How to Calculate the Energy Stored in an Inductor
An inductor stores energy in the magnetic field created by current flowing through its coil. The amount of energy depends on two things: how effectively the coil concentrates that field per amp (its inductance, L) and how much current flows through it (I). This calculator applies the standard magnetic energy formula, E = ½LI², along with the related flux linkage formula λ = LI, to convert your inductance and current into stored energy, flux linkage, and — if you provide a collapse time — the average power released as that current dies away.
Where the ½LI² formula comes from
An inductor's voltage relates to how fast its current changes: v = L(di/dt). The instantaneous power delivered to the inductor is p = vi = Li(di/dt). Integrating that power over time as current ramps from 0 up to its final value I gives the total energy stored: W = ∫Li di, evaluated from 0 to I, which is W = ½LI². The factor of ½ appears for the same reason it does in ½mv² or ½CV²: the quantity being built up (current, and the flux linked with it) grows linearly, so the average "cost" over the buildup is half the final value.
Reading the results: energy units, flux linkage, and collapse power
The calculator reports energy in joules (auto-scaled to µJ, mJ, or kJ as needed) and also converts it to watt-hours so you can compare an inductor's storage capacity to a battery's — inductors typically store far less energy than batteries or even capacitors of similar size, which is why they excel at brief energy transfer (as in switching power supplies) rather than long-term storage. Flux linkage (λ = LI) is reported in weber-turns (Wb), auto-scaled to µWb or mWb for typical component values. If you enter a collapse time, the tool divides total energy by that time to estimate average power (P = E/t) released while the current dies away — this is an average over the whole collapse, not the instantaneous peak, which depends on the circuit and is highest at the moment the current is interrupted.
Practical notes and safety
- Energy scales with the square of current, so doubling the current through an inductor quadruples the stored energy — current rating matters as much as inductance for total energy storage.
- Interrupting current through an inductor abruptly (opening a switch on a relay coil or motor winding) produces a large back-EMF voltage spike, since V = L(dI/dt) grows without bound as dt shrinks; always use a flyback diode or snubber circuit to give that energy a safe path.
- Iron- or ferrite-core inductors store more energy per unit volume than air-core inductors of the same size, but pushing current too high saturates the core and causes inductance to collapse — check the manufacturer's saturation current rating.
- Real inductors have series resistance (DCR) that dissipates some energy as heat during charge/discharge and limits true peak collapse power below the idealized P = E/t estimate.