Calculate the energy needed to heat water from one temperature to another or find saturated liquid water specific enthalpy at a given temperature.
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Enthalpy of Water Formula
The calculator has two modes. The first mode estimates the energy needed to heat or cool liquid water using the specific heat equation.
- q = heat energy transferred, usually in kJ
- m = mass of water in kg
- c = specific heat capacity of liquid water, 4.184 kJ/(kg·°C)
- ΔT = temperature change in °C
- T1 = starting temperature
- T2 = target temperature
The second mode estimates the specific enthalpy of saturated liquid water, hf, from steam-table values. If the entered temperature falls between two table values, the calculator uses linear interpolation.
- hf = specific enthalpy of saturated liquid water in kJ/kg
- T = entered water temperature
- T1, T2 = the nearest lower and upper steam-table temperatures
- h1, h2 = the corresponding saturated liquid enthalpy values
In the energy mode, the calculator converts your water amount to kilograms, converts temperatures to °C when needed, then applies q = m × c × ΔT. A positive result means heat is added to the water. A negative result means heat is removed as the water cools.
In the specific enthalpy mode, the calculator treats the water as saturated liquid water and returns hf. This is not the enthalpy of steam vapor, superheated steam, or compressed liquid water at an arbitrary pressure.
Common Water Heating and Enthalpy Values
These values are useful for checking whether your result is in the expected range.
| Water amount | Temperature change | Energy needed | Approximate Wh |
|---|---|---|---|
| 250 mL | 20 °C to 100 °C | 83.7 kJ | 23.2 Wh |
| 500 mL | 20 °C to 100 °C | 167.4 kJ | 46.5 Wh |
| 1 L | 20 °C to 100 °C | 334.7 kJ | 93.0 Wh |
| 1 kg | 0 °C to 100 °C | 418.4 kJ | 116.2 Wh |
| Temperature | Saturated liquid enthalpy, hf | Typical interpretation |
|---|---|---|
| 0 °C | 0.0 kJ/kg | Reference point used for liquid water enthalpy |
| 25 °C | 104.8 kJ/kg | Room-temperature liquid water |
| 80 °C | 334.9 kJ/kg | Hot liquid water |
| 100 °C | 419.0 kJ/kg | Saturated liquid at normal boiling temperature |
| 200 °C | 852.4 kJ/kg | Pressurized saturated liquid |
Example Calculations
Example 1: Energy to heat 500 mL of water
Find the energy required to heat 500 mL of water from 20 °C to 100 °C.
The energy required is about 167.4 kJ.
Example 2: Specific enthalpy at 80 °C
Find the saturated liquid water enthalpy at 80 °C.
The specific enthalpy is about 334.9 kJ/kg.
FAQs
Is enthalpy the same as heat energy?
No. Heat energy, q, is energy transferred because of a temperature difference. Enthalpy, h, is a thermodynamic property of a substance. In the heating mode, you calculate the amount of heat needed to change the water temperature. In the specific enthalpy mode, you get a property value for saturated liquid water at a chosen temperature.
Why is the enthalpy of water 0 kJ/kg at 0 °C?
The value is based on a reference point. Steam tables commonly set the specific enthalpy of saturated liquid water to 0 kJ/kg at 0 °C. This does not mean the water has no internal energy. It means enthalpy values are measured relative to that chosen reference.
Does this include the energy needed to boil water into steam?
No, the energy-to-heat mode only uses the liquid water specific heat equation. It estimates heating or cooling within the liquid range. To convert boiling water into steam, you also need the latent heat of vaporization, which is separate from the sensible heating calculation.
