Water Heater Efficiency Calculator Formula
Understand the math behind the water heater efficiency calculator. Each variable explained with a worked example.
Formulas Used
Annual Water Heating Cost
annual_cost = annual_btu / 3412 * fuel_costDaily Energy Needed
daily_btu = btu_needed / energy_factorMonthly Cost
monthly_cost = annual_btu / 3412 * fuel_cost / 12Heat Pump WH Savings (UEF 3.5)
heat_pump_savings = annual_btu / 3412 * fuel_cost - (btu_needed * 365 / 3.5) / 3412 * fuel_costVariables
| Variable | Description | Default |
|---|---|---|
daily_hot_gal | Daily Hot Water Usage(gallons) | 50 |
temp_rise | Temperature Rise(degrees F) | 60 |
energy_factor | Energy Factor (UEF)(ratio) | 0.92 |
fuel_cost | Fuel Cost(USD/therm or USD/kWh) | 0.15 |
btu_needed | Derived value= daily_hot_gal * temp_rise * 8.33 | calculated |
annual_btu | Derived value= btu_needed * 365 / energy_factor | calculated |
How It Works
How to Calculate Water Heater Costs
Water heating is typically the second largest energy expense in a home (14-18% of energy bills).
Formula
Daily BTU = Gallons * Temp Rise * 8.33
Annual Energy = Daily BTU * 365 / Energy Factor
Energy Factors
Worked Example
50 gal/day hot water, 60F temp rise, 0.92 energy factor, $0.15/kWh.
- 01Daily BTU: 50 * 60 * 8.33 = 24,990 BTU
- 02Daily energy with EF: 24,990 / 0.92 = 27,163 BTU
- 03Annual energy: 27,163 * 365 = 9,914,526 BTU
- 04Annual cost: 9,914,526 / 3,412 * $0.15 = $436
Frequently Asked Questions
What is the most efficient water heater?
Heat pump water heaters are the most efficient, with energy factors of 2.0-4.0, meaning they use 2-4x less energy than electric resistance heaters. They cost $1,200-2,500 installed.
What temperature should my water heater be set to?
The recommended setting is 120F, balancing safety and efficiency. Every 10F reduction saves 3-5% in water heating costs.
How much of my energy bill is water heating?
Water heating accounts for about 14-18% of the average home energy bill, or $400-600 per year. This makes it the second largest energy expense after HVAC.
Ready to run the numbers?
Open Water Heater Efficiency Calculator