Incoming cold water temperature by state
Temperature rise is the number that sizes a tankless heater, and it starts with how cold your supply arrives. Shallow groundwater sits close to the mean annual air temperature of its location, which is the basis for the figures below.
These are guidance, not measurement. A supply drawn from a surface reservoir can run colder in winter than groundwater would. Measuring your own cold tap with a thermometer in the coldest month beats any table, and takes a minute.
| State | Incoming water | Rise to 120 F | 199,000 BTU unit delivers | Showers at once |
|---|---|---|---|---|
| Alabama | 62 F | 58 F | 6.4 GPM | 3 |
| Alaska | 38 F | 82 F | 4.5 GPM | 2 |
| Arizona | 65 F | 55 F | 6.7 GPM | 3 |
| Arkansas | 58 F | 62 F | 6.0 GPM | 2 |
| California | 60 F | 60 F | 6.2 GPM | 2 |
| Colorado | 47 F | 73 F | 5.1 GPM | 2 |
| Connecticut | 45 F | 75 F | 4.9 GPM | 2 |
| Delaware | 52 F | 68 F | 5.4 GPM | 2 |
| District of Columbia | 52 F | 68 F | 5.4 GPM | 2 |
| Florida | 72 F | 48 F | 7.7 GPM | 3 |
| Georgia | 62 F | 58 F | 6.4 GPM | 3 |
| Hawaii | 75 F | 45 F | 8.2 GPM | 3 |
| Idaho | 47 F | 73 F | 5.1 GPM | 2 |
| Illinois | 47 F | 73 F | 5.1 GPM | 2 |
| Indiana | 49 F | 71 F | 5.2 GPM | 2 |
| Iowa | 45 F | 75 F | 4.9 GPM | 2 |
| Kansas | 52 F | 68 F | 5.4 GPM | 2 |
| Kentucky | 54 F | 66 F | 5.6 GPM | 2 |
| Louisiana | 66 F | 54 F | 6.8 GPM | 3 |
| Maine | 42 F | 78 F | 4.7 GPM | 2 |
| Maryland | 52 F | 68 F | 5.4 GPM | 2 |
| Massachusetts | 44 F | 76 F | 4.9 GPM | 2 |
| Michigan | 43 F | 77 F | 4.8 GPM | 2 |
| Minnesota | 40 F | 80 F | 4.6 GPM | 2 |
| Mississippi | 64 F | 56 F | 6.6 GPM | 3 |
| Missouri | 53 F | 67 F | 5.5 GPM | 2 |
| Montana | 42 F | 78 F | 4.7 GPM | 2 |
| Nebraska | 48 F | 72 F | 5.1 GPM | 2 |
| Nevada | 55 F | 65 F | 5.7 GPM | 2 |
| New Hampshire | 43 F | 77 F | 4.8 GPM | 2 |
| New Jersey | 49 F | 71 F | 5.2 GPM | 2 |
| New Mexico | 57 F | 63 F | 5.9 GPM | 2 |
| New York | 44 F | 76 F | 4.9 GPM | 2 |
| North Carolina | 58 F | 62 F | 6.0 GPM | 2 |
| North Dakota | 39 F | 81 F | 4.6 GPM | 2 |
| Ohio | 48 F | 72 F | 5.1 GPM | 2 |
| Oklahoma | 58 F | 62 F | 6.0 GPM | 2 |
| Oregon | 50 F | 70 F | 5.3 GPM | 2 |
| Pennsylvania | 47 F | 73 F | 5.1 GPM | 2 |
| Rhode Island | 45 F | 75 F | 4.9 GPM | 2 |
| South Carolina | 63 F | 57 F | 6.5 GPM | 3 |
| South Dakota | 43 F | 77 F | 4.8 GPM | 2 |
| Tennessee | 57 F | 63 F | 5.9 GPM | 2 |
| Texas | 66 F | 54 F | 6.8 GPM | 3 |
| Utah | 50 F | 70 F | 5.3 GPM | 2 |
| Vermont | 41 F | 79 F | 4.7 GPM | 2 |
| Virginia | 55 F | 65 F | 5.7 GPM | 2 |
| Washington | 48 F | 72 F | 5.1 GPM | 2 |
| West Virginia | 51 F | 69 F | 5.4 GPM | 2 |
| Wisconsin | 41 F | 79 F | 4.7 GPM | 2 |
| Wyoming | 43 F | 77 F | 4.8 GPM | 2 |
The last two columns are the largest residential gas unit available, at 0.93 efficiency. They show how much the same equipment varies across the country.
Why the spread matters so much
The coldest and warmest states differ by more than 37 degrees of incoming water. Against a 120 F target that is the difference between a 45 degree rise and a 82 degree rise, which is to say the same heater does roughly 1.8 times as much work in one place as the other.
This is why a national recommendation for "the best tankless water heater for a 3 bathroom house" cannot be right. The question has no answer without a location.