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Capacitor and Inductor Stored Energy Calculator

Capacitors store energy in an electric field, inductors in a magnetic one. Both formulas are squared in the quantity that cannot change instantaneously — voltage for a capacitor, current for an inductor.

Stored Energy

Energy stored in a capacitor's electric field: E = ½·C·V².

Stored energy
Charge Q
  1. 1.Given: C = 1000 µF, V = 12 V
  2. 2.FormulaEnergy E = ½·C·V²SubstituteE = ½ × 0.001 × 12²ResultE = 72 mJ
  3. 3.FormulaCharge Q = C·VSubstituteQ = 0.001 × 12ResultQ = 12 mC
Common trap: Energy grows with the squareof voltage (or current) — doubling the voltage on a capacitor stores 4× the energy. That's why a charged high-voltage capacitor is dangerous long after the supply is off, and why it must be bled through a resistor before you touch it.

The formula

E = ½ × C × V² (capacitor) ; E = ½ × L × I² (inductor) ; Q = C × V

E
stored energy (joules)
C
capacitance (farads)
V
voltage across the capacitor (volts)
L
inductance (henries)
I
current through the inductor (amperes)

Worked example

A 1000 µF capacitor charged to 25 V, and a 10 mH inductor carrying 2 A.

  1. Capacitor: E = ½ × 1000×10⁻⁶ × 25² = ½ × 0.001 × 625
  2. Charge held: Q = 0.001 × 25 = 25 mC
  3. Inductor: E = ½ × 0.01 × 2² = ½ × 0.01 × 4

0.3125 J in the capacitor, 0.02 J in the inductor.

Where you'll use it

Sizing smoothing and hold-up capacitors, flash circuits, and switching supplies. The inductor formula is also the safety one: interrupting inductor current forces that energy out as a voltage spike, which is why relay coils need flyback diodes.

The laws behind it

Parts this applies to

Capacitor (ceramic / MLCC)Capacitor (aluminium electrolytic)Capacitor (tantalum)Capacitor (film)SupercapacitorQuartz Crystal / ResonatorMOSFET555 Timer IC

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