Capacitor

How Capacitor Charge and Energy Are Calculated

A capacitor stores electrical energy as an electric field between two conductive plates, separated by an insulator. Enter its capacitance and the voltage across it, and this calculator finds the energy stored and the electric charge it holds.

The more capacitance a capacitor has and the higher the voltage across it, the more energy and charge it can hold. Charge grows directly with voltage, but energy grows with the square of voltage — doubling the voltage doubles the charge but quadruples the stored energy.

The Formula

• Charge: $Q = C \times V$
• Energy: $E = 0.5 \times C \times V^2$

Capacitance is entered in microfarads (µF), the practical unit most real-world capacitors are actually labeled in, since a full farad is an enormous amount of capacitance for a typical component.

Worked Example

A 100 µF capacitor charged to 12 V:

1. Charge: $0.0001 \times 12 = 0.0012$ C, or 1,200 µC.
2. Energy: $0.5 \times 0.0001 \times 12^2 = 0.0072$ J, or 7.2 mJ.

Charging the same capacitor to 24V instead (double the voltage) would double the charge to 2,400 µC, but quadruple the stored energy to 28.8 mJ.

Key Factors to Consider

• A capacitor has a maximum rated voltage that should never be exceeded. Exceeding a capacitor’s voltage rating can cause it to fail, sometimes dramatically (electrolytic capacitors in particular can rupture or vent under overvoltage) — always check the printed voltage rating before applying a given voltage in a real circuit.
• A charged capacitor can retain a dangerous charge even after power is removed. Larger capacitors, especially at higher voltages, can hold enough stored energy to deliver a real electric shock well after being disconnected from a power source — always treat a capacitor as potentially charged until it’s confirmed safely discharged.
• Capacitors in series and parallel combine differently than resistors do. Capacitance in parallel adds directly (like resistors in series), while capacitance in series combines via the reciprocal-sum formula (like resistors in parallel) — this calculator handles a single capacitor’s own energy and charge, not a network of multiple capacitors.
• Real capacitors have some internal resistance and leakage, which this ideal formula ignores. A real-world capacitor slowly loses some charge over time even with no external circuit connected, and its internal resistance affects how quickly it can actually charge or discharge — this calculator’s formulas describe the ideal, textbook capacitor.

Interpreting Your Results

• The energy figure describes total stored energy, not how fast it can be released. Unlike a battery, which delivers its energy gradually over minutes or hours, a capacitor can release nearly all of its stored energy in a tiny fraction of a second when short-circuited — which is why even a modest, joule-scale reading from this calculator represents a real risk, not just an abstract number.
• The charge figure tells you how much current the capacitor can supply, and for how long. Since current multiplied by time equals charge, a higher charge reading means the capacitor can either deliver more current for the same duration, or the same current for longer — useful when sizing a capacitor meant to smooth a power supply’s output or bridge a brief interruption.
• Leave real-world headroom below the voltage you enter. If you’re checking a real capacitor against a circuit, many engineers deliberately keep the working voltage to roughly 80% of a capacitor’s printed rating rather than running right up to the limit — especially for electrolytic types — since voltage spikes and ripple in a real circuit can otherwise briefly exceed what the printed rating alone would suggest is safe.

Useful to Know

Never discharge a charged capacitor by shorting its leads together with a screwdriver or bare wire. A large or high-voltage capacitor — like those found in microwave ovens, camera flash units, CRT televisions, and power supplies — can release its full stored charge almost instantly, producing a current spike large enough to weld metal, damage the capacitor, or throw a dangerous spark. The safe way to discharge a capacitor is gradually, through a resistor sized to limit the discharge current (often called a “bleeder resistor”), or with a purpose-built capacitor discharge tool — never a direct short.