Tank vs tankless water heaters
The two technologies solve different capacity questions: delivery over an hour and flow at a temperature rise.
| Question | Storage tank | Tankless |
|---|---|---|
| Sizing unit | First-hour rating in gallons, plus recovery and operating mode. | GPM at your winter temperature rise. |
| Demand input | Hot-water events in the busiest hour, even if sequential. | Fixtures running simultaneously and their actual flow. |
| A four-person example | Four worksheet showers + shaving + dishes = 46 gallons peak-hour demand. | Two 2 GPM showers + a 1 GPM faucet = 5 GPM; heat depends on inlet temperature. |
| Installation check | Space, recovery, operating mode, drainage and manufacturer instructions. | Fuel or electrical capacity, venting, minimum flow and manufacturer flow curves. |
| What capacity cannot tell you | Tank gallons alone do not establish first-hour delivery or annual cost. | Advertised maximum GPM does not establish cold-weather delivery. |
Bring the actual inputs.
Compare bids that identify the measured demand, the exact model, operating conditions and installation requirements. A headline capacity or broad household average is not enough to choose equipment.
Planning estimate. A licensed contractor must confirm final size and installation. Electrical work needs a licensed electrician.
Sources and methods.
Read the model methods and assumptions and primary source register. The linked calculators show their working and relevant sources for your selected inputs.