Inductor Energy Storage Calculator
Calculate the energy stored in an inductor from its inductance and current.
Shows energy in joules and millijoules.
An inductor stores energy in its magnetic field when current flows through it. The stored energy is:
E = ½LI²
Where:
- E = Energy in joules (J)
- L = Inductance in henries (H)
- I = Current in amperes (A)
Key observations:
- Energy scales with the square of current — doubling the current quadruples the stored energy
- Energy scales linearly with inductance — doubling L doubles stored energy
- This is the magnetic analog of the capacitor formula E = ½CV²
What happens to stored energy?
When current through an inductor is suddenly interrupted, the stored energy must go somewhere. The inductor will create a back-EMF voltage spike to maintain the current. This can be calculated from:
V = L × dI/dt
A 1 H inductor carrying 1 A that is switched off in 1 ms will generate a voltage spike of V = 1 × (1/0.001) = 1,000 V — even if the supply was only 12 V! This is why inductive loads require protection diodes (flyback diodes) in electronic circuits.
Practical examples:
- Relay coil (0.1 H, 0.05 A): E = ½ × 0.1 × 0.0025 = 0.000125 J = 0.125 mJ
- Car ignition coil (~10 mH, 5 A): E = ½ × 0.01 × 25 = 0.125 J — enough to create a spark
- MRI superconducting magnet (~50 H, 400 A): E = ½ × 50 × 160,000 = 4,000,000 J = 4 MJ!