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
| Field | What to enter |
|---|---|
| Amount given as | Choose one: Volume, Mass |
| Volume | Units: L, mL, m³, US gal, ft³ |
| Mass | Units: kg, g, mg, t, lb, oz |
| Initial temperature | Units: °C, °F, K |
| Final temperature | Units: °C, °F, K |
| Heater power | Units: kW, W |
| Efficiency (%) | A number |
| Electricity price per kWh | Amount 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) |
|---|---|
| 0.75 | 2.466 |
| 1.13 | 3.6989 |
| 1.5 | 4.9319 |
| 1.88 | 6.1649 |
| 2.25 | 7.3979 |
| 3 | 9.8638 |
Formula
Q = m·c·ΔT (c = 4,186 J/(kg·K)), t = Q / (P·η)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).
Frequently asked questions
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).