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Capacitor Energy Calculator

Free capacitor energy calculator — stored energy from ½CV² in joules and millijoules, with the charge-voltage triangle drawn out.

Capacitance (µF)

Voltage (V)

Capacitance must be greater than zero.

Formula

E = ½CV², equivalently ½QV or Q²/2C. The half is there because voltage rises from zero to V as the charge accumulates, so the average voltage during charging is only half the final value.

⚠️ Large capacitors hold a dangerous charge long after the power is removed. Anything above roughly a joule at mains voltage deserves respect and a proper bleeder resistor — discharge before handling.

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Everyday Uses

Sizing a flash capacitor

Flash energy is quoted in joules or watt-seconds. Work backwards to the capacitance and voltage needed for the output you want.

Judging discharge risk

Above roughly a joule at mains voltage a capacitor can hurt you. Calculating first tells you what needs a bleeder resistor and a meter check.

Supercapacitor run time

Energy divided by load power gives how long a supercapacitor can hold a circuit up during a brownout or a battery swap.

Amplifier headroom

Reservoir energy determines whether a bass transient sags the supply. It is why serious amplifiers carry far more capacitance than average current suggests.

Regenerative braking buffers

Capacitors absorb a braking surge that would damage a battery, then feed it back gently. Trams and hybrid buses use them for exactly this.

Coil guns and pulse discharge

Physics club projects live or die on stored energy. The square law means voltage is the lever, not capacitance.

Frequently Asked Questions

How do you calculate the energy stored in a capacitor?

E = ½CV², with capacitance in farads and voltage in volts, giving energy in joules. The same value can be written ½QV or Q²/2C depending on which quantities you have. A 2,200 µF capacitor at 50 V holds 0.5 × 0.0022 × 2,500 = 2.75 joules, which is enough to be genuinely dangerous and easily enough to weld a screwdriver tip if shorted.

Why is there a half in the formula?

Because the voltage is not constant while the capacitor charges — it starts at zero and climbs to its final value as charge accumulates. The energy is the area under the graph of voltage against charge, and since that graph is a straight line the area is a triangle, giving half of Q times V rather than the full rectangle. It is the same reason the energy in a stretched spring is ½kx² rather than kx².

What happens to the energy if I double the voltage?

It quadruples, because energy depends on voltage squared. Going from 12 V to 24 V on the same capacitor takes the stored energy from one unit to four. This is the most practically important fact on the page: it means voltage rating matters far more than capacitance for safety, and it is why high-voltage capacitor banks are treated with such caution even when their capacitance looks modest.

Are charged capacitors dangerous?

They can be, and they stay dangerous after the power is switched off. Large electrolytics in power supplies, amplifiers, microwave ovens and camera flashes can hold a lethal charge for minutes or longer. Anything storing more than about one joule deserves respect; microwave oven capacitors store far more than that and have killed people. Proper equipment includes a bleeder resistor to discharge safely, but never assume one is fitted or working — check with a meter before touching anything.

How does capacitor energy compare with a battery?

Very unfavourably by volume. A typical AA battery holds around 10,000 joules; a large 10,000 µF capacitor at 50 V holds about 12.5 joules — roughly a thousandth as much in a similar physical size. What the capacitor offers is the ability to release that energy in microseconds and to be recharged millions of times without degrading. Capacitors win on power delivery and cycle life, batteries win overwhelmingly on stored energy.