Tankless Water Heater Operating Cost Calculator

Tankless water heaters only consume energy while water is flowing.
That eliminates the standby losses of a tank model (40-60 gallons sitting at 130°F all day, even when nobody is home).
The catch is they need much higher peak BTU input to deliver hot water on demand, and that raises operating cost during use.

The math:

annual_BTU = gallons_per_day × 8.34 × (T_set − T_inlet) × 365 annual_fuel = annual_BTU / efficiency / fuel_BTU_per_unit

The 8.34 is the weight of a gallon of water in pounds, and one BTU raises one pound of water by one degree Fahrenheit. Multiply the two and you have the BTU to heat a gallon by a degree.
A 4-person household uses about 60 gallons of hot water per day per the EPA.
Inlet water temperature averages 50-70°F depending on climate; setpoint is usually 120°F (the recommended max for safety, especially with kids in the house).

Worked example for a typical setup.
4 people, 60 gal/day, inlet 55°F, setpoint 120°F.
Annual BTU need: 60 × 8.34 × 65 × 365 = 11.9 million BTU.

Gas tankless at 90% efficiency: 11.9 / 0.90 = 13.2 million BTU input = 132 therms/year.
At $1.30/therm = $172/year.

Electric tankless at 99% efficiency: 11.9 / 0.99 = 12.0 million BTU = 3,520 kWh/year.
At $0.16/kWh = $563/year.

Propane is the odd one out on units. It is sold by the gallon rather than the therm, and a gallon carries about 91,500 BTU against the therm’s 100,000, so the same 13.2 million BTU input is 144 gallons rather than 132 therms. Enter the price straight off the propane invoice and the calculator handles the difference.

Electric tankless costs 3× more to operate than gas in this scenario, even though it’s more efficient at converting input fuel to hot water.
The reason: electricity per BTU costs about 4× more than natural gas.

Compared to a tank water heater:

• Tank gas (50 gal, 60% efficient with standby): $230-300/year
• Tankless gas: $150-180/year
• Tank electric: $480-600/year
• Tankless electric: $500-580/year (barely cheaper than tank electric)

The break-even on tankless versus tank.
A tankless gas unit costs $1,800-3,500 installed; a tank gas unit is $1,000-1,800.
Annual savings of $80 to $100 means a 10 to 25 year payback, which is about the lifespan of a tank, so the arithmetic comes out roughly neutral.
Tankless wins on never running out of hot water, taking less floor space, and lasting 20+ years vs 10-12 for tanks.
It loses on cold-weather performance: many gas tankless units struggle at full flow when inlet water is below 40°F.

Three practical points.
Whole-home electric tankless needs 20 to 40 kW of electrical service, which usually means a panel upgrade: fine in a new build, expensive in a retrofit.
Recirculation pumps add convenience but defeat much of the standby-loss advantage; a recirc pump on a tankless adds 10-20% to annual operating cost.
And hard water shortens tankless life dramatically: scale buildup in the heat exchanger requires annual descaling for water above 7 grains per gallon hardness, or warranty often becomes void.