The right tankless water heater size depends on two numbers: your peak flow rate in gallons per minute (GPM) and the temperature rise your climate requires. A typical family of 4 running a shower and a dishwasher simultaneously needs a unit rated for at least 5.0 GPM at a 60°F temperature rise — which translates to a gas unit rated around 150,000–199,000 BTU or an electric unit drawing 27–36 kW.
Undersizing a tankless unit is the most common installation mistake, and the result is a lukewarm shower the moment someone turns on the kitchen faucet. This guide gives you the exact sizing methodology HVAC professionals and plumbers use.
The Two Numbers You Need: Flow Rate and Temperature Rise
Flow Rate (GPM)
Flow rate is how many gallons of hot water you draw per minute across all fixtures running simultaneously. Every fixture in your home has a typical flow rate:
| Fixture | Typical Flow Rate (GPM) |
|---|---|
| Low-flow showerhead | 1.5–2.0 |
| Standard showerhead | 2.0–2.5 |
| Rain/luxury showerhead | 2.5–3.5 |
| Bathroom faucet | 0.5–1.5 |
| Kitchen faucet | 1.0–1.5 |
| Dishwasher | 1.0–1.5 |
| Washing machine | 1.5–2.0 |
| Bathtub fill | 3.0–4.0 |
| Utility sink | 1.0–2.0 |
| High-flow sprayer (kitchen) | 2.0–2.5 |
To find your peak demand, add the flow rates of fixtures you expect to run at the same time. Be realistic — most households rarely run more than 2–3 hot water fixtures simultaneously.
Temperature Rise
Temperature rise is the difference between your incoming cold water temperature (groundwater temperature) and your desired hot water output (typically 120°F).
Temperature Rise = Desired Output Temp − Incoming Water Temp
Incoming water temperatures vary dramatically by location and season:
| Region | Winter Groundwater Temp (°F) | Required Rise to 120°F |
|---|---|---|
| Northern US (Minnesota, Montana, Maine) | 35–42°F | 78–85°F |
| Upper Midwest/Northeast (Chicago, Boston, NYC) | 42–50°F | 70–78°F |
| Mid-Atlantic/Midwest (DC, St. Louis, Denver) | 47–55°F | 65–73°F |
| Southeast (Atlanta, Charlotte, Nashville) | 52–60°F | 60–68°F |
| South/Southwest (Dallas, Phoenix, LA) | 55–65°F | 55–65°F |
| Deep South/Hawaii (Miami, Honolulu) | 65–77°F | 43–55°F |
Always size for winter temperatures. Your incoming water is coldest in January–February, and that's when you need the most hot water. A unit that works fine in August (when groundwater is 65°F) may deliver lukewarm water in January (when groundwater drops to 42°F). Use your winter groundwater temperature for sizing.
Tankless Water Heater Sizing Chart
Cross-reference your peak flow rate and temperature rise to find the minimum unit size:
Gas Tankless (BTU Rating)
| Peak GPM | 45°F Rise | 55°F Rise | 65°F Rise | 75°F Rise | 85°F Rise |
|---|---|---|---|---|---|
| 2.0 | 75,000 | 92,000 | 108,000 | 125,000 | 142,000 |
| 3.0 | 112,000 | 138,000 | 162,000 | 187,000 | 213,000 |
| 4.0 | 150,000 | 183,000 | 216,000 | 250,000 | — |
| 5.0 | 187,000 | 229,000 | — | — | — |
| 6.0 | 225,000 | — | — | — | — |
| 7.0+ | 262,000+ | — | — | — | — |
Cells marked "—" exceed residential single-unit capacity. Use two units in parallel or reduce simultaneous demand.
The formula: BTU required = GPM × Temperature Rise × 500 / Efficiency Factor (0.80–0.96)
Electric Tankless (kW Rating)
| Peak GPM | 45°F Rise | 55°F Rise | 65°F Rise | 75°F Rise | 85°F Rise |
|---|---|---|---|---|---|
| 1.5 | 11 kW | 14 kW | 16 kW | 19 kW | 21 kW |
| 2.0 | 15 kW | 18 kW | 22 kW | 25 kW | 28 kW |
| 2.5 | 19 kW | 23 kW | 27 kW | 31 kW | 36 kW |
| 3.0 | 22 kW | 27 kW | 32 kW | 37 kW | — |
| 3.5 | 26 kW | 32 kW | 38 kW | — | — |
| 4.0+ | 30 kW+ | 36 kW+ | — | — | — |
Electric tankless units max out around 36 kW for residential. In cold climates requiring 75°F+ rise, gas is often the only practical option for whole-home use.
