Circuit Analysis & Fundamentals
Capacitor & Inductor Energy Calculator
Energy stored in a capacitor (½CV²) or an inductor (½LI²), plus the stored charge or flux linkage — with the quadratic energy curve drawn.
About this calculator
A capacitor stores energy in the electric field between its plates; an inductor stores it in the magnetic field around its winding. Both store an amount that grows with the square of the driving quantity: a capacitor holds E = ½·C·V², and an inductor holds E = ½·L·I². Because the relationship is quadratic, doubling the voltage on a capacitor or the current in an inductor quadruples the stored energy — a fact that drives everything from flash-capacitor design to the voltage spike an inductor produces when its current is interrupted.
This calculator returns the stored energy for whichever element you choose, along with the companion quantity: the charge Q = C·V held on a capacitor, or the flux linkage λ = L·I set up in an inductor. It plots the energy as a function of the driving quantity so the square-law is visible and your operating point is marked on the curve.
These numbers matter for practical reasons. Capacitor energy sets how much a supply-hold-up or snubber capacitor can deliver during a dropout, and it is the energy that must be discharged safely before servicing. Inductor energy is the energy that has to go somewhere when a switch opens — into a flyback diode, a snubber, or an arc — which is why interrupting inductive current needs care. The tool is for sizing and understanding stored energy in an ideal single element; it does not model leakage, series resistance, or the dynamics of charging and discharging (use the RC/RL time-constant tool for those).
Design notes & common mistakes
- Energy is quadratic: E ∝ V² for a capacitor, E ∝ I² for an inductor. Doubling the drive quadruples the stored energy.
- The three capacitor-energy forms are identical: ½CV² = Q²/(2C) = ½QV. Pick whichever variables you have.
- Stored inductor energy has to go somewhere when the switch opens — size the flyback/snubber path for ½LI², not just for the steady current.
- A charged capacitor is a shock and short-circuit hazard long after power is removed; bleed it before servicing.
Assumptions
- Ideal energy-storage element: no series resistance, dielectric leakage, or core loss removes energy.
- The stated voltage or current is the instantaneous value at which the stored energy is evaluated.
- Linear element — capacitance and inductance do not change with voltage, current, or bias.
When to use this calculator
Appropriate for
- Sizing hold-up, snubber, or flash capacitors by the energy they must store or release
- Estimating the energy a switch must interrupt in an inductive load
- Teaching the square-law nature of stored field energy
Not suitable for
- Modelling charge/discharge timing or waveforms (use the RC/RL time-constant tool)
- Real components where ESR, leakage, or saturation change the stored energy materially
- Cycle-averaged AC energy or resonant energy exchange between L and C
What this calculator does not cover
- Single ideal element only — it does not sum energy across a network or account for how the element is connected.
- No charging/discharging dynamics; for the time behaviour of energy transfer use the RC/RL time-constant tool.
- Ignores dielectric absorption, leakage, ESR/core loss, voltage-dependent capacitance, and inductor saturation.
- Assumes a single instantaneous operating point, not an AC waveform's cycle-averaged or peak-to-peak energy swing.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
How much energy is stored in a capacitor?
E = ½·C·V², with C in farads and V in volts giving energy in joules. Because it depends on the square of the voltage, doubling the voltage stores four times the energy. The same energy can be written as Q²/(2C) or ½QV, where Q = CV is the stored charge.
How much energy is stored in an inductor?
E = ½·L·I², with L in henries and I in amperes giving energy in joules. It depends on the square of the current. This is the energy that must be absorbed when the current is interrupted — which is why opening an inductive circuit produces a voltage spike unless a flyback path is provided.
Why do capacitors and inductors store energy differently?
A capacitor stores energy in the electric field between its plates, set by the voltage across it. An inductor stores energy in the magnetic field created by the current through it. That is why capacitor energy is written in terms of voltage (½CV²) and inductor energy in terms of current (½LI²).
References
- Nilsson, J. & Riedel, S., Electric Circuits, 11th ed. (energy stored in capacitors and inductors)
- Irwin, J. D., Basic Engineering Circuit Analysis, 11th ed. (energy storage elements)
Related calculators
RC / RL Time Constant Calculator
Time constant τ of an RC or RL circuit, the fraction charged/discharged at any time t, and the time to reach a target percentage — with a live charge/discharge curve.
RLC Resonance Calculator
Resonant frequency, quality factor Q, bandwidth, and damping for a series or parallel RLC circuit — with the resonance curve and half-power points drawn.
Series/Parallel Capacitance & Inductance Calculator
Equivalent capacitance or inductance for series or parallel combinations — with the inverse-vs-resistor rules made explicit so you never mix them up.
Ohm's Law Calculator & Power Wheel
Solve for voltage, current, resistance, or power from any two known values.
Get the ComputeVerse.ai quick reference
A free engineering quick-reference PDF, plus a note when new calculators launch across the library — one or two emails a month, nothing else.