What this calculator does
A capacitor stores charge in proportion to the voltage across it, and energy in proportion to the square of that voltage. The squared term is the important one: doubling the voltage doubles the charge but quadruples the stored energy.
That is why capacitor voltage ratings matter so much, and why a charged high-voltage capacitor is genuinely dangerous long after the power has been switched off.
The formula
Charge is capacitance times voltage, with microfarads converted to farads first. Energy is half the capacitance times the square of the voltage, which is the area under the charge against voltage line.
| Term | Meaning |
|---|---|
| Capacitance (C) | How much charge a capacitor holds per volt applied, in farads. A microfarad is a millionth of a farad. |
| Coulomb (C) | The unit of electric charge. One coulomb is one amp flowing for one second. |
| Farad (F) | One coulomb stored per volt. A very large unit, so practical capacitors are usually rated in micro, nano or picofarads. |
The inputs explained
| Field | What to enter |
|---|---|
| Capacitance (µF) | The capacitance in microfarads, as printed on the component. |
| Voltage (V) | The voltage across the capacitor. This must stay within the capacitor's rated voltage in any real circuit. |
When to use it
Sizing a smoothing or hold-up capacitor
The energy stored determines how long a capacitor can support a load when the supply is interrupted.
Understanding a voltage rating
Since energy rises with the square of voltage, running a capacitor near its limit stores far more energy than running it at half that voltage.
Estimating a discharge current
The stored charge divided by a discharge time gives the average current that discharge represents.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does stored energy grow with voltage?
The same capacitor charged to a range of voltages.
| Voltage | Stored energy | Stored charge |
|---|---|---|
| 6 V | 0.001800 J | 600.000 µC |
| 12 V | 0.007200 J | 1,200.000 µC |
| 24 V | 0.028800 J | 2,400.000 µC |
| 48 V | 0.115200 J | 4,800.000 µC |
Questions
Why is energy proportional to voltage squared?
Because charging a capacitor is progressive: the first charge goes on easily, and each additional bit has to be pushed against the voltage already built up. Integrating that rising opposition gives the half CV squared term.
What happens if I exceed the voltage rating?
The dielectric breaks down, often destructively. The rating is a hard limit rather than a guideline, and it should be derated further for reliability in anything that matters.
Do capacitors hold charge after power is removed?
Yes, sometimes for a long time. Large capacitors in power supplies can remain dangerously charged after the equipment is unplugged, which is why bleed resistors are fitted and why service procedures require discharging them.
How does this compare with a battery?
A capacitor stores far less energy for its size but can deliver and absorb it far faster. That trade-off is why capacitors handle smoothing and bursts while batteries handle sustained supply.
For the magnetic equivalent, see the inductor stored energy calculator. For how quickly a capacitor charges through a resistor, see the RC circuit calculator.