In short: heating 50 gallons of water from 50°F to 120°F — a typical cold-supply-to-water-heater rise — takes about 8.56 kWh (29,194 BTU). Energy scales directly with volume and with the size of the temperature rise, so doubling either one roughly doubles the energy needed.
This is the theoretical energy delivered to the water itself, a planning aid based on standard physics. It does not include your heater's efficiency or standby losses — for a cost estimate, use the shower cost or bath cost calculators.
How to calculate the energy to heat water
Heating water is a straightforward application of the specific heat equation: the energy needed depends on how much water you have, how much you need to raise its temperature, and one physical constant — the specific heat of water.
This calculator converts your volume to cubic meters, looks up the density of pure water at your starting temperature, multiplies by the specific heat and the temperature rise, and returns the result in kWh, BTU, MJ, kJ, kcal and Wh.
Worked example. Take the default scenario: 50 US gallons heated from 50°F to 120°F. Fifty gallons is 189.27 liters, or 0.18927 m³. At 50°F (10°C), water has a density of about 999.70 kg/m³, so the mass is 0.18927 × 999.70 ≈ 189.2 kg (417.1 lb). The temperature rise is 70°F, which is 38.9°C. Multiply: 189.2 kg × 4,186 J/(kg·°C) × 38.9°C ≈ 30.8 million joules — about 8.56 kWh, or 29,194 BTU.
Where the energy actually goes. This is the energy transferred into the water as heat. It assumes a sealed, well-mixed volume with no evaporation and no heat lost to the tank, pipes or surrounding air — real installations lose some of that energy, which is why an electric water heater's actual electricity use runs a bit higher than this figure.
Why the temperature rise (and starting point) changes the answer
The single biggest factor is ΔT — how far you need to raise the temperature. Energy scales directly with it: doubling the temperature rise doubles the energy required, all else equal. That is why heating water from near-freezing takes noticeably more energy than a small top-up of already-warm water.
The starting temperature matters in two ways. First, and by far the largest effect, a colder start usually means a bigger rise to reach the same target. Second, and much smaller, water is slightly denser when cold, so a fixed volume of cold water has a touch more mass to heat — the density used here comes from the same Kell equation used across this site, and it changes by well under half a percent between 0°C and 60°C. This calculator applies density at your starting temperature so the mass figure is accurate either way. You can read the full method on our methodology page.
Energy to heat 1 US gallon of water by common temperature rises
Using a 60°F starting point. Because energy scales linearly with ΔT, you can scale these figures by your actual volume — for example, double them for 2 gallons, or multiply by 50 for a 50-gallon tank.
| Temperature rise | Energy (kWh) | Energy (BTU) | Energy (kJ) |
|---|---|---|---|
| 10°F (5.6°C) | 0.0244 | 83.4 | 87.9 |
| 20°F (11.1°C) | 0.0489 | 166.7 | 175.9 |
| 30°F (16.7°C) | 0.0733 | 250.1 | 263.8 |
| 40°F (22.2°C) | 0.0977 | 333.4 | 351.8 |
| 50°F (27.8°C) shower rise | 0.1221 | 416.8 | 439.7 |
| 60°F (33.3°C) | 0.1466 | 500.1 | 527.7 |
| 70°F (38.9°C) tank rise | 0.1710 | 583.5 | 615.6 |
| 80°F (44.4°C) | 0.1954 | 666.8 | 703.6 |
| 90°F (50.0°C) | 0.2199 | 750.2 | 791.5 |
| 100°F (55.6°C) | 0.2443 | 833.6 | 879.5 |
Worked examples
Refilling a 50-gallon water heater
8.56 kWh
Raising 50 gallons from a 50°F cold supply to a 120°F tank setting takes about 8.56 kWh (29,194 BTU) — the tool's default scenario.
A 10-minute shower
1.22 kWh
10 gallons raised from 55°F to 105°F takes about 1.22 kWh (4,169 BTU) — roughly a seventh of a full tank refill.
Boiling a liter for tea
0.093 kWh
1 liter raised from 20°C to 100°C takes about 0.093 kWh (317 BTU, 80 kcal), before accounting for any water that turns to steam.
What else affects the energy you actually use
Heater efficiency. Electric resistance heaters convert nearly all their input energy into heat, so actual use is close to this figure. Gas water heaters typically run 80–95% efficient, and heat pump water heaters can exceed 100% efficiency (measured against electricity input) because they move heat rather than generate it — so real energy use differs from the physics-only number here.
Standby and distribution losses. Stored hot water slowly loses heat to its surroundings, and pipes lose some heat on the way to the fixture. Insulated tanks and pipe wrap reduce this loss but rarely eliminate it.
Mixing with cold water at the tap. If you are estimating a shower or bath, remember that some of the water at the fixture may be blended with cold water to reach a comfortable temperature — that changes the effective volume actually heated, not the physics of this calculation.
Vaporization. This calculator only covers heating liquid water to a target temperature. Turning water into steam at the boiling point takes substantially more energy on top of this — see the heat of vaporization calculator for that step.
Frequently asked questions
How much energy does it take to heat a gallon of water?
Near room temperature, it takes about 8.3 BTU (0.0024 kWh) to raise one US gallon of water by 1°F. Raising a gallon by 70°F — a typical cold-to-hot-tap rise — takes about 0.17 kWh (583 BTU). Energy scales directly with both the volume and the temperature rise.
How much energy does a 50-gallon water heater use to heat a full tank?
Heating 50 gallons of water from 50°F (a typical cold-water supply) to 120°F (a common water heater setting) takes about 8.56 kWh, or roughly 29,200 BTU. This is the energy delivered to the water itself, before accounting for the heater's efficiency or standby losses.
Does the starting temperature of the water matter?
Mostly through the temperature difference: colder incoming water needs a bigger rise to reach the same target, so it takes more energy. The density of water itself changes only slightly with temperature — well under half a percent between 0°C and 60°C — so its direct effect on the answer is small compared with the size of the temperature rise.
How much energy does it take to boil a liter of water?
Raising 1 liter of water from 20°C to 100°C takes about 0.093 kWh, or roughly 317 BTU (about 80 kcal). That figure only covers heating the liquid to its boiling point — it does not include the extra energy needed to actually vaporize any of the water into steam.
Does this include water heater efficiency and standby losses?
No. This calculator gives the theoretical energy transferred to the water itself. The electricity or gas you actually use is higher, because no heater is perfectly efficient and tanks lose some heat while water sits. For a cost estimate that accounts for your rates and equipment, see the shower cost or bath cost calculators.
Why does this calculator only give energy, not a dollar cost?
Because converting energy into cost depends on your utility rate, fuel type and heater efficiency, none of which are physical constants. This page keeps to the physics — the energy transferred to the water. The shower cost and bath cost calculators build on this same formula and add rate and efficiency inputs.
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, chapter on thermophysical properties of water.
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)