About the Energy to Capacitance Calculator
This tool converts a desired stored energy and capacitor voltage into the capacitance required to store that energy. It is useful for electronics designers, students, and anyone sizing an ideal capacitor or capacitor bank for pulsed power, backup, or energy storage applications.
How to use this calculator
- Enter the stored energy value.
- Select the energy unit: mJ, J, kJ, Wh, or kWh.
- Enter the capacitor voltage in volts.
- Click Calculate Capacitance to view the required capacitance and stored charge.
- Click Reset to restore the default 10 J and 12 V inputs.
How it works
The calculator uses the ideal capacitor energy equation E = 1/2 C V². Solving for capacitance gives C = 2E / V², where C is in farads, E is energy in joules, and V is voltage in volts.
If you choose an energy unit other than joules, the calculator first converts the entered value to joules. It then divides twice that energy by the square of the entered voltage to find the required capacitance.
The result is shown in the most convenient capacitance unