The most common water heater mistake is not buying the wrong brand; it is buying the wrong size. Go too small and you run out of hot water during the morning rush. Go too big and you pay to keep water hot that nobody uses. And the number most people shop by, tank gallons, is not even the figure that predicts whether you will run out. Sizing a heater correctly is about matching the unit to how your household actually draws hot water, and it is worth getting right because you live with the choice for a decade.
This guide explains the number that matters, how to estimate your real peak demand, and what our climate and water add to the decision, so your replacement fits the house instead of the showroom.
The number that matters: first-hour rating
For a tank heater, the figure that predicts real-world performance is the first-hour rating, or FHR, not the raw gallon count. The FHR is how much hot water the unit can actually deliver in a busy hour, combining what is stored in the tank with how fast it can heat more. The first-hour rating is a standardized figure manufacturers publish on the unit's energy guide label precisely so you can compare heaters on delivered hot water rather than tank size, and two heaters with the same tank size can carry different first-hour ratings depending on how powerful the burner or element is. When you shop by gallons alone, you are ignoring the recovery speed, which is half the story of whether you run out.
The goal is simple: your heater's first-hour rating should meet or slightly exceed your household's peak-hour hot water demand. Match those two numbers and cold showers stop being a thing.
Estimating your peak-hour demand
Peak demand is not your daily total; it is the busiest hour, usually a weekday morning when showers, a shave, and maybe a dishwasher or laundry load stack up. Walk through your own worst hour and add up the hot water it uses. A shower runs in the neighborhood of ten to twenty gallons depending on length and showerhead, shaving and hand washing a couple of gallons, a dishwasher and a clothes washer several gallons each. Add up everything that could realistically run in that one hour, and that total is the first-hour rating you are shopping for.
Pick your busiest hour
Usually the weekday morning. Picture everyone getting ready at once, plus any appliance that tends to run then.
Add up the hot water it uses
Count each shower, each sink use, and any dishwasher or laundry that overlaps. Use realistic shower lengths, not best case.
That total is your target FHR
Shop for a heater whose first-hour rating meets or slightly exceeds that number, not one chosen by tank gallons alone.
Add margin for growth
Planning more people, longer showers, or a new bathroom soon? Size for the household you are about to have, not just today's.
Recovery is slower on some fuels
How fast a heater reheats, its recovery rate, depends on the fuel and the burner or element. Gas units generally recover faster than standard electric ones, which is why a gas heater can hit a given first-hour rating with a smaller tank, while an electric one may need more stored gallons to reach the same number. This is part of why a like-for-like swap by gallons alone can still leave you short if the fuel or the model changed.
| Household | Typical peak-hour demand | General direction |
|---|---|---|
| 1 to 2 people | Lower, one or two showers close together | A modest tank or a small tankless |
| 3 to 4 people | Moderate, stacked morning showers | A mid to larger tank sized by FHR |
| 5 or more people | High, several overlapping draws | A high-FHR tank or a sized tankless |
| Frequent guests or a rental | Spiky and unpredictable | Size for the busy weekend, not the average |
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CALL NOWCALL A PLUMBER NOW(251) 299-5221Where a Foley home adds wrinkles
Two local factors bend the sizing math. Homes with big soaking tubs, which are common in newer Foley builds, need enough hot water to fill the tub without draining the house, and a tub is a large single draw that a too-small heater cannot keep up with. And seasonal rentals and second homes see demand that swings hard, empty for weeks then full for a holiday weekend, so they are best sized for the busy stretch rather than the quiet average. Undersizing a rental to save on the average leaves guests with cold showers exactly when the house is full.
Bigger is not automatically better
It is tempting to oversize to be safe, but a tank far larger than you need heats and reheats water you never use, which costs you every month for a decade. The right size is the one that comfortably covers your real peak with a little margin, not the biggest one on the shelf. Matching the unit to the household is what saves money and prevents cold showers at the same time, and it is the part of a replacement worth slowing down for.
Questions Foley owners ask about this
Should I just buy the same size tank I have now?
Only if your current heater actually meets your needs. If you run out of hot water now, matching the old size just buys the same frustration. And if you are switching fuel or model, the same gallons can deliver a different first-hour rating, so size by demand rather than by copying the old label.
Is a bigger water heater always better?
No. An oversized tank keeps more water hot than you use, which wastes energy every month for the life of the unit. The right size covers your real peak-hour demand with modest margin. Bigger than that is money spent heating water that goes down the drain lukewarm.
How does a big soaking tub change the size I need?
A large tub is a single big hot-water draw that can empty a modest tank on its own. If filling the tub matters to you, the heater has to be sized to deliver that fill without leaving the rest of the house cold, which usually means a higher first-hour rating than the room count alone would suggest.
Ready to size a replacement the right way?
Call and describe your household and your busiest hour. We will size the unit to what you actually use, not to the showroom.