In short: boiling one US gallon of water from room temperature (20 °C) takes about 0.35 kWh (1,265 kJ, roughly 1,199 BTU) of heat energy — around $0.06 of electricity and 15 minutes in a typical 1,500 W kettle. Colder starting water needs more energy; warmer water needs less, because you are only paying to close the remaining gap to 100 °C.
These figures are planning aids based on published thermodynamic data and typical appliance efficiencies. For metering, billing disputes or commercial equipment sizing, confirm with your utility or a licensed engineer.
How to calculate the energy to boil water
Bringing water to a boil is a straightforward heat-transfer problem: you are raising the temperature of a known mass of water by a known number of degrees. The amount of heat that takes is given by the sensible-heat equation:
This calculator converts your volume to a mass using the temperature-dependent density of water, then multiplies by the specific heat and the temperature gap to 100 °C. The result converts into kWh, BTU, kJ and kcal — see the specific heat of water calculator if you want to explore the constant itself, or the energy to heat water calculator if your target temperature isn't 100 °C.
Worked example. Take 1 US gallon of water at 20 °C. That's 3.7854 liters, which at 20 °C has a density of 998.2 kg/m³, giving a mass of 3.7786 kg. The temperature gap to boiling is 80 °C. Multiply: 3.7786 × 4,186 × 80 = 1,265,382 J, which is 1,265.4 kJ, 0.3515 kWh, or 1,199 BTU. That single multiplication is everything the calculator is doing — the work is in getting the mass and the gap right.
Boiling vs. fully vaporizing, in one line. Reaching 100 °C and turning all of the water into steam are very different amounts of energy. Fully vaporizing water takes an additional 2,257 kJ per kilogram — the latent heat of vaporization — because that energy breaks molecular bonds rather than raising temperature. Tick "also add energy to fully vaporize" above to include it, or use the dedicated heat of vaporization calculator.
Why starting temperature — and mass — change the answer
The energy needed is directly proportional to two things: how much water you have, and how far it has to travel to reach 100 °C. Double the volume and you double the energy. Halve the temperature gap — say by starting with warm tap water instead of cold — and you roughly halve the energy, because you are simply doing less thermal work.
Water's density also shifts slightly with temperature, which changes how much mass a fixed volume represents. This calculator accounts for that using the same Kell equation used across this site's density and weight tools, so the mass behind your result is accurate to a few parts per million. In practice, the temperature gap matters far more than the small density shift: water starting at 5 °C needs roughly 60% more energy to boil than water already at 40 °C.
Energy to boil water by starting temperature
Sensible-heat energy only (no vaporization), for two common volumes. Notice how the energy needed shrinks as the starting temperature climbs toward 100 °C.
| Starting temp. | 1 US gallon (kWh) | 1 US gallon (BTU) | 1 liter (kWh) | 1 liter (BTU) |
|---|---|---|---|---|
| 5 °C / 41 °F (fridge) | 0.418 | 1,427 | 0.111 | 377 |
| 10 °C / 50 °F (cold tap) | 0.396 | 1,351 | 0.105 | 357 |
| 15 °C / 59 °F | 0.374 | 1,275 | 0.099 | 337 |
| 20 °C / 68 °F (room) | 0.351 | 1,199 | 0.093 | 317 |
| 25 °C / 77 °F | 0.329 | 1,123 | 0.087 | 297 |
| 30 °C / 86 °F | 0.307 | 1,047 | 0.081 | 277 |
| 40 °C / 104 °F (warm tap) | 0.262 | 894 | 0.069 | 236 |
Quick reference: energy to boil common volumes
Starting from room temperature (20 °C / 68 °F), sensible heat only. Handy when you just need a number without opening the calculator.
| Volume | kWh | BTU | kcal |
|---|---|---|---|
| 1 US quart | 0.088 | 300 | 76 |
| 1 liter | 0.093 | 317 | 80 |
| 1 US gallon | 0.351 | 1,199 | 302 |
| 5 US gallons | 1.757 | 5,997 | 1,511 |
| 55-gallon drum | 19.33 | 65,964 | 16,626 |
Worked examples
Tea kettle: 1 liter from 15 °C
0.099 kWh
337 BTU of heat energy — about 4 minutes on a 1,500 W electric kettle at 95% efficiency, costing roughly $0.02 at $0.16/kWh.
Stockpot: 1 gallon from 20 °C on the stove
0.351 kWh
1,199 BTU — about 14 minutes on a 2,000 W electric stovetop burner at 75% efficiency, costing roughly $0.08.
Catering urn: 5 gallons from 10 °C
1.98 kWh
6,757 BTU — about 44 minutes in a 3,000 W electric urn at 90% efficiency, costing roughly $0.35.
What else affects the real energy and cost
Altitude. Water boils below 100 °C at higher elevations because atmospheric pressure is lower, so there's a smaller temperature gap to close and slightly less sensible-heat energy is needed. The effect on energy is modest — a few percent at typical mountain elevations — but the lower boiling point changes cooking times more than it changes your energy bill. See the boiling point at altitude calculator for the exact figure at your elevation.
Heater efficiency. Not all input energy reaches the water. Electric kettles are highly efficient because the element sits in the water; open stovetop burners lose more heat to the surrounding air, and gas burners lose the most, since much of the flame's heat escapes around the pot rather than through it.
Lids and insulation. An uncovered pot loses heat to evaporation and convection well before boiling, which is why a lid measurably shortens boiling time and reduces real-world energy use versus the idealized figure this calculator reports.
Cookware and burner match. A pot smaller than the burner, or poor contact with an induction element, wastes energy around the edges. Matching pot size to heat source recovers some of this loss.
Frequently asked questions
How much energy does it take to boil a gallon of water?
Heating one US gallon of water from room temperature (20 °C / 68 °F) to boiling takes about 0.35 kWh — roughly 1,265 kJ or 1,199 BTU. Starting from colder tap water takes a little more; starting from warm water takes less, because you are only supplying the heat needed to close the remaining gap to 100 °C.
How much does it cost to boil a gallon of water?
At the U.S. average electricity price of about $0.16 per kWh and a typical electric kettle's 95% efficiency, boiling one gallon of room-temperature water costs roughly $0.06 and takes about 15 minutes at 1,500 watts. A less efficient stovetop burner costs a bit more for the same water because more of the input energy escapes as waste heat.
Does altitude change how much energy it takes to boil water?
Yes, slightly. Water boils below 100 °C at higher elevations because atmospheric pressure is lower, so there is a smaller temperature gap to close and the sensible-heat energy needed is a little less. The energy saving is modest — a few percent at typical mountain elevations — but the lower boiling point itself matters more for cooking times than for energy use.
Is boiling water the same as fully vaporizing it?
No. Bringing water to a boil only requires sensible heat to raise its temperature to 100 °C. Turning that same water into steam takes far more energy — about 2,257 kJ per kilogram, called the latent heat of vaporization — because it goes into breaking molecular bonds rather than raising temperature. This calculator reports the energy to reach boiling by default, with an option to add full vaporization.
Why does starting temperature matter so much?
The energy required is directly proportional to the temperature gap between your starting point and 100 °C. Water starting at 5 °C needs about 60% more energy to boil than water already at 40 °C, because there is a bigger gap to close. This is why preheating water, or starting from warm tap water, measurably cuts the energy and time needed.
Sources & references
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- ASHRAE. ASHRAE Handbook — Fundamentals (thermophysical properties of water, specific heat and latent heat of vaporization).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)