Magnetic field energy
What is energy stored in an inductor?
An inductor stores energy in the magnetic field created when electric current flows through its coil.
The stored energy depends on the inductance of the component and the square of the current passing through it.
Main energy equation
Inductor energy formula
Stored magnetic energyE = ½LI²
- E is stored energy in joules.
- L is inductance in henries.
- I is electric current in amperes.
The equation can be rearranged as L = 2E/I² to calculate inductance and I = √(2E/L) to calculate current.
Worked example
Calculate energy from inductance and current
Suppose an inductor has an inductance of 0.5 H and carries a current of 4 A.
- Use E = ½LI².
- Substitute L = 0.5 H and I = 4 A.
- E = ½ × 0.5 × 4².
- The stored energy is 4 J.
Current effect
Why current strongly affects energy
Current is squared in the energy equation. Doubling current increases stored energy by a factor of four when inductance stays constant.
This is important in switching circuits because interrupting a large inductor current can produce a high induced voltage.
SI unit guidance
Inductor energy units
- Energy: joules (J).
- Inductance: henries (H).
- Current: amperes (A).
- 1 millihenry = 1 × 10⁻³ H.
- 1 microhenry = 1 × 10⁻⁶ H.
- 1 millijoule = 1 × 10⁻³ J.
Practical applications
Where inductor energy matters
- Switching power supplies and converters.
- Transformers, motors, and relays.
- Electromagnets and magnetic actuators.
- Energy transfer in LC and RLC circuits.
- Flyback circuits and transient protection.
Model limitations
Assumptions and limitations
- The inductor is treated as an ideal linear component.
- Coil resistance is not included.
- Magnetic core saturation is ignored.
- Hysteresis and eddy-current losses are not modelled.
- Inductance is assumed constant at the specified current.
Common questions
Inductor energy calculator FAQ
What formula does the inductor energy calculator use?
It uses E = 1/2 LI², where E is stored magnetic energy, L is inductance, and I is current.
What is the SI unit of energy stored in an inductor?
Inductor energy is measured in joules, written J.
How do I calculate inductance from stored energy?
Multiply stored energy by two and divide by current squared using L = 2E/I².
How do I calculate current from energy and inductance?
Divide twice the stored energy by inductance and take the square root using I = √(2E/L).
Why does inductor energy depend on current squared?
The magnetic field becomes stronger as current rises, so stored energy increases in proportion to the square of current.