Contents
How the capacitor energy calculator works
Enter the capacitance, choose its unit, and add the voltage across the terminals. The calculator returns the ideal stored energy plus the corresponding electric charge.
The two formulas behind the result
A capacitor stores energy in its electric field. The calculation uses capacitance in farads and voltage in volts:
E is energy in joules, C is capacitance in farads, V is voltage, and Q is charge in coulombs. Capacitance affects both results linearly. Voltage affects charge linearly but energy quadratically.
Pick the prefix printed on the component
Most capacitors are marked in picofarads, nanofarads, microfarads, or millifarads because one farad is large for many circuits. The calculator converts the selected unit internally: 1 mF = 10⁻³ F, 1 μF = 10⁻⁶ F, 1 nF = 10⁻⁹ F, and 1 pF = 10⁻¹² F.
A prefix error changes the answer sharply. Entering 100 μF as 100 mF makes the capacitance, energy, and charge 1,000 times too large.
Three scale checks
- 1,000 μF at 12 V stores 0.072 J. The same result is 72 mJ; the charge is 0.012 C. This is the default example and a quick way to check the units.
- 100 μF at 300 V stores 4.5 J. Its charge is 0.03 C. The much higher voltage matters because voltage is squared in the energy formula.
- 1 F at 5 V stores 12.5 J. Its charge is 5 C. Raising the voltage to 10 V would produce 50 J, four times as much energy.
Stored energy is not the same as usable energy
The formula describes an ideal capacitor at the entered voltage. A real circuit loses energy through equivalent series resistance, leakage, wiring, and the load. Many devices also stop working before the voltage falls to zero, so they cannot use the entire calculated amount.
The calculator does not determine runtime, discharge current, polarity, or a safe working voltage. Check the component data sheet and never exceed its rated voltage. A charged high-voltage capacitor can remain dangerous after power is removed.
Questions about capacitor energy
These answers separate energy from charge, power, and the limits printed on the component.
Why is there a factor of one half in the formula?
Voltage rises from zero to its final value while the capacitor charges. The work accumulated over that process gives E = QV/2, which is the same as CV²/2.
What happens when voltage doubles?
Charge doubles, but stored energy becomes four times larger because voltage is squared in the energy formula.
Can the result tell me how long a device will run?
Not by itself. Runtime also depends on the load, converter efficiency, resistance, and the minimum voltage at which the device still works.
Is a joule the same as a watt?
No. A joule measures energy, while a watt measures the rate at which energy is transferred. One watt equals one joule per second.
May I use a higher voltage to store more energy?
Only within the capacitor’s rated voltage and the circuit design. Exceeding the rating can damage the dielectric and create a serious safety hazard.
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