Inductor Energy Storage Calculator
Enter the inductance and current of any inductor to find the energy stored in its magnetic field using the formula E = 1/2 LI^2. Switch between henrys, millihenrys, microhenrys, and nanohenrys for inductance, and amps or milliamps for current. Choose a solve mode to reverse-calculate inductance from a known energy and current, or to find the required current for a target energy. Results update instantly as you type.
How the inductor energy storage formula works
An inductor stores energy in the magnetic field that builds up around its coil whenever current flows through it. The governing equation is E = 1/2 x L x I^2, where E is the stored energy in joules, L is the inductance in henrys, and I is the current in amperes. Because current appears squared, energy grows much faster with increasing current than with increasing inductance: doubling the inductance doubles the energy, but doubling the current quadruples it. This squared relationship is why even small inductors can store surprisingly large amounts of energy when they carry high currents, and it is also why the magnetic field pushes back hard when anything tries to interrupt the current suddenly.
Solving for inductance or current (reverse calculation)
The same formula can be rearranged to find the inductance needed to store a target energy at a known current: L = 2E / I^2. It can also be rearranged to find the current needed to reach a target energy in a known inductor: I = sqrt(2E / L). These reverse modes are useful when designing a circuit: you might start with a required energy budget and a current limit, then work out the minimum inductance you need, or start with a particular off-the-shelf inductor and check what current it takes to reach the desired energy. Select the solve-for mode at the top of the calculator to use either rearrangement.
Practical considerations: saturation, losses, and core material
The formula E = 1/2 x L x I^2 assumes the inductance L stays constant, which holds only while the magnetic core is not saturated. Every real inductor has a saturation current rating: once current exceeds this level, the core saturates and inductance falls sharply, often by 50 to 80 percent, so the actual stored energy is much less than the formula predicts. Winding resistance also wastes energy as heat (I^2 x R losses). Core material choice governs the saturation flux density and therefore the usable energy density: ferrite cores are common in switching power supplies operating at high frequency, powdered iron handles higher currents before saturation, and laminated silicon steel is used in large low-frequency inductors. For accurate design, always consult the inductor datasheet and factor in the rated saturation current and DC resistance.
Where inductor energy storage matters in real circuits
Inductors store and release energy in virtually every switching power supply. In a buck (step-down) converter the inductor absorbs energy when the switch is on and delivers it to the load when the switch is off, smoothing the output current. Boost (step-up) converters reverse this: the inductor stores energy during the on-time and releases it at a higher voltage. Flyback converters, motor drivers, and class-D audio amplifiers all depend on carefully chosen inductance values for efficiency and stability. At the other end of the scale, superconducting magnetic energy storage (SMES) systems use massive inductors carrying thousands of amperes to store megajoules of electrical energy for grid balancing. Understanding the energy relationship helps engineers size components correctly, predict transient voltage spikes when current is interrupted (V = L x dI/dt), and meet efficiency targets.
Typical inductor types and energy storage ranges
| Inductor type | Typical inductance | Typical current | Energy range | Common application |
|---|---|---|---|---|
| RF/chip inductor | 1 nH - 100 nH | 0.1 - 2 A | < 1 nJ | RF matching, filters |
| Power inductor (SMD) | 0.1 uH - 100 uH | 0.5 - 20 A | 1 nJ - 10 mJ | DC-DC converters, SMPS |
| Toroidal power inductor | 10 uH - 10 mH | 1 - 50 A | 10 uJ - 10 J | Power supplies, inverters |
| Common-mode choke | 10 uH - 100 mH | 0.1 - 10 A | < 1 mJ | EMI suppression |
| Ferrite-core inductor | 1 mH - 1 H | 0.01 - 5 A | 0.1 uJ - 10 mJ | Audio filters, low-frequency |
| Air-core coil | 1 nH - 10 uH | 1 - 100 A | < 100 uJ | High-frequency, pulsed circuits |
| Energy storage inductor | 0.1 H - 10 H | 10 - 1000 A | 1 J - 1 kJ | SMES, pulsed power systems |
Approximate inductance and energy storage characteristics across common inductor families. Actual values depend on current rating, core material, and winding.
Frequently asked questions
What is the formula for energy stored in an inductor?
The energy stored in an inductor is E = 1/2 x L x I^2, where E is the energy in joules, L is the inductance in henrys, and I is the current in amperes. Because current is squared, a small increase in current has a large effect on the energy stored.
What units does inductor energy come in?
The SI unit is the joule (J). In electronics, the energies are usually small, so millijoules (mJ = 0.001 J), microjoules (uJ = 0.000001 J), and nanojoules (nJ = 10^-9 J) are commonly used. This calculator displays results in the most appropriate sub-unit automatically.
Does doubling the inductance double the stored energy?
Yes, energy is directly proportional to inductance, so doubling L doubles E (at the same current). By contrast, doubling the current quadruples the energy, because current enters the formula squared. In most designs, increasing current is the more powerful lever for raising stored energy, but it also raises resistive losses and saturation risk.
What happens to the stored energy when current is switched off?
When the current through an inductor is interrupted suddenly, the magnetic field collapses and the stored energy must go somewhere. It drives a voltage spike (V = L x dI/dt) that can be very large if dI/dt is rapid, potentially damaging other components. Freewheeling diodes (also called flyback or snubber diodes) are placed across inductive loads to provide a safe path for this energy to dissipate when the switch opens.
How do I convert microhenrys to henrys for the calculation?
Divide by 1,000,000: 1 uH = 10^-6 H. Similarly, 1 mH = 10^-3 H and 1 nH = 10^-9 H. This calculator accepts inductance in H, mH, uH, or nH and converts automatically, so you can enter the value as it appears on the component datasheet.
Why does the stored energy formula assume constant inductance?
The formula E = 1/2 x L x I^2 is derived by integrating the power (V x I) over time while assuming L is constant: the integral of L x I x dI gives (1/2) x L x I^2. In reality, inductance depends on the permeability of the core material, which changes with flux density. When the core saturates, permeability drops and so does L, so the real stored energy is less than the formula predicts. For accurate results near saturation, you need the full B-H curve of the core material.
Can I use this calculator for transformers?
Only for the magnetising inductance of the primary, which stores energy in the same way as a standalone inductor. An ideal transformer does not store energy; it transfers it. In a real transformer the leakage inductance stores energy temporarily and contributes to ringing and voltage spikes in switching circuits. Flyback converters deliberately use the transformer leakage inductance for energy storage.