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.
About this calculator
Heating water requires energy proportional to the volume heated and the temperature rise. The core formula is: Annual Cost ($) = (dailyHotWater × (targetTemp − inletTemp) × 8.33 × 365 × energyCost) ÷ 100,000. Here, 8.33 is the weight of one gallon of water in pounds, and 100,000 converts BTUs to therms (for gas) or aligns units for electricity. The factor (targetTemp − inletTemp) represents the temperature rise in °F needed to heat cold incoming water to your desired output temperature. Dividing by 100,000 converts total BTUs into therms so you can multiply by the cost per therm. For electric heaters, the same structure applies with energy cost expressed per kWh and an appropriate unit conversion. Efficiency ratings (EF or UEF) should be applied to get actual consumed energy rather than theoretical minimum energy.
How to use
A household uses 60 gallons of hot water daily, heating cold water from 55 °F to 120 °F, with natural gas at $1.20 per therm. Temperature rise = 120 − 55 = 65 °F. Annual Cost = (60 × 65 × 8.33 × 365 × 1.20) ÷ 100,000 = (60 × 65 × 8.33 × 438) ÷ 100,000 = 14,226,444 ÷ 100,000 ≈ $142.26 per year in theoretical heat energy. Dividing by a typical gas heater efficiency of 0.67 gives a real-world cost of about $212 per 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.