Compare estimated water heater operating costs, fuel-switch savings, and simple payback using your hot-water use, UEF ratings, and local fuel prices.
Advanced options
Table of contents
How to use our Water Heater UEF Operating Cost Calculator
- Enter the delivered Hot water used each day, plus the Water heater setting and Incoming cold-water temperature.
- Choose the Current heater fuel, enter its published Current heater UEF, and enter the price in the fuel unit named by the calculator.
- Enter the matching fuel, UEF, and price for the proposed heater.
- Open Advanced options to add Proposed heater price, Installation cost, and confirmed Rebates and incentives if you want a net project cost and payback estimate.
- Click Calculate, then check that a higher proposed UEF or lower proposed fuel price produces a lower Proposed heater estimated yearly operating cost.
Definitions
UEF: Uniform Energy Factor. A water-heater rating that compares useful hot-water energy with the energy used by the heater. A higher UEF generally means less fuel energy is needed for the same hot-water demand. [1]
Therm: A natural-gas energy unit equal to 100,000 Btu. [2]
Btu: British thermal unit, a unit of heat energy used to compare electricity and heating fuels. [2]
Temperature rise: The water heater setting minus the incoming cold-water temperature. More temperature rise requires more energy.
Simple payback time: Net upfront project cost divided by estimated yearly operating savings. It is the estimated time for savings to equal the upfront cost.
Common mistakes and quick fixes
Mistake: Entering total household water use in Hot water used each day.
Fix: Enter only hot water delivered after heating, in gallons per day.
Mistake: Setting Water heater setting at or below Incoming cold-water temperature.
Fix: Enter a water heater setting that is higher than the incoming cold-water temperature.
Mistake: Using a gas price per cubic foot in Current fuel price or Proposed fuel price.
Fix: For natural gas, convert or obtain the price in $ per therm before entering it.
Mistake: Entering an efficiency percent, such as 90, for Current heater UEF.
Fix: Enter the published UEF rating, such as 0.90, rather than a percent.
Mistake: Treating Estimated yearly operating savings as guaranteed utility-bill savings.
Fix: Use it as a simplified estimate and update fuel prices and hot-water use to test your situation.
Mistake: Forgetting rebates in Estimated net upfront project cost.
Fix: Enter only confirmed amounts in Rebates and incentives so the project cost and Simple payback time stay realistic.
Limitations & Key Assumptions / Boundary Conditions
- Operating costs are simplified estimates, not utility bills. Actual hot-water use, incoming-water temperature, standby losses, and fuel prices can change over time.
- The calculation treats the entered UEF as useful hot-water energy divided by fuel energy used. Published UEF values are based on standardized test conditions that may differ from your home. [1]
- Fuel prices must match the selected fuel unit: $ per kWh for electricity, $ per therm for natural gas, and $ per gallon for propane or fuel oil.
- Simple payback is shown only when estimated yearly operating savings and estimated net upfront project cost are both positive. It does not include financing, maintenance, repair costs, taxes, fuel-price changes, or the value of replacing a failed heater.
- Rebates can have eligibility rules, deadlines, and installation requirements. Enter only rebates you reasonably expect to receive.
Methodology
Operating-cost calculation
The calculator first estimates the heat delivered to hot water each day from gallons used and temperature rise. Water is treated as weighing 8.34 pounds per gallon.
daily_output_btu = daily_gallons * 8.34 * (set_temp_f - inlet_temp_f)
It annualizes that useful heat and divides by the entered UEF to estimate fuel energy needed each year.
annual_input_btu = daily_output_btu * 365 / uef
That energy is converted into the unit for the selected fuel, then multiplied by the entered fuel price. Electricity uses 3,412.142 Btu per kWh; natural gas uses 100,000 Btu per therm; propane uses 91,502 Btu per gallon; and fuel oil uses 138,500 Btu per gallon. [2]
annual_cost = annual_input_btu / fuel_btu_per_unit * fuel_price
Comparison and project cost
Estimated yearly operating savings are current estimated yearly cost minus proposed estimated yearly cost. A positive value means the proposed heater is estimated to cost less to run.
annual_savings = current_annual_cost - proposed_annual_cost
monthly_savings = annual_savings / 12
Net upfront project cost adds the proposed heater price and installation cost, then subtracts rebates and incentives.
net_project_cost = equipment_cost + installation_cost - rebate
simple_payback_years = net_project_cost / annual_savings
Mini-example
With 50 gallons per day, a 120 F setting, and 50 F incoming water, daily useful heat is 29,190 Btu. An electric current heater with UEF 0.90 and electricity at $0.15 per kWh has an estimated annual operating cost of about $520.41. With the same inputs, a proposed electric heater with UEF 3.00 has an estimated annual operating cost of about $156.12, for estimated yearly operating savings of about $364.29.