater. A local unit avoids both the wait and the heat lost down a long cold pipe every time the tap is opened. Where use is genuinely occasional, an instant heater eliminates standby loss entirely.

Commercial Sizing

The same principle applies, but the peak is sharper and the cost of getting it wrong is higher.

Commercial buildings also carry loads that are easy to omit: kitchens, on-site laundry, staff facilities and cleaning. Sizing on guest or occupant count alone regularly produces systems that underperform. See our commercial hot water systems guide for the full method.

Three Factors That Shift the Answer

• Incoming water temperature. Colder mains water requires more energy and more time to reach your set temperature, which effectively reduces usable capacity in winter.
• Fixture flow rates. A rainfall shower head can use nearly double a standard head. Two of them change the calculation substantially.
• How compressed your peak is. The same total demand in twenty minutes rather than sixty needs materially more stored volume, because recovery has no time to help.

Why Bigger Is Not Safer

Oversizing feels like the cautious choice. It is not free.

A tank holds its full volume at temperature continuously, losing heat through its walls every hour of every day whether you use that water or not. An oversized tank pays that penalty permanently, in exchange for capacity you never draw. It also costs more to buy and occupies space you may need.

Correct sizing beats generous sizing. If you are genuinely between two capacities, size up modestly rather than dramatically, and put the difference into better insulation and an adequate power rating instead.

Questions fréquemment posées

What size water heater for a family of four?

With one bathroom used in sequence, 80 to 100 litres is a common starting point. If two bathrooms run simultaneously, move to a central system of 200 to 300 litres rather than adding a second small tank.

What size do I need for 6 bathrooms?

At that scale you are specifying a central system, typically 500 litres and above, with the exact figure driven by how many of those bathrooms are used at the same time. Recovery rate matters as much as capacity here.

Is a 6 gallon (about 25 litre) heater enough?

For a single handbasin, kitchen sink or guest washroom, yes. For a shower, no. Capacities in that range are point-of-use units, not bathroom heaters.

My hot water runs out. Do I need a bigger tank?

Not necessarily. It may be a recovery-rate problem, or sediment insulating the element and extending heating times. Check both before assuming you need more litres.

Does capacity mean the same as usable hot water?

Not exactly. Stored water is usually hotter than what leaves the tap, and it mixes with cold at the outlet, so usable output can exceed nominal capacity. But incoming cold water also dilutes the tank as you draw, so treat capacity as a guide rather than a precise ration.

Have Us Size It Properly

Send us your fixture count, how many outlets run simultaneously, occupancy and building type, and our engineers will calculate both the capacity and the power rating you need, rather than rounding to the nearest stocked size.

Courriel sales@eauchauffage.com ou appeler +1 (905) 299-3382. Related: how to choose a water heater in 6 steps et 5 buying mistakes to avoid.

Once you know your capacity, see central vs individual water heaters to decide whether one unit or several costs less.

Sizing a solar system? It has a second dimension. See how to size a solar water heater for your roof.