Estimate the fuel cost to run a natural gas water heater from hot water use, temperature rise, efficiency, and gas price.
Table of contents
How to use our Gas Water Heater Cost Calculator
- Enter Hot water used each day (gallons), using only heated water, not all water used in the home.
- Enter Cold water temperature (degrees F) and Hot water setting (degrees F); the hot setting must be higher.
- Enter Efficiency rating (UEF or decimal) from the heater label and Natural gas price ($ per therm) from your gas bill.
- Open Advanced options if you want Billing period (days), Long-term estimate (years), or Yearly gas price change (percent) to match your bill or planning period.
- Click Calculate, then sanity-check the result by comparing Estimated cost for your billing period with your gas bill after allowing for other gas appliances, fixed fees, and taxes.
Definitions
UEF: Uniform Energy Factor, the efficiency score used for water heaters. A higher UEF means more of the gas heat becomes useful hot water [1].
BTU: British thermal unit, a small unit of heat energy. This calculator uses BTU to estimate the heat needed to warm water.
Therm: A natural gas billing unit equal to 100,000 BTU.
Temperature rise: The hot water setting minus the cold water temperature. A bigger rise means the heater must add more heat to each gallon.
Billing period: The number of days covered by one gas bill. Matching this helps compare the estimate with a real bill.
Yearly gas price change: The percent increase or decrease applied once per year in the long-term estimate.
Direct carbon dioxide: The carbon dioxide from burning the natural gas used by the water heater, not from producing or delivering the gas.
Common mistakes and quick fixes
Mistake: Using total household water for Hot water used each day (gallons) .
Fix: Enter only the gallons that are heated by the water heater.
Mistake: Setting Hot water setting (degrees F) at or below Cold water temperature (degrees F) .
Fix: Make the hot water setting higher than the cold water temperature.
Mistake: Typing 67 instead of 0.67 for Efficiency rating (UEF or decimal) .
Fix: Enter the efficiency as a decimal from 0.01 to 1.00, such as 0.67 for 67 percent.
Mistake: Entering a monthly bill total in Natural gas price ($ per therm) .
Fix: Use the price per therm from the bill, not the full bill amount.
Mistake: Leaving Billing period (days) at 30 when your bill covers 28, 31, or 32 days.
Fix: Enter the exact number of days on the bill you want to compare.
Mistake: Treating Long-term gas cost as a replacement or repair estimate.
Fix: Use it only for fuel cost; it does not include a new heater, labor, permits, maintenance, or repairs.
Limitations & Key Assumptions / Boundary Conditions
- This estimates natural gas fuel cost for water heating only. It does not include the heater purchase price, installation, permits, repairs, maintenance, taxes, or fixed utility charges.
- The calculation assumes the same average hot water use every day. Guests, vacations, long showers, laundry habits, and dishwasher use can change real use.
- The cold water temperature is an estimate. Seasonal inlet water changes can move the actual cost above or below the result.
- The efficiency input is treated as one steady decimal value. Real water heater performance can vary with tank standby losses, pipe losses, venting, age, and use pattern.
- The bill-period result is easiest to compare with a gas bill when you account for furnace, dryer, stove, fireplace, fixed fees, and taxes on the same bill.
- The long-term estimate keeps hot water use and efficiency constant and applies the gas price change once per year.
- The carbon dioxide estimate covers direct combustion only and uses a rounded factor of 11.7 lb CO2 per therm.
Methodology
How the fuel cost is calculated
The calculator first finds the temperature rise, which is the amount the heater must warm the incoming water.
temperature_rise_f = hot_water_temp_f - cold_water_temp_f
Next, it estimates useful heat delivered to the water. The factor 8.34 is the approximate number of BTU needed to raise 1 gallon of water by 1 degree F.
useful_btu_day = daily_hot_water_gal * 8.34 * temperature_rise_f
It then adjusts for heater efficiency. The efficiency value may be the UEF from the heater label or the same efficiency written as a decimal [1].
therms_day = useful_btu_day / (heater_efficiency * 100000)
Cost is found by multiplying daily therms by the gas price per therm, then scaling that daily cost to the selected time periods.
daily_cost = therms_day * gas_price_per_therm
annual_cost = daily_cost * 365
monthly_cost = annual_cost / 12
billing_period_cost = daily_cost * billing_days
Long-term and CO2 calculations
If the yearly gas price change is 0, the long-term cost is the yearly cost times the number of years. If the price changes, the calculator uses a yearly growth series.
projected_total_cost = annual_cost * projection_years
projected_total_cost = annual_cost * (((1 + yearly_change_rate) ^ projection_years - 1) / yearly_change_rate)
Yearly natural gas use and direct carbon dioxide are estimated from daily therms.
annual_therms = therms_day * 365
annual_co2_lbs = annual_therms * 11.7
Mini-example
With 50 gallons per day, 55 degrees F cold water, a 120 degrees F hot setting, 0.67 efficiency, and $1.50 per therm, the temperature rise is 65 degrees F. Useful heat is 50 * 8.34 * 65 = 27,105 BTU per day. Gas use is 27,105 / (0.67 * 100,000) = 0.405 therms per day. At $1.50 per therm, the daily cost is about $0.61, the yearly cost is about $221.49, and a 30-day billing period is about $18.20.