Water Heater Cost Calculator Formula
Understand the math behind the water heater cost calculator. Each variable explained with a worked example.
Formulas Used
Monthly Cost
monthly_cost = daily_therms * 30.44 * energy_cost_per_thermAnnual Cost
annual_cost = daily_therms * 365 * energy_cost_per_thermDaily Energy Used
daily_therms_display = daily_thermsVariables
| Variable | Description | Default |
|---|---|---|
gallons_per_day | Hot Water Usage(gallons/day) | 50 |
temp_rise_f | Temperature Rise(°F) | 60 |
efficiency | Heater Efficiency(%) | 92 |
energy_cost_per_therm | Energy Cost (gas)(USD/therm) | 1.2 |
daily_btu | Derived value= gallons_per_day * 8.33 * temp_rise_f | calculated |
daily_therms | Derived value= daily_btu / 100000 / (efficiency / 100) | calculated |
How It Works
Water Heater Operating Cost
Water heating is the second largest energy expense in most homes, accounting for 14-18% of utility bills.
Formula
Daily BTU = Gallons/Day x 8.33 (lbs/gal) x Temp Rise (°F)
Daily Therms = Daily BTU / 100,000 / Efficiency
Monthly Cost = Daily Therms x 30.44 x Cost per Therm
8.33 is the weight of one gallon of water in pounds. One BTU raises one pound of water by 1°F.
Worked Example
50 gallons/day, 60°F temperature rise, 92% efficient gas heater at $1.20/therm.
gallons_per_day = 50temp_rise_f = 60efficiency = 92energy_cost_per_therm = 1.2
- 01Daily BTU = 50 x 8.33 x 60 = 24,990 BTU
- 02Daily therms = 24,990 / 100,000 / 0.92 = 0.272 therms
- 03Monthly cost = 0.272 x 30.44 x $1.20 = $9.93
- 04Annual cost = 0.272 x 365 x $1.20 = $119.07
Ready to run the numbers?
Open Water Heater Cost Calculator