TryBuildCalcElectric Water Heater Sizing Calculator (2-Person Household, Quick Sizing)
Size an electric water heater for a small household.
🕒 Last updated: August 1, 2026
Water Heater Type
ℹ️Storage Tank is sized by First Hour Rating; Tankless is sized by simultaneous flow rate and BTU/hr input.
ℹ️Electric and Heat Pump storage heaters recover slower than gas — the target FHR is the same, but consider sizing up one class for the same real-world performance.
Sizing Method
ℹ️Quick uses a household-size lookup table; Detailed tallies your actual fixture use during the busiest hour.
Household Size
ℹ️Number of people the water heater serves — sourced from A.O. Smith's published household-size guidance.
🔧 Recommended tank size: 30–40 gallons
Fuel: Electric (Resistance)
Household Size
Occupants: 2
Recommended Tank Size: 30–40 gal
Source: A.O. Smith household-size sizing guidance (single manufacturer source — see FAQ).
Model-Size Class: ~40 gal class
Design Notes
First Hour Rating (FHR) — printed on every storage heater's EnergyGuide label — already accounts for tank volume and recovery rate together. A gas heater typically reaches a given FHR with a smaller tank than an electric heater, since gas recovers hot water faster.
Household-size quick sizing is a single manufacturer's (A.O. Smith) published guidance — the Detailed method (your actual fixture tally) is more precise for an unusual household or high simultaneous-use pattern.
This target FHR was computed the same way regardless of fuel — but electric and heat pump storage heaters recover slower than gas, so consider sizing to the next model class up from what's shown here for the same real-world performance.
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Water Heater Sizing Calculator.
2-person household electric sizing example
This page is set up for a 2-person household, which the quick household-size method recommends a 30-40 gallon tank for — but the fuel-type note matters more here than for most sizes, since electric water heaters recover roughly 2-3x slower than gas.
For an electric unit specifically, lean toward the higher end of the recommended range (closer to 40 gallons) compared to an equivalent gas installation, since a smaller electric tank's slower recovery has a bigger practical impact on back-to-back hot water draws within the same hour.
- 2-person household → 30-40 gallon recommended range.
- Electric recovers roughly 2-3x slower than gas — lean toward the higher end of the range.
- First Hour Rating (not tank gallons) is the figure to match on the EnergyGuide label.
How Water Heater Sizing Is Calculated
The calculation differs by water heater type and, for storage tanks, by sizing method.
Step 1 — Storage Tank Quick Sizing (Household Size)
A direct lookup from A.O. Smith's published household-size guidance — no entry beyond 5 occupants, so larger households are directed to the Detailed method instead of an extrapolated guess.
Step 2 — Storage Tank Detailed Sizing (Peak-Hour Tally)
Sum every fixture used during your busiest hot-water hour (not necessarily simultaneous — cumulative within one hour), then shop for a storage heater with a First Hour Rating at or slightly above the total.
Step 3 — Tankless Flow Rate & BTU/hr
Temperature Rise (°F) = Desired Output Temp − Groundwater Temp
Required BTU/hr = (GPM × Rise × 500) ÷ Efficiency
500 comes from water's weight (8.33 lb/gal) × 60 minutes/hour. This sums the actual simultaneous fixtures you expect running at once — a different diversity model than the WSFU demand-curve method used on this site's Pipe Sizing by Fixture Units calculator, since a tankless unit must supply real-time flow rather than a statistically-averaged peak. If you already know your required flow rate, enter it directly instead of building a fixture list.
Step 4 — Tankless Electric Load (kW & Amperage)
Amps = (kW × 1000) ÷ 240V
Recommended Minimum Breaker = next standard size ≥ (Amps × 1.25)
For an electric fuel type, the same BTU/hr requirement converts to kW using the standard 3412.14 BTU-per-kWh constant, then to amperage at the standard 240V US residential circuit voltage. A 125% continuous-load safety margin (consistent with NEC continuous-load practice) is applied before rounding up to a real, purchasable breaker size — a planning estimate only, not a substitute for a licensed electrician's circuit design.
Step 5 — Model-Size Class & Feasibility
Margin % = (Model Class − Requirement) ÷ Requirement × 100
The requirement (target FHR for storage, BTU/hr or kW for tankless) is matched against common off-the-shelf model-size classes to show what to actually shop for, with a margin percentage classified as comfortable (15-50% headroom), borderline (under 15% — consider the next size up), or oversized (over 50% — more capacity than this specific requirement needs). If the requirement exceeds every listed class, this calculator flags a feasibility warning rather than guessing a bigger, unlisted size — a single residential unit may not cover very high demand, and multiple units or a commercial-grade unit should be considered.
Worked Example
This example uses the active inputs above and follows the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Water Heater Type | Storage Tank |
| Fuel Type | Electric (Resistance) |
| Occupants | 2 |
Step 1 — Tank Size Lookup
| Calculation | Substitution | Result |
|---|---|---|
| Tank Size | Lookup(2 occupants) | 30-40 gal |
| Model-Size Class | Nearest class ≥ 40 gal | ~40 gal (borderline) |
Therefore, a household of 2 should target a storage tank in the 30-40 gallon range — closest to a ~40 gallon standard class (borderline).
This does not cover venting, gas/electrical sizing, or code compliance. Cross-check against the Reference Tables in the complete Water Heater Sizing Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Water Heater Sizing Calculator.