The most common sizing mistake is choosing by tank capacity alone. FHR for tanks and GPM at temperature rise for tankless predict whether hot water runs out.
Tank sizing by household
Match peak hour demand to First Hour Rating, not just capacity.
| People | Gas | Electric | FHR | Notes |
|---|---|---|---|---|
| 1-2 | 40 gal | 50 gal | 40-60 gal | A single person or couple with moderate usage. 50-gallon electric covers most needs. |
| 3-4 | 50 gal | 65 gal | 60-80 gal | Most common household size. 80-gallon tanks recommended for high-demand homes (multiple showers in morning). Most common |
| 5+ | 75 gal | 80 gal | 80-120 gal | Large households should prioritize FHR over tank size. Consider tankless or multiple units for 6+ people. |
FHR explained. First Hour Rating (FHR) measures how many gallons of hot water a tank can deliver in the first hour starting with a full tank of hot water. It accounts for both stored hot water and recovery rate.
Peak demand worksheet
List all hot water activities during your household's peak one-hour period (usually morning). Sum the gallons. The total is your peak hour demand. Choose a water heater with an FHR within 1-2 gallons of that number.
| Fixture | Gal/Use | Notes |
|---|---|---|
| Shower | 20 gal | Based on 2.0 GPM showerhead for 10 minutes. Low-flow heads reduce to 15 gallons. |
| Shaving | 2 gal | Running faucet while shaving. |
| Hand Dishwashing | 3 gal | Per load of hand-washed dishes. |
| Automatic Dishwasher | 7 gal | Per full cycle. ENERGY STAR models may use as little as 3-4 gallons. |
| Clothes Washer Top Load | 25 gal | Traditional top-loading machines use more hot water. |
| Clothes Washer Front Load | 15 gal | H-axis (front-load) machines use roughly 40% less hot water than top-loaders. |
| Hand Washing | 2 gal | Kitchen or bathroom faucet for hand/face washing. |
| Bath | 20 gal | Full bathtub fill. Varies 15-30 gallons by tub size. |
Tankless sizing
Sized by simultaneous flow rate and temperature rise.
| Fixture | GPM | Notes |
|---|---|---|
| Shower | 2.0 GPM | Low-flow: 1.5 GPM. Standard: 2.0-2.5 GPM. |
| Bathroom Faucet | 1.0 GPM | Typical 0.5-1.5 GPM. |
| Kitchen Faucet | 1.5 GPM | Typical 1.0-2.0 GPM. |
| Dishwasher | 1.5 GPM | Intermittent draw during cycle. |
| Clothes Washer | 2.0 GPM | Hot water fill cycles only. |
Temp rise by region
| Region | Inlet Temp | Rise Needed | Difficulty |
|---|---|---|---|
| Southern US (FL, TX, AZ, LA, GA) | 72°F | 48°F | Easy |
| Mid-Atlantic / Midwest (OH, PA, IL, MO) | 55°F | 65°F | Easy |
| Northern US (MN, WI, ME, MT, ND) | 42°F | 78°F | Moderate |
| Pacific Northwest (WA, OR) | 50°F | 70°F | Moderate |
| Mountain West (CO, UT, WY) | 47°F | 73°F | Moderate |
Heat pump sizing
Need 1,000 cu ft min, ambient 40-90F.
| People | Size | Notes |
|---|---|---|
| 1-2 | 50 gal | 50-gallon HPWH covers most 1-2 person households. |
| 3-4 | 65 gal | 65-gallon recommended. 80-gallon if high demand patterns. |
| 5+ | 80 gal | 80-gallon minimum. Consider hybrid mode settings for peak demand. |
When to upsize
- In cold climates where the unit will frequently operate in hybrid mode, go one size up (e.g., 65-gallon instead of 50).
- If installed in an unheated space like a garage that drops below 50F in winter, upsize by one tier.
- Households with concentrated morning peak demand (multiple showers within 1 hour) should upsize to avoid triggering electric resistance backup.
- If replacing a gas tank water heater, upsize the HPWH by 10-20 gallons because heat pump recovery is slower than gas.
The federal tax credit of up to $2,000 under IRA Section 25C expired December 31, 2025. State and utility rebates may still apply.
Once sized, your state rate determines savings. See installation accessories
Climate zone adjustments
| Zone | States | Inlet | Factor |
|---|---|---|---|
| Hot | FL, TX, AZ, LA, HI, Southern CA | 68-78F | 1.0x |
| Mixed | NC, TN, MO, KS, NM, Northern CA, OR | 50-62F | 1.0x |
| Cold | OH, PA, IL, NY, MA, CO, WA, MI | 42-55F | 1.15x |
| Very Cold | MN, WI, ND, MT, ME, AK, WY, VT | 35-45F | 1.25x |
A gas tankless rated at 9.5 GPM at 35F rise may only deliver 5.5 GPM at 77F rise in a cold-climate home. Always check the manufacturer's GPM-at-temperature-rise chart, not just the headline GPM rating.
Annual cost by state
Heat pump (~1,100 kWh/yr) vs standard electric (~4,700 kWh/yr). EIA rates. Sorted by electricity cost.
| State | Rate | Heat Pump | Std Electric | Annual Savings |
|---|---|---|---|---|
| ND | $11.02/kWh | $121/yr | $518/yr | $397/yr |
| NE | $11.57/kWh | $127/yr | $544/yr | $417/yr |
| ID | $11.87/kWh | $131/yr | $558/yr | $427/yr |
| MO | $11.91/kWh | $131/yr | $560/yr | $429/yr |
| OK | $12.25/kWh | $135/yr | $576/yr | $441/yr |
| FL | $15.02/kWh | $165/yr | $706/yr | $541/yr |
| ME | $30.39/kWh | $334/yr | $1428/yr | $1094/yr |
| MA | $30.88/kWh | $340/yr | $1451/yr | $1111/yr |
| RI | $31.15/kWh | $343/yr | $1464/yr | $1121/yr |
| CA | $34.71/kWh | $382/yr | $1631/yr | $1249/yr |
| HI | $41.62/kWh | $458/yr | $1956/yr | $1498/yr |
Showing cheapest 5, median, and most expensive 5 states. Savings = standard electric cost minus heat pump cost.
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Sources. U.S. DOE, EIA.Methodology