Capacitor Energy and Charge Calculator
Enter any two of the four quantities - capacitance (C), voltage (V), stored charge (Q), or stored energy (E) - and this calculator solves for the remaining two. It supports every common unit prefix from picofarads to farads and nanojoules to kilowatt-hours, shows the full step-by-step working, and draws a chart of stored energy vs. voltage for the capacitor you have specified.
How a capacitor stores energy
A capacitor is formed by two conducting plates separated by an insulating dielectric. When a voltage is applied, positive charge accumulates on one plate and negative charge on the other, creating an electric field in the dielectric layer. Work is done against the increasing field each time more charge is added, and that work is stored as potential energy in the field. The total energy stored equals the integral of V dQ from 0 to the final charge, which works out to E = (1/2) x C x V^2. The factor of one-half appears because the voltage rises linearly from 0 to V as the capacitor charges, so the average voltage during charging is V/2.
The three equivalent energy formulas
Three algebraically equivalent formulas describe the stored energy, and each is most convenient depending on which two quantities you have measured. If you know capacitance C and voltage V, use E = (1/2) x C x V^2. If you know charge Q and capacitance C, use E = Q^2 / (2C). If you know charge Q and voltage V, use E = (1/2) x Q x V. All three give the same answer because they are related by Q = C x V. This calculator accepts any two of the four quantities - C, V, Q, and E - and solves for the other two, so you never have to rearrange the formula by hand.
Unit prefixes and practical scales
Capacitance spans about 15 orders of magnitude in real electronics. A ceramic decoupling capacitor might be 100 pF (100 x 10^-12 F) while a supercapacitor (ultracapacitor or EDLC) can reach thousands of farads. Voltage ranges from millivolts in sensor circuits to tens of kilovolts in pulse-power equipment. Energy similarly spans from nanojoules in logic-level bypass caps to megajoules in industrial capacitor banks. Because of this wide range, the calculator lets you pick the appropriate unit prefix for each quantity. The underlying arithmetic always works in SI base units (farads, volts, coulombs, joules) and converts the result to your chosen display unit.
Capacitor discharge current and safety
A charged capacitor can release its energy almost instantaneously through a short circuit, producing a peak current limited only by circuit resistance. Even a modest 100 uF capacitor charged to 400 V stores 8 J and can deliver thousands of amperes in a short pulse, enough to cause severe burns or cardiac arrhythmia. Before handling any capacitor in a powered circuit, confirm it has been discharged using a suitably rated resistor bleed or discharge tool, and verify with a voltmeter. High-voltage capacitors (above roughly 50 V) should always be treated as potentially lethal until confirmed discharged. The energy stored scales with voltage squared, so doubling the voltage quadruples the hazard.
Time constant and charging time
In a simple RC (resistor-capacitor) series circuit, the time constant is tau = R x C. After one time constant, the capacitor has charged to approximately 63.2% of the supply voltage; after 5 time constants it is considered fully charged (99.3%). The energy stored increases during charging, and the charge delivered from the supply is Q = C x V_supply, but because half of that energy is lost as heat in the resistance, only E = (1/2) x C x V^2 is stored - regardless of the value of R. This 50% charging efficiency is a fundamental consequence of the linear voltage-rise relationship and is not improved by lowering resistance (though switching regulators and more sophisticated circuits can do better).
Common capacitor types and typical energy storage
| Capacitor type | Typical capacitance | Typical voltage | Typical stored energy |
|---|---|---|---|
| Ceramic (SMD decoupling) | 100 nF | 5 V | 1.25 uJ |
| Film (signal/audio) | 100 nF | 63 V | 199 uJ |
| Electrolytic (power supply) | 1000 uF | 25 V | 312 mJ |
| Electrolytic (bulk/filter) | 10,000 uF | 50 V | 12.5 J |
| Supercapacitor (EDLC) | 1 F | 2.7 V | 3.65 J |
| Supercapacitor (large) | 3000 F | 2.7 V | 10.9 kJ |
| High-voltage pulse cap | 100 uF | 10 kV | 5000 J |
Approximate values. Actual energy depends on capacitance and voltage rating.
Frequently asked questions
What is the formula for the energy stored in a capacitor?
The most common form is E = (1/2) x C x V^2, where E is energy in joules, C is capacitance in farads, and V is voltage in volts. Equivalently, E = Q^2 / (2C) if you know the charge Q in coulombs, or E = (1/2) x Q x V if you know both Q and V. All three give identical results because Q = C x V.
How do I calculate the charge stored on a capacitor?
Charge Q equals capacitance C times voltage V: Q = C x V. For example, a 470 uF capacitor charged to 16 V holds Q = 470 x 10^-6 x 16 = 7.52 mC (millicoulombs). Enter C and V in this calculator and the charge is computed automatically.
Why does energy scale with voltage squared?
As charge accumulates on a capacitor, the voltage across it rises in proportion (V = Q/C). Each additional increment of charge must be pushed against an increasingly large voltage, so the work done per unit charge increases linearly with charge, and the total energy is the integral - which is proportional to V^2. Practically this means doubling the voltage quadruples the stored energy, and this is why high-voltage capacitor banks are so energetic.
Can I use this calculator in reverse to find capacitance or voltage?
Yes. Enter any two of the four quantities (C, V, Q, E) and leave the others at zero. The calculator solves for the missing two using the appropriate rearrangement: if you enter Q and V it computes C = Q/V and E = (1/2) x Q x V; if you enter E and V it computes C = 2E/V^2 and Q = CV; and so on.
What is the difference between a farad and a microfarad?
One farad (F) is a very large capacitance - it would describe a capacitor that stores one coulomb of charge at one volt. Most electronic capacitors are in the microfarad (uF, 10^-6 F), nanofarad (nF, 10^-9 F), or picofarad (pF, 10^-12 F) range. Supercapacitors and ultracapacitors are the exception, reaching from 0.1 F to thousands of farads in a single component.
How much energy can a supercapacitor store?
A typical 3000 F supercapacitor rated at 2.7 V stores about (1/2) x 3000 x 2.7^2 = 10,935 J, or roughly 3 Wh. That is much less than even a small lithium-ion battery cell, but supercapacitors can charge and discharge extremely quickly and tolerate millions of cycles, making them useful for energy harvesting, regenerative braking, and power buffering applications.