Estimate first-hour rating for a tank from your busiest hour, or flow and temperature rise for a tankless unit.
How it works
For tanks, adds the published gallons per use for your busiest hour. For tankless, adds simultaneous flows and multiplies by temperature rise and 500 to get BTU per hour.
Before you order
- Check the first-hour rating on the label, because tank size alone misleads.
- A cold inlet in winter makes tankless capacity much tighter.
- Heat pump water heaters list their first-hour rating in hybrid mode, so check that figure.
Common questions
Why do I enter showers twice?
The same field means showers in your busiest hour for tanks and showers at the same moment for tankless units.
Where do the gallons come from?
The shower, shaving and clothes washing figures come from the U.S. Department of Energy worksheet. Add anything else you use in that hour.