What size water heater do I need?
Size a tank or heat pump unit by first-hour rating against your peak-hour demand, not by gallons; size tankless by simultaneous flow at your coldest incoming water temperature.
The DOE’s worksheet adds up hot water used in your busiest hour: about 20 gallons per shower, 7 per automatic dishwasher, 25 per top-loading wash. Its example totals 66 gallons, so that home needs a first-hour rating of 66 or more.
For tankless, add the flow rates you run at once and size for the temperature rise from your winter inlet water. Gas units typically deliver about 5 gpm at a 70°F rise; electric about 2 gpm.
Heat pump tanks recover slowly, so installers typically go one size larger than the tank being replaced.
Confidence: multi-source · Based on 2 cited sources.
Sources
- Sizing a New Water Heater · U.S. Department of Energy, Energy Saver
- Tankless Water Heaters: Gas vs Electric (2026) · HVACBase
Read the complete guide
Best Water Heater for the Money: Tank, Tankless or Heat PumpBy Purchasd Research. We used DOE and ENERGY STAR material for sizing and heat pump requirements; an independent HVAC reference model for running costs; contractor and cost-guide ranges for installed and repair prices; manufacturer warranty documents for transfer terms; city utility guides for serial-number dating; and owner and installer forums, a Reddit aggregation, and 18 Reddit threads read in full (about 2,500 top-level comments) for brand sentiment, recurring complaints and reported prices. Three sources were read in full; the rest were read from search excerpts and are marked unverified. Forum evidence was used to find recurring complaints and disagreements, never to estimate failure rates. Prices are published ranges, not quotes, and no specific model was priced. Purchasd received no manufacturer payment, review units or affiliate consideration for this research. Several cost guides earn from contractor leads or product sales, and one brand-reliability source sells anode rods; each source is labeled by bias. Purchasd has not installed or tested any of these units.