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Peak-Hour Demand vs Heater Capacity
Result Summary
| Recommended | — |
| Demand | — |
| Temperature rise | — |
| Annual operating cost | — |
| Configuration | — |
Typical Tank Size by Household
| People | Tank (gas) | Tank (electric) | First-hour rating |
|---|---|---|---|
| 1 – 2 | 30 – 40 gal | 30 – 50 gal | ~35 – 50 gal |
| 2 – 3 | 40 gal | 50 gal | ~50 – 60 gal |
| 3 – 4 | 40 – 50 gal | 50 – 80 gal | ~60 – 70 gal |
| 5+ | 50 – 75 gal | 80 gal | ~70 – 90 gal |
Typical Replacement Cost (installed)
| Type | Approx. installed cost |
|---|---|
| Electric tank | $1,000 – 2,000 |
| Gas tank | $1,200 – 2,800 |
| Heat pump tank | $2,500 – 4,000 |
| Tankless (gas) | $2,500 – 4,500 |
| Tankless (electric) | $1,500 – 3,000 |
Ranges vary by region, venting, and electrical upgrades. Use the Project Cost Calculator for a detailed estimate.
What Size Water Heater Do I Need?
Water heater sizing is about meeting your busiest hour of hot water use, not your daily total. For a tank, that means matching the first-hour rating to your peak-hour demand: add up the hot water used during the busiest hour, such as several back-to-back showers plus a dishwasher, and choose a tank whose first-hour rating meets or beats it. For a tankless unit, sizing is about flow: it must deliver enough gallons per minute at your required temperature rise to run every fixture you expect to use at once. The calculator above handles both, starting from your household and fixture inputs.
Tank vs Tankless Water Heater Sizing
A tank heater stores a reservoir of hot water and is sized by first-hour rating, the gallons it can deliver in one hour from a full, hot tank. It is cheaper to buy but runs out during heavy use and loses standby heat. A tankless heater never runs out but is limited by flow: it heats water as it passes through, so the higher your temperature rise and the more fixtures running at once, the bigger the unit you need. In cold climates with high incoming-water temperature rise, tankless capacity drops, so size for your coldest groundwater.
First Hour Rating Explained
The first-hour rating (FHR) is the most useful number on a tank water heater label. It combines the usable stored hot water with how fast the heater can reheat in that hour, the recovery rate. A 50 gallon gas heater might have an FHR of 80 to 90 gallons because gas recovers quickly, while a 50 gallon electric heater might show an FHR closer to 60 because it reheats slowly. Always compare your peak-hour demand to the FHR, not just the tank gallons, which is why gas and electric heaters of the same tank size are not interchangeable.
Frequently Asked Questions
Is a 40 or 50 gallon tank right for a family of four? Often a 40 gallon gas or 50 gallon electric, but it depends on your peak-hour demand, which the calculator estimates.
Why does gas need fewer gallons than electric? Gas recovers faster, so a smaller gas tank delivers the same first-hour rating as a larger electric one.
What size tankless do I need? Size by simultaneous flow at your temperature rise. Two showers at once in a cold climate can need 7 or more GPM.
Do I need an expansion tank? On a closed plumbing system, yes. See the Expansion Tank Calculator to size it.