Capacitor Energy Calculator
A capacitor stores energy in the electric field between its plates, and the amount depends on its capacitance and the voltage across it. This calculator applies the standard relations of electrostatics: the stored energy is one half the capacitance times the voltage squared, and the stored charge is the capacitance times the voltage. Enter the capacitance in farads and the voltage in volts, and it returns the energy in joules, the energy in millijoules, and the charge in coulombs. Remember that energy rises with the square of voltage, so voltage ratings matter.
Capacitor energy formula
Stored energy = 0.5 * C * V^2
Stored charge = C * V
Energy (mJ) = energy (J) * 1,000
(C in farads, V in volts, energy in joules, charge in coulombs)
The energy is the work done to move charge onto the plates against the rising voltage, which integrates to one half C V squared. The charge relation Q equals C V is the definition of capacitance.
Electronics context
- The farad is a large unit; practical capacitors are often rated in microfarads (1e-6 F) or picofarads (1e-12 F).
- Energy scales with the square of voltage, so never exceed a capacitor's voltage rating.
- The charge Q equals C times V defines capacitance in the SI system.
- Capacitor banks store large energies and can discharge dangerously; discharge them safely before handling.
- One joule equals one watt-second; this links stored energy to power delivered over time.
Capacitor energy: frequently asked questions
How do you calculate the energy stored in a capacitor?
The energy stored in a capacitor is one half times the capacitance times the voltage squared: E equals 0.5 times C times V squared. Capacitance is in farads, voltage in volts, and the energy comes out in joules. The voltage is squared, so doubling the voltage quadruples the stored energy.
What is the charge on a capacitor?
The charge stored on a capacitor is the capacitance times the voltage: Q equals C times V, in coulombs. This is the defining relationship of capacitance. Together with the energy formula it fully describes a charged capacitor's electrical state at a given voltage.
What units should I enter?
Enter capacitance in farads and voltage in volts. Many real capacitors are rated in microfarads or picofarads, so convert first: one microfarad is 0.000001 farads and one picofarad is 0.000000000001 farads. The energy result is in joules and the charge in coulombs.
Why does voltage matter more than capacitance for energy?
Energy depends on the square of the voltage but only the first power of capacitance. Doubling capacitance doubles the energy, but doubling voltage quadruples it. This is why high-energy capacitor banks are charged to high voltages, and why capacitor voltage ratings must never be exceeded.
Is a charged capacitor dangerous?
It can be. A large capacitor charged to a high voltage stores significant energy that can discharge rapidly and cause injury or damage, even after the power source is removed. Always observe the capacitor's voltage rating, and discharge large capacitors safely through an appropriate resistor before handling.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.