# Water Heating Calculator

URL: https://speedycalculator.com/calculators/physics/water-heating/
Last updated: 2026-10-30

> The water heating calculator finds the energy and time needed to heat a quantity of water from one temperature to another with a heater of given power and efficiency, and the energy cost.

## Inputs

- Amount given as
- Volume (L, mL, m³, US gal, ft³)
- Mass (kg, g, mg, t, lb, oz)
- Initial temperature (°C, °F, K)
- Final temperature (°C, °F, K)
- Heater power (kW, W)
- Efficiency (%)
- Electricity price per kWh

## Results

- Energy needed
- Heating time
- Energy cost

## Formula

```
Q = m·c·ΔT (c = 4,186 J/(kg·K)),  t = Q / (P·η)
```

## Explanation

Heating water is one of the biggest energy uses at home. A 2 kW kettle boils a litre in about three minutes; a 150 L tank needs several kWh to heat from cold.

### How to use the Water Heating Calculator

- **Amount given as**
- **Volume**
- **Mass**
- **Initial temperature**
- **Final temperature**
- **Heater power**
- **Efficiency (%)**
- **Electricity price per kWh**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the amount of water, start and target temperatures, the heater power, efficiency and your electricity price.

### Tips

- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.

### Common mistakes to avoid

- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).

### Inputs explained

FieldWhat to enterAmount given asChoose one: Volume, MassVolumeUnits: L, mL, m³, US gal, ft³MassUnits: kg, g, mg, t, lb, ozInitial temperatureUnits: °C, °F, KFinal temperatureUnits: °C, °F, KHeater powerUnits: kW, WEfficiency (%)A numberElectricity price per kWhAmount in your currency

### What each result means

- **Energy needed** = [mass]×4186×([Final temperature] - [Initial temperature])
- **Heating time** = [mass]×4186×([Final temperature] - [Initial temperature])÷([Heater power]×[Efficiency (%)]÷100)
- **Energy cost** = [mass]×4186×([Final temperature] - [Initial temperature])÷([Efficiency (%)]÷100)÷3.6e6×[Electricity price per kWh]

### Example values

How the main result changes when **Volume** changes and everything else stays at the example values above:

Volume (L)Heating time (min)0.752.4661.133.69891.54.93191.886.16492.257.397939.8638

## Worked example

Inputs — Amount given as: Volume; Volume: 1.5 L; Initial temperature: 15 °C; Final temperature: 100 °C; Heater power: 2 kW; Efficiency (%): 90; Electricity price per kWh: $0.25.

Results — Energy needed = 0.14796 kWh; **Heating time = 4.9319 min**; Energy cost = $0.041.

## Key terms

- **Specific heat of water**: About 4,186 J/(kg·K).

## FAQ

**How long does a kettle take to boil 1 L?**
About 3 minutes at 2 kW.

**Why include efficiency?**
Some heat is lost to the surroundings and the kettle itself.

**Does this include boiling off?**
No – evaporating water needs 2,257 kJ/kg more.

**How much energy heats a 150 L tank by 45 °C?**
About 7.8 kWh.

**What formula does the Water Heating Calculator use?**
Q = m·c·ΔT (c = 4,186 J/(kg·K)),  t = Q / (P·η).


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)