Water Heater Sizing Calculator
Determine the appropriate water heater capacity for your household based on peak hour demand, number of occupants, and usage patterns.
Formulas Used
Peak Hour Demand (PHD) at reference 120°F:
PHD = (Showers × Flow Rate × Duration) + (Baths × 20 gal) + (Dishwasher × 6 gal) + (Laundry × 25 gal) + (Hand Washing × 2 gal)
Temperature Correction:
PHDcorrected = PHD × (Tdesired − Tinlet) / (120 − Tinlet)
Storage Tank — First Hour Rating (FHR):
FHRneeded = PHDcorrected | Tank Capacity = PHDcorrected / 0.70
Tankless — Required Flow Rate & Capacity:
Flow Rate (GPM) = PHDcorrected / 60
BTU/hr = GPM × 8.33 lb/gal × 60 min/hr × ΔT
kW = BTU/hr / 3,412
Annual Energy Cost:
Annual BTU = Daily Usage × 365 × 8.33 × ΔT
Gas Cost = (Annual BTU / EF / 100,000) × $1.10/therm
Electric Cost = (Annual BTU / EF / 3,412) × $0.13/kWh
Assumptions & References
- Shower flow rate: 2.0 GPM (low-flow showerhead, EPA WaterSense standard)
- Bath fill: 20 gallons of hot water per standard bathtub fill
- Dishwasher: 6 gallons per cycle (Energy Star certified dishwasher)
- Laundry (hot/warm): 25 gallons per load (top-load washer average)
- Hand/face washing: 2 gallons per event
- Usable hot water in storage tank: 70% of total capacity (DOE standard)
- Storage tank Energy Factors: Gas 0.67, Electric 0.92 (DOE minimum standards)
- Tankless Energy Factors: Gas 0.82, Electric 0.98 (DOE typical values)
- Energy prices: Natural gas $1.10/therm, Electricity $0.13/kWh (U.S. EIA averages)
- Daily hot water use: ~16 gallons per person (DOE / ENERGY STAR baseline)
- Reference: U.S. Department of Energy — Water Heating; ENERGY STAR Water Heater Sizing Guide
- Reference: American Society of Plumbing Engineers (ASPE) — Plumbing Engineering Design Handbook