The 64 models in this registry rated near 65 gallons (±5 gal) file First Hour Ratings from 76 to 100 gallons — the same nominal tank size can answer very differently depending on fuel type and recovery speed, which is why a fixed "65 gallons = enough for N people" rule has no single source behind it.
DOE's own method, not a per-person guess
First Hour Rating (FHR) must be >= peak hour demand — the sum of what you actually use within the busiest single hour, not a headcount. DOE Energy Saver -- Sizing a New Water Heater (peak hour demand worksheet) publishes gallons per use for six fixtures:
| Use | Gallons |
|---|---|
| One shower | 20 |
| Shaving | 2 |
| Hand dishwashing / food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer (top-loader) | 25 |
| Clothes washer (front-loader) | 15 |
Add up what your household actually uses in one busy hour on the water heater selector to get your own First Hour Rating target, then compare it to the range above.