Water Heater Recovery Time Calculator
Estimate how long it takes your water heater to reheat a full tank of cold water based on tank size, BTU rating or wattage, and temperature rise.
Typical gas: 60–80 %; electric: 90–98 %
Formulas Used
BTU Required:
BTU = Gallons × 8.3454 (lb/gal) × ΔT (°F)
Where ΔT = Target Temp − Inlet Temp
Effective Heat Output (Gas):
Effective BTU/hr = Burner BTU/hr × (Efficiency / 100)
Effective Heat Output (Electric):
Effective BTU/hr = Watts × 3.41214 × (Efficiency / 100)
Recovery Time:
Time (hours) = BTU Required / Effective BTU/hr
First-Hour Delivery (FHD) Estimate:
FHD ≈ 0.70 × Tank Size + (Effective BTU/hr / (8.3454 × ΔT))
The 0.70 factor accounts for the ~70 % of stored hot water usable before cold dilution drops temperature below acceptable levels.
Assumptions & References
- Water density is taken as 8.3454 lb/gallon (at ~60 °F), per NIST standard values.
- Specific heat of water = 1 BTU / (lb·°F).
- Electric conversion: 1 Watt = 3.41214 BTU/hr (exact thermodynamic conversion).
- Typical gas water heater efficiency: 60–80 % (EF 0.60–0.80); high-efficiency condensing units can reach 90–96 %.
- Typical electric water heater efficiency: 90–98 %; heat-pump water heaters can exceed 200 % COP but use a different model.
- The FHD formula follows the DOE first-hour rating methodology (10 CFR Part 430, Appendix E).
- Heat losses through tank walls and pipes are not modelled; actual recovery time may be slightly longer.
- Formula source: ASHRAE Fundamentals Handbook; DOE Energy Efficiency & Renewable Energy — Water Heater Basics.