Inductor Stored Energy Calculator
An inductor stores energy in the magnetic field created by current flowing through it. The amount depends on the inductance and the square of the current. This calculator returns the stored energy in joules, plus the same value in millijoules and microjoules, from an inductance in henries and a current in amps. Understanding stored inductor energy is essential for designing switching power supplies, sizing snubbers and flyback protection, and predicting the voltage spikes that appear when current through an inductor is interrupted suddenly.
Inductor energy formula
stored energy E = 0.5 * L * I^2
L in henries, I in amps, E in joules
1 joule = 1,000 millijoules = 1,000,000 microjoules
energy scales with the square of the current
The factor of one-half comes from integrating the inductor's voltage times current as the current ramps from zero to its final value. Doubling the current quadruples the energy.
Practical notes
- Energy rises with the square of current, so current is the dominant factor.
- 1 millihenry equals 0.001 henry; 1 microhenry equals 0.000001 henry.
- Interrupting inductor current releases this energy fast, causing voltage spikes.
- Switching converters cycle this energy between the inductor and the load.
- One joule equals one watt held for one second.
Inductor energy: frequently asked questions
How do you calculate the energy stored in an inductor?
The energy stored in an inductor's magnetic field is E = 0.5 * L * I^2, where L is the inductance in henries and I is the current in amps. The result is in joules. The energy rises with the square of the current, so doubling the current quadruples the stored energy.
Why does the energy depend on the square of the current?
Energy is the integral of voltage times current over time as the current builds up. Because the inductor voltage is L times the rate of change of current, integrating gives one-half L times current squared, which is why the relationship is quadratic in current.
What units does this calculator use?
Enter inductance in henries and current in amps to get energy in joules. Convert first if needed: 1 millihenry is 0.001 henry and 1 microhenry is 0.000001 henry. One joule equals one watt-second, so power times duration gives energy in the same unit.
Where does this stored energy go when the current stops?
An inductor resists sudden changes in current, so interrupting it releases the stored energy quickly, often as a voltage spike. Snubber circuits, flyback diodes, and clamps are used to absorb or redirect this energy safely in switching circuits.
Is this the same energy used in a switching power supply?
Yes. Inductor energy transfer is central to buck, boost, and flyback converters: energy is stored in the inductor during one phase and released to the load during the next. E = 0.5 * L * I^2 gives the peak energy held each switching cycle.
Official sources
- NIST: SI units (joule, henry, ampere).
- NASA Glenn Research Center: Inductor and magnetic energy fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.