Example Data Table
| Scenario |
Peak fixtures |
Peak gallons |
Suggested tank |
Best use |
| Small home |
1 shower, faucet use |
34 gal |
40 gal |
Light daily demand |
| Family rush |
2 showers, laundry, dishwasher |
75 gal |
60 gal |
Busy morning use |
| Large bath load |
1 bath, 2 showers |
92 gal |
75 gal |
High peak comfort |
Formula Used
Peak mixed gallons = shower gallons + bath gallons + faucet gallons + laundry gallons + dishwasher gallons.
Shower gallons = showers × minutes per shower × shower flow rate.
Mixing ratio = (target outlet temperature − inlet temperature) ÷ (storage temperature − inlet temperature).
Stored hot gallons needed = peak mixed gallons × mixing ratio.
Recommended tank size = stored hot gallons needed ÷ usable drawdown percent.
Recovery gallons per hour = output BTU per hour ÷ (8.34 × temperature rise).
First hour delivery = usable mixed gallons from tank + recovery gallons per hour.
Annual cost = daily energy units × 365 × entered energy cost.
How To Use This Calculator
- Enter the number of people and the busiest hot water window.
- Add showers, baths, faucets, laundry, and dishwasher loads.
- Enter inlet, outlet, and tank storage temperatures.
- Add current tank size, drawdown, input rating, and efficiency.
- Select the energy type and enter the correct energy price.
- Press calculate to view sizing, recovery, cost, and shortfall.
- Use CSV or PDF export to save the current result.
Why This Water Heater Calculator Helps
Choosing a water heater is not only about tank gallons. Peak demand matters. Recovery speed matters too. A small household can still need strong delivery when showers, laundry, and dishes overlap. This calculator converts normal usage into mixed hot water demand. It then estimates storage needs, first hour delivery, recovery, and yearly energy cost.
How Peak Demand Is Estimated
Peak demand is built from fixture use. Shower gallons come from shower count, minutes, and flow rate. Bath, faucet, washer, and dishwasher values are added to that peak window. The tool treats these gallons as usable mixed water at the target outlet temperature. That makes the result easier to read because people use mixed water, not tank temperature water.
Why Temperature Rise Matters
Cold inlet water changes the answer. A heater in a warm climate works less than one fed by cold winter water. Temperature rise is the target outlet temperature minus inlet temperature. A larger rise needs more stored energy and slower recovery. The storage temperature also affects how much mixed water one tank can deliver.
Understanding Tank Size And Recovery
A tank does not deliver every gallon at the set temperature. The calculator uses a drawdown percentage to allow for mixing and cooling inside the tank. It also estimates recovery from input power and efficiency. Faster recovery can support a smaller tank, while low recovery often needs more storage.
Using The Result Wisely
Use the recommended tank size as a planning number. Compare it with model ratings. The first hour result is useful for families. The yearly cost estimate is only a guide because rates, standby losses, and habits vary.
Good Data Improves Results
Measure flow rates when possible. Count real fixture use during the busiest hour. Use a lower inlet temperature for conservative sizing. Keep the outlet temperature safe. High storage settings may require a mixing valve. The calculator is not an official product selector. It is a helpful planning step before reviewing manuals, warranties, and professional advice. It can compare gas, electric, and heat pump choices. Change one input at a time. This shows which habit or specification affects capacity most. Save exported results. Share them with installers for clearer discussions.
FAQs
Is this an official AO Smith selector?
No. It is a planning calculator for estimating demand, recovery, and cost. Always compare results with product documents, local rules, and installer guidance before buying equipment.
What does first hour delivery mean?
First hour delivery estimates how many mixed gallons the heater can supply in one hour. It combines usable stored water and recovery during that hour.
Why does inlet temperature matter?
Cold inlet water needs more heat. Lower inlet temperature increases temperature rise, reduces recovery speed, and can increase the tank size recommendation.
What drawdown percent should I use?
Many tank estimates use about 70 percent. Use a lower value for conservative planning. Use manufacturer ratings when you know the exact model.
Can this compare gas and electric heaters?
Yes. Select the energy type and enter the correct input rating. Gas uses BTU per hour. Electric and heat pump entries use kilowatts.
How accurate is the yearly cost?
It is an estimate. Real cost depends on water habits, standby loss, incoming temperature, energy rates, maintenance, and actual equipment performance.
Why is storage temperature different from outlet temperature?
Tank water may be stored hotter than delivered water. A mixing valve can blend hot and cold water to provide safer outlet temperatures.
When should I call a professional?
Call one when changing fuel type, venting, wiring, tank location, safety valves, or capacity. Hot water systems must meet local code.