Capacitor Energy Calculator
Calculate energy stored and charge on a capacitor from capacitance and voltage.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator computes two key properties of a charged capacitor: the energy stored in its electric field and the charge on its plates.
- Enter capacitance. Select the unit (F, mF, µF, nF, or pF) and enter the capacitance value. Electrolytic capacitors used in power supplies are typically 10 to 10,000 µF. Ceramic decoupling capacitors are typically 0.1 to 100 µF. Supercapacitors can be 1 to 3000 F.
- Enter voltage. The voltage across the capacitor in volts. For a capacitor in a circuit, this is the voltage at the operating point, not the maximum rated voltage.
- Read the results. Energy in joules (or mJ, µJ, kJ as appropriate) and charge in coulombs (or mC, µC, nC).
About Capacitor Energy Storage
The energy stored in a capacitor is E = ½ × C × V², where C is in farads and V is in volts. The charge is Q = C × V. Doubling the voltage quadruples the stored energy; doubling the capacitance doubles the energy. This quadratic relationship with voltage is why high-voltage capacitors in power supplies store surprisingly large amounts of energy and can be dangerous even when the power is off.
Supercapacitors (also called ultracapacitors or electric double-layer capacitors) can store much more energy than standard capacitors, reaching thousands of farads. A 3000 F supercapacitor at 2.7 V stores about 10,950 J — comparable to a small battery. They are used in regenerative braking, flash photography, and backup power applications.
Frequently Asked Questions
What is the formula for energy stored in a capacitor?
The energy stored is E = ½ × C × V², where E is in joules, C is in farads, and V is in volts. This can also be written as E = Q²/(2C) = ½QV, where Q = CV is the charge in coulombs. The energy is stored in the electric field between the capacitor plates.
How dangerous is the energy stored in a large capacitor?
Large charged capacitors are genuinely dangerous. A 1000 µF capacitor at 400 V stores 80 J — enough to cause a severe shock or cardiac arrest. CRT television sets, camera flash units, and switching power supplies contain capacitors that remain charged long after power is removed. Always discharge large capacitors through a resistor (not a short circuit, which can destroy the capacitor) before working on equipment. A 10 kΩ resistor is typical for safe discharge.
Why does energy scale with voltage squared?
As a capacitor charges, each additional increment of charge must be pushed against an increasing voltage (the voltage already on the capacitor). The work done (energy) is the integral of V × dQ from 0 to Q. Since V = Q/C, integrating gives ½Q²/C = ½CV². This is analogous to stretching a spring: the force (voltage) increases with displacement (charge), and the stored energy (analogous to spring potential energy) scales as the square of the displacement.
What are typical capacitor values for power supply filtering?
Power supply bulk filter capacitors are typically 1000 to 10,000 µF at the supply voltage plus 20% headroom. A 12 V supply might use a 2200 µF 25 V capacitor. Decoupling capacitors near integrated circuits are typically 0.1 µF to 10 µF. High-frequency decoupling uses 1 nF to 100 nF ceramic capacitors placed as close as possible to the power pins of the IC. Multiple capacitor values in parallel cover a broad frequency range.