Gas vs. Electric Tankless: Which to Choose
| Factor | Gas Tankless | Electric Tankless |
|---|---|---|
| Max flow rate | 5–11 GPM | 2–5 GPM |
| Best for | Whole-home, cold climates | Point-of-use, mild climates, small homes |
| Installation cost | $1,500–$3,500 | $800–$2,500 |
| Unit cost (2026) | $600–$2,000 | $200–$900 |
| Venting required | Yes (direct vent or power vent) | No |
| Gas line upgrade | Often required (¾" minimum) | N/A |
| Electrical upgrade | 120V outlet for controls | 200–300A panel may be needed |
| Uniform Energy Factor (UEF) | 0.80–0.96 | 0.96–0.99 |
| Annual operating cost | $150–$350 | $200–$500 |
| Lifespan | 15–20 years | 8–15 years |
The electric panel bottleneck: A 36 kW electric tankless draws 150 amps at 240V. Many older homes have only 100- or 150-amp electrical panels. Upgrading to a 200-amp panel costs $1,500–$4,000 and must be done before the tankless installation. Factor this into your decision.
Sizing Examples for Common Households
Example 1: Couple in Phoenix, AZ (2-person, 1-bathroom condo)
- Peak demand: 1 shower (2.0 GPM) + kitchen faucet (1.0 GPM) = 3.0 GPM
- Incoming water temp: 62°F winter
- Temperature rise: 120°F − 62°F = 58°F
- Gas unit needed: 3.0 × 58 × 500 / 0.90 = 96,667 BTU → 100,000–120,000 BTU gas unit
- Electric option: 3.0 GPM at 58°F rise → 24 kW electric unit (feasible)
Recommendation: Either a mid-range gas unit (Rinnai RU130iN, 130,000 BTU) or a whole-home electric unit (EcoSmart ECO 24, 24 kW). The mild climate makes electric viable here.
Example 2: Family of 4 in Chicago, IL (3-bathroom home)
- Peak demand: 2 showers (4.0 GPM) + dishwasher (1.0 GPM) = 5.0 GPM
- Incoming water temp: 44°F winter
- Temperature rise: 120°F − 44°F = 76°F
- Gas unit needed: 5.0 × 76 × 500 / 0.90 = 211,111 BTU → 199,000+ BTU gas unit
- Electric option: 5.0 GPM at 76°F rise requires ~63 kW → Not feasible with single electric unit
Recommendation: A premium gas unit (Navien NPE-240A, 199,000 BTU, condensing) or (Rinnai RU199iN, 199,000 BTU). The high temperature rise and flow rate make electric impractical for this scenario. Even the 199K unit may limit you to 4.5 GPM in the coldest weeks — install low-flow showerheads (2.0 GPM) to stay within capacity.
Example 3: Family of 6 in Minneapolis, MN (4-bathroom home)
- Peak demand: 3 showers (6.0 GPM) + washing machine (1.5 GPM) = 7.5 GPM
- Incoming water temp: 38°F winter
- Temperature rise: 120°F − 38°F = 82°F
- Gas unit needed: 7.5 × 82 × 500 / 0.90 = 341,667 BTU → Exceeds single-unit capacity
Recommendation: Two gas tankless units in parallel. A pair of Navien NPE-240A units (199,000 BTU each) with a recirculation loop provides 398,000 BTU total capacity and handles the extreme temperature rise. Alternatively, a hybrid approach: one tankless unit for the master bath zone and one for the rest of the home, each sized for their zone's peak demand.
Example 4: Single-person apartment in Atlanta, GA (point-of-use)
- Peak demand: 1 shower only = 2.0 GPM
- Incoming water temp: 55°F winter
- Temperature rise: 110°F − 55°F = 55°F (lower output for single shower)
- Electric unit needed: 2.0 × 55 × 500 / 0.98 = 56,122 BTU → 16–18 kW electric point-of-use unit
Recommendation: An EcoSmart ECO 18 or Stiebel Eltron Tempra 20 Plus. At 18 kW, this handles a single shower with room to spare and requires only a 75-amp circuit.
Household Size Quick Reference
| Household | Bathrooms | Typical Peak GPM | Gas BTU Needed (Cold Climate) | Gas BTU Needed (Warm Climate) | Electric kW (Warm Climate Only) |
|---|---|---|---|---|---|
| 1 person | 1 | 1.5–2.0 | 100,000 | 75,000 | 14–18 kW |
| 2 people | 1–2 | 2.5–3.5 | 140,000–160,000 | 100,000–120,000 | 22–27 kW |
| 3–4 people | 2–3 | 4.0–5.5 | 180,000–199,000 | 140,000–160,000 | 27–36 kW* |
| 5–6 people | 3–4 | 5.5–7.5 | 199,000+ (or 2 units) | 180,000–199,000 | Not practical |
| 6+ people | 4+ | 7.0+ | 2 units in parallel | 199,000+ | Not practical |
*Electric whole-home is borderline for 3–4 people in warm climates and typically not feasible in cold climates.
Point-of-Use vs. Whole-Home Tankless
Point-of-use (POU) tankless units serve a single fixture or small group of fixtures. They're compact (shoe-box sized), require smaller electrical circuits, and eliminate pipe heat loss. Here's when each makes sense:
| Scenario | Best Choice | Why |
|---|---|---|
| New whole-home install, gas available | Whole-home gas tankless | Highest capacity, lowest per-gallon cost |
| Retrofit, no gas line, mild climate | Whole-home electric tankless | No venting, lower install cost |
| Distant bathroom (40+ ft from heater) | POU electric under sink | Eliminates wait time and pipe loss |
| Guest house or ADU | POU electric or small gas | Independent system, simple install |
| Kitchen sink only (supplement existing heater) | POU electric (4–8 kW) | Instant hot water, $150–$400 |
| Commercial hand-washing stations | POU electric (3–6 kW) | Code-required, minimal demand |
Maintenance and Longevity by Size
Larger tankless units (180,000+ BTU or 27+ kW) require more maintenance due to higher mineral deposit rates from higher water volume. Descaling frequency depends on your water hardness:
| Water Hardness | Descale Frequency | Annual Descaling Cost |
|---|---|---|
| Soft (0–3 GPG) | Every 2–3 years | $100–$150 |
| Moderate (4–7 GPG) | Annually | $100–$200 |
| Hard (8–12 GPG) | Every 6 months | $200–$400 |
| Very hard (12+ GPG) | Every 3–4 months (or install water softener) | $400–$800+ |
A water softener ($500–$2,500 installed) dramatically reduces descaling needs and extends unit lifespan from 15 to 20+ years. For households with water hardness above 10 GPG, a softener is practically required to make a tankless investment worthwhile.
2026 Efficiency Ratings and Rebates
The DOE's Uniform Energy Factor (UEF) replaced the older Energy Factor (EF) for water heaters. In 2026, ENERGY STAR certified tankless water heaters must meet:
| Fuel Type | Minimum UEF for ENERGY STAR |
|---|---|
| Gas-fired | ≥ 0.87 |
| Gas-fired condensing | ≥ 0.92 (typical 0.92–0.96) |
| Electric | ≥ 0.96 |
Gas condensing tankless units recover heat from exhaust gases, achieving 92–96% thermal efficiency compared to 80–85% for non-condensing models. The extra 10–15% efficiency saves $50–$100/year on a typical household's gas bill and allows PVC venting (cheaper and more flexible than stainless steel venting for non-condensing units).
Federal tax credits under the Inflation Reduction Act (IRA) remain available in 2026: up to $2,000 per year for qualifying ENERGY STAR certified water heaters (including heat pump water heaters and high-efficiency gas tankless units with UEF ≥ 0.95). Many state and utility rebates stack on top of this, totaling $500–$3,000 in combined incentives.
Key Takeaways
- Find your peak GPM (simultaneous fixtures) and temperature rise (120°F minus winter groundwater temp) — these two numbers determine your unit size
- Gas tankless units handle 5–11 GPM and work in any climate. Electric units top out at 3–5 GPM and struggle in cold climates
- Always size for winter groundwater temperatures — a unit that works in summer may fail in January
- Families of 5+ in cold climates often need two gas units in parallel or a hybrid approach
- Low-flow fixtures reduce peak GPM and let you use a smaller (cheaper) unit without sacrificing comfort
- 2026 federal tax credits cover up to $2,000 for qualifying ENERGY STAR tankless water heaters
Frequently Asked Questions
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