Water Heater Usage Calculator
Estimate your water heater's annual energy cost based on hot water demand and temperature rise. Ideal for comparing heater types or finding savings before buying a new unit.
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 the volume heated and the temperature rise. Annual heat = dailyHotWater × (targetTemp − inletTemp) × 8.33 × 365 BTU, where 8.33 is the weight of one gallon of water in pounds and 1 BTU raises 1 lb of water by 1 °F. For the gas heaters that heat is divided by 100,000 to get therms and priced per therm; for the electric heaters it is divided by 3,412 to get kWh and priced per kWh. Finally the energy is divided by the heater's efficiency (95% for the tankless units, 80% for a gas storage tank, 90% for an electric storage tank) to get the energy actually purchased: Annual Cost = heat ÷ (100,000 or 3,412) × energyCost ÷ efficiency. Enter energyCost in $/therm for gas and $/kWh for electric. Real bills also include standby losses that vary with tank insulation and location, so treat the result as an estimate.
How to use
A household uses 60 gallons of hot water daily, heating from 55 °F to 120 °F (ΔT 65 °F), with a gas storage tank (80% efficient) at $1.20/therm. Annual heat = 60 × 65 × 8.33 × 365 = 11,857,755 BTU = 118.58 therms, costing 118.58 × $1.20 ≈ $142.29 before losses. Dividing by the 0.80 tank efficiency gives ≈ $177.87/year. A 95%-efficient gas tankless unit would cut that to ≈ $149.78. The same hot water from an electric storage tank (90%) at $0.16/kWh: 11,857,755 ÷ 3,412 = 3,475.3 kWh, ÷ 0.90 × $0.16 ≈ $617.83/year.
Frequently asked questions
How much does it cost to run a water heater per year for a family of four?
A family of four typically uses 60–80 gallons of hot water daily. Using the formula above with average U.S. gas prices around $1.20/therm, annual costs range from $200 to $600 depending on heater efficiency and local inlet temperatures. Tankless and heat-pump water heaters can cut those costs by 30–50% compared to a standard storage tank unit.
What temperature should I set my water heater to save energy?
The U.S. Department of Energy recommends 120 °F as the optimal setting — hot enough to prevent Legionella bacteria growth while minimizing standby heat loss. Every 10 °F reduction in water heater temperature saves roughly 3–5% on water heating costs. Setting the temperature above 140 °F significantly increases both energy use and scalding risk.
How does a tankless water heater compare in energy cost to a storage tank heater?
Tankless (on-demand) water heaters eliminate standby heat loss, which accounts for 10–20% of a traditional tank's energy consumption. For homes using less than 41 gallons of hot water daily, tankless units are 24–34% more energy-efficient than conventional storage models, according to the DOE. However, tankless units have higher upfront costs and may require upgraded gas lines or electrical panels, so payback periods typically range from 5 to 10 years.