Size to the peak hour, not the headcount
Most “what size water heater do I need” advice starts with bedrooms or people, but a storage heater does not care how big your house is — it cares about the single busiest hour, when showers, laundry and the dishwasher pile up at once. Size to that hour and everyone gets a warm shower; size to the daily average and someone gets a cold one. The figure that matters is your peak-hour demand in gallons (or litres), and the spec you match it to is the heater’s first-hour rating.
First-hour rating beats nominal gallons
A 50-gallon tank does not deliver 50 gallons of hot water in a busy hour. As you draw hot water off the top, cold water flows in at the bottom and mixes, so only about 70% of the stored volume comes out genuinely hot before the temperature sags. The first-hour rating (FHR) captures this: it is roughly 70% of the tank volume plus whatever the burner or element can reheat within that hour. That is why two tanks of identical gallons can perform very differently — a fast-recovering gas model adds far more during the hour than a slow electric one.
first-hour rating ≈ 0.70 × tank gallons + recovery (gal/hr)
size so FHR ≥ peak-hour demand
Recovery rate, and why gas is faster than electric
Recovery rate is how many gallons the heater can reheat per hour. It comes straight from the energy in the water: raising a gallon one degree Fahrenheit takes about 8.33 BTU, so the gallons reheated per hour are the input energy times efficiency divided by 8.33 times the temperature rise.
recovery (gal/hr) = (input BTU/hr × eff) ÷ (8.33 × ΔT°F)
electric: input BTU/hr = kW × 3412
A gas burner pours in far more BTU per hour than an electric element’s equivalent, so gas recovers more water per hour at the same temperature rise — which is exactly why a smaller fast gas tank can out-deliver a larger slow electric one. And because a colder inlet means a bigger rise, every gallon costs more energy, so recovery slows in winter for either fuel.
Worked example
Take a 40,000 BTU/hr gas burner at 0.95 efficiency, raising water 80 °F (a 40 °F inlet to 120 °F):
recovery = (40,000 × 0.95) ÷ (8.33 × 80) ≈ 57 gal/hr
FHR on a 50-gallon tank ≈ 0.70 × 50 + 57 ≈ 92 gal
So that 50-gallon tank behaves like a 92-gallon delivery in the first hour — comfortably covering a medium-use family of four (around 72 gallons of peak demand), with headroom to spare. Switch the same job to a 4.5 kW element (about 15,400 BTU/hr) and recovery drops to roughly 22 gal/hr, dragging the FHR down to about 57 gallons — the same tank, far less delivered, because electric reheats so much more slowly.
A rough people-to-size guide (estimate, not a manufacturer chart)
These are computed from the peak-hour rule of thumb — light, medium and heavy use at roughly 12, 18 and 24 gallons per person — not lifted from any manufacturer’s sizing sheet. Treat them as a sanity check, then match a real unit’s first-hour rating to the demand.
| Household | Light (12/person) | Medium (18) | Heavy (24) |
|---|---|---|---|
| 1–2 people | ~24 gal | ~36 gal | ~48 gal |
| 3 people | ~36 gal | ~54 gal | ~72 gal |
| 4 people | ~48 gal | ~72 gal | ~96 gal |
| 5 people | ~60 gal | ~90 gal | ~120 gal |
| 6 people | ~72 gal | ~108 gal | ~144 gal |
The figures are first-hour-rating targets in gallons, not tank capacities — a 50-gallon gas tank can meet a 72-gallon target because its recovery tops up the stored volume during the hour. For a hot water cylinder, switch the tool to litres and read everything through the same lens.
Bigger is not automatically better
An oversized tank costs more up front and loses more heat sitting idle, because a larger volume is kept hot around the clock whether you use it or not. Size to your real peak hour with modest headroom rather than rounding far up. If your hot water is spread through the day instead of concentrated in one rush, a smaller tank with brisk recovery often serves better than a large, slow one — and however you land, confirm the choice and the gas or electrical supply with a qualified installer and local code.