How the numbers are worked out
For tank heaters, the calculator estimates the first-hour rating your home needs from the number of people and bathrooms, then picks the smallest common tank size whose typical rating covers it. Electric tanks recover more slowly than gas, so electric homes are often pointed one size up. Heat pump heaters are sized on their first-hour rating in hybrid mode.
For tankless heaters, it adds up the flow of fixtures running at once: about 2 gallons per minute per shower, 1.5 for a kitchen faucet and 1.5 for a dishwasher or washer. Units are rated at different temperature rises, so compare models at a rise of about 70 degrees, which is what winter groundwater in Greenville needs to reach a comfortable shower.
Read more in What size water heater does your home need?
Sizing questions
Does the calculator work for both gas and electric homes?
Yes. Pick your fuel; electric and heat pump tanks are sized differently from gas because they recover more slowly.
Why size a tankless heater for winter?
Incoming water is much colder in January, so a tankless unit has to heat it more and delivers less flow. Sizing for winter keeps the second shower hot.