How to size a water heater tank for your household
The right hot water tank size depends on how much hot water your household uses in its busiest hour, not on the number of people alone. The first-hour rating on the unit's label, which combines tank volume and recovery speed, is the number to compare against that peak.
What does tank size actually mean?
Tank capacity is the volume of water the tank holds, usually shown in litres or gallons on the label. It is only part of the story. A smaller tank with a fast burner can deliver more hot water in an hour than a larger tank with a slow element, because it reheats as it is drawn down.
That is why sizing guides use the first-hour rating, which estimates how much hot water the unit can supply in the first hour of heavy use starting from a full, hot tank. Compare that figure with your busiest hour and you have a sound basis for choosing. Capacity and recovery also vary by fuel, so see natural gas versus electric heating when the fuel is still open.
How do you estimate your peak hour demand?
List what usually runs together in the busiest hour, typically a weekday morning. Give each use a rough volume of hot water, then add them up. The numbers below are assumptions for illustration, and your fixtures may differ.
- Two showers of about eight minutes each at a modest flow rate, roughly 130 L of hot water in total once cold water is mixed in.
- A dishwasher cycle and a hand-washed pan, roughly 20 to 30 L.
- A washing machine on a warm setting, roughly 30 to 40 L.
That adds to around 180 to 200 L in the peak hour. A tank with a first-hour rating at or just above that figure would cope. Plumbing fixtures with low-flow ratings reduce the total, and a soaker tub raises it sharply. Your own list will give a better answer than any rule of thumb.
What happens if the tank is too small or too big?
A tank that is too small runs out during the morning rush, and the water turns lukewarm halfway through a shower. Raising the thermostat to compensate is not a good fix, because it raises scald risk and energy use. A tank that is too large holds extra heated water that loses heat through its walls all day, so you pay for standby loss and often a higher purchase price.
Household changes matter. Children growing up, a new bathroom or a move to low-flow showerheads all shift the answer. If a household's demand is highly variable, tank or tankless is a decision worth revisiting, since an on-demand unit is sized by flow rather than storage.
Does a solar or heat pump system change the size?
Yes. A solar preheat tank is sized against the collector area and the household's daily use, and it normally sits in front of a regular backup heater. Our guide to solar hot water cost in Canada describes how storage and collectors fit together. Heat pump water heaters have slower recovery in heat pump mode, so installers often choose a slightly larger tank, as described in heat pump water heaters in Canada.
Drake Landing Solar Community stored heat in two 120 cubic metre tanks and a borehole field to carry the supply across the seasons, which is a very different scale. See the Energy Centre page for how that storage was used.
Quick answers
What size hot water tank does a family of four need?
It depends on peak-hour use, not just head count. Add up what runs together in your busiest hour and pick a tank whose first-hour rating covers it.
What is a first-hour rating?
It is the volume of hot water a tank can supply in one hour of heavy use starting full. It is shown on the unit's label or in the manufacturer's data.
Can I use a bigger tank to be safe?
A tank that is too large costs more to buy and loses more heat while idle. Sizing to your real peak is usually the better choice.