Water Heater Efficiency (EF/UEF) Calculator
Calculate your water heater's Energy Factor (EF) or Uniform Energy Factor (UEF), estimate annual operating costs, and compare savings between models.
$/kWh (electricity)
Compare with a Second Model (optional)
Formulas Used
Annual Energy Consumption (AEC):
AEC (fuel units/yr) = Q_ref / (UEF × BTU_per_fuel_unit) Annual Cost ($) = AEC × fuel_rate Lifetime Cost ($) = Annual Cost × lifespan_years Annual Savings = Annual_Cost_Model_A − Annual_Cost_Model_B Payback Period (yr) = (Cost_B − Cost_A) / Annual_Savings EF ≈ UEF + 0.0005 × tank_volume_gal [storage tanks, DOE approximation]
Reference Heat Loads (Q_ref) per DOE 10 CFR Part 430 Appendix E:
- Very Small (10 gal/day): 3,078,000 Btu/yr
- Low (38 gal/day): 12,266,000 Btu/yr
- Medium (55 gal/day): 18,719,000 Btu/yr
- High (84 gal/day): 28,518,000 Btu/yr
Fuel Energy Content: Natural Gas = 100,000 Btu/therm; Electricity = 3,412 Btu/kWh
Assumptions & References
- UEF (Uniform Energy Factor) is the current DOE standard (effective April 2017); EF (Energy Factor) is the legacy rating — both measure delivered energy vs. consumed energy.
- Reference heat loads and draw patterns follow DOE 10 CFR Part 430, Subpart B, Appendix E (2017 test procedure).
- ENERGY STAR thresholds per ENERGY STAR Version 4.0 specification: Gas storage ≥0.67, Electric storage ≥0.93, HPWH ≥2.20, Tankless gas ≥0.87.
- Fuel prices are user-supplied; U.S. averages: electricity ~$0.13/kWh, natural gas ~$1.20/therm (EIA 2024).
- The EF ≈ UEF + 0.0005 × V conversion is a DOE-published approximation for storage-type heaters only.
- Typical lifespans: gas storage 8–12 yr, electric storage 10–15 yr, HPWH 10–15 yr, tankless 15–20 yr (DOE/ENERGY STAR).
- Solar water heater UEF reflects the combined system (solar + backup); actual savings depend on solar fraction and climate.
- This calculator does not account for installation costs, rebates, or time-value of money (NPV).