Capacitor Energy Calculator
Energy and charge stored in a capacitor from its capacitance and voltage.
Exact: 0.072 J from E = ½ × C × V².
A capacitor stores energy E = ½ × C × V² (joules) and charge Q = C × V (coulombs). A 1000 µF capacitor at 12 V holds Q = 0.001 × 12 = 0.012 C (12 mC) and E = ½ × 0.001 × 12² = 0.072 J (72 mJ). Because energy scales with V², doubling the voltage quadruples it.
What a capacitor stores
A capacitor holds energy in the electric field between its plates. Two quantities describe that store: the charge Q, in coulombs, which is how much electric charge sits on the plates, and the energy E, in joules, which is the work done to move that charge across the plate voltage. Charge grows linearly with voltage, but energy grows with the square of it, so a small rise in voltage stores disproportionately more energy.
energy in joules; charge is Q = C × V in coulombs (C in farads, V in volts)
Worked example
A 1000 µF electrolytic capacitor charged to 12 V.
- 1 Convert capacitance to farads. 1000 µF = 1000 × 10⁻⁶ = 1.0 × 10⁻³ F (0.001 F).
- 2 Find the charge (Q = C × V). Q = 0.001 × 12 = 0.012 C, which is 12 mC.
- 3 Square the voltage. V² = 12² = 144 volts squared.
- 4 Apply E = ½ × C × V². E = ½ × 0.001 × 144 = 0.072 J, which is 72 mJ.
Capacitance unit prefixes
How each prefix converts to farads, the SI base unit of capacitance.
| Unit | Name | In farads |
|---|---|---|
| F | farad | 1 F |
| mF | millifarad | 10⁻³ F = 0.001 F |
| µF | microfarad | 10⁻⁶ F |
| nF | nanofarad | 10⁻⁹ F |
| pF | picofarad | 10⁻¹² F |
Why voltage matters most, and staying safe
Energy scales with V², not V. Because the V² term dominates, doubling the voltage on the same capacitor quadruples the stored energy, while doubling the capacitance only doubles it. Voltage is the lever with the biggest effect — and the reason a capacitor’s voltage rating is chosen with margin.
Charged capacitors bite. A large electrolytic or a high-voltage capacitor can hold a dangerous charge long after power is removed, because there is often no path to discharge it. Before touching a circuit, discharge big capacitors through a suitable resistor (never a bare screwdriver, which arcs and can damage the part) and confirm the voltage has fallen to zero.