Three numbers, one useful figure
A storage tank carries three published volumes. Nominal volume (40 US gallons, 151 L) is the model's marketing size. Rated storage volume (38 US gallons, 144 L on the RG240T6N) is the tested volume used by regulators, often printed only on the manufacturer's full data sheets. First-hour rating is the figure that matters for sizing: how much hot water the tank can deliver in a busy hour starting from a full, hot tank.
The first-hour rating is not a mystery number. It is approximately the stored volume plus the burner's recovery during that hour. The RG240T6N stores about 38 to 40 gallons and recovers about 43 US gal/h at a 90 °F (50 °C) rise, which is how Bradford White's Canadian sheet arrives at a 75-gallon first-hour figure. A high-input tank such as the RG275H6N pushes the same arithmetic to a 135 US gallon first-hour rating by recovering 82 US gal/h.
Matching the tank to the busiest hour
Sizing starts with the hour of the week when the most hot water leaves the house at once: weekday mornings usually, or evenings when showers, laundry and dishes overlap. A planning rule of thumb is 150 to 190 L of first-hour delivery for one or two people, 230 to 280 L for three or four, and 300 L or more for five plus, but the rule is only a starting point. The assessment counts the actual fixtures that run at once and the flow of each.
A 40-gallon tank with a strong burner often covers a four-person household because recovery refills it during the hour. A 50-gallon tank adds stored headroom, not unlimited headroom: its first-hour figure on the same family of tanks is only about 23 L higher, because the burner, not the tank, limits the first hour.
Tankless sizing is a different calculation
A tankless unit stores nothing, so its capacity is a flow rate at a temperature rise, and the rise in January decides the number. Metro Vancouver's incoming water can sit below 8 °C, so delivering 49 °C at a shower is roughly a 40 °C rise. At that rise a 199,000 BTU/h condensing unit delivers far less than its brochure maximum, which is why the demand assessment checks the manufacturer's flow-at-rise table rather than the GPM on the box.
Minimum flow matters on the low end too: a tankless unit needs enough draw to fire, about 0.4 US GPM (1.5 L/min) on the Rinnai RU series, so a trickle from a low-flow fixture may not activate it.
Where recovery shows up
Recovery is the rate at which the burner reheats the tank, published in gallons per hour at a stated rise. It separates two tanks of the same nominal volume: the RG250T6N and RG250S6N are both 50-gallon tanks, but the S6N's 50,000 BTU/h input recovers 204 L/h against 163 L/h for the T6N, so it comes back faster between draws even though its first-hour test figure is lower. Homes with long, spread-out evening use value recovery; homes with one crushing morning peak value first-hour delivery.
What a CANRO assessment does
CANRO's free written estimates for replacements and installations start from this arithmetic: fixtures and simultaneous use, the winter rise, the existing flue or vent route, gas supply and floor space. The instant estimator on this site gives a CAD ballpark first, and the written estimate follows the site visit.