Hot Water Energy Cost Calculator
Estimates your monthly water heating bill based on daily hot water use, temperature rise, energy source, and heater efficiency. Useful when comparing electric vs. gas heaters or auditing home energy costs.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Heating water requires energy proportional to its mass, specific heat and temperature change: one gallon weighs 8.33 lb and needs 1 BTU per lb per °F, so daily heat = gallons × 8.33 × ΔT BTU. For electric heaters that heat is converted to kWh (1 kWh = 3,412 BTU) and priced per kWh: Cost = gallons/day × 8.33 × ΔT ÷ 3,412 ÷ (efficiency/100) × rate × 30. For natural gas and propane the heat is converted to therms (1 therm = 100,000 BTU) and the rate is entered in $/therm (propane is sold by the gallon; 1 gallon ≈ 0.915 therm, so divide the price per gallon by 0.915). Dividing by efficiency accounts for heat lost to exhaust or standby losses — a 90% efficient heater uses 11% more energy than the theoretical minimum. A heat-pump water heater can be entered at its rated efficiency of 300-400% (COP 3-4). Multiplying by 30 converts the daily cost into a monthly figure.
How to use
Suppose you use 50 gallons/day, the cold inlet is 55°F and you want 120°F (ΔT = 65°F), you have an electric heater at 92% efficiency, and your rate is $0.13/kWh. Daily heat = 50 × 8.33 × 65 = 27,072.5 BTU = 7.93 kWh. Monthly cost = 7.93 ÷ 0.92 × 0.13 × 30 ≈ $33.64. The same water from a natural-gas heater at 80% efficiency and $1.30/therm: 27,072.5 ÷ 100,000 ÷ 0.80 × 1.30 × 30 ≈ $13.20 per month. Enter each value in the corresponding field and the calculator displays the result instantly.
Frequently asked questions
How does water heater efficiency affect my monthly energy bill?
Efficiency is the percentage of input energy that actually heats water; the rest is lost to exhaust gases, standby heat loss, or jacket losses. A heater rated at 80% efficiency consumes 25% more energy than one rated at 100% for the same hot water output. Modern heat-pump water heaters can exceed 300% efficiency (COP > 3) by moving heat rather than generating it. Upgrading from a 60% to a 90% efficient unit can cut your water heating bill by roughly 33%.
What temperature rise should I use when calculating hot water energy costs?
Temperature rise (ΔT) is the difference between your cold water inlet temperature and your desired hot water delivery temperature. Typical cold inlet temperatures range from 45°F in winter to 65°F in summer; standard delivery temperature is 120°F. A conservative annual average ΔT of 60°F is commonly used for annual cost estimates. Using your actual measured inlet temperature gives the most accurate result.
Is it cheaper to heat water with electricity or natural gas?
Natural gas is typically cheaper per BTU of heat delivered in most U.S. regions, but the comparison depends on local rates and heater efficiency. At typical U.S. residential rates (about $0.16/kWh for electricity and $1.40/therm for natural gas), a gas heater often costs only a third to a half as much per month as a standard electric resistance heater. However, a heat-pump water heater running on electricity can outperform gas due to its high COP. Always plug your local utility rates into the calculator to get a region-specific comparison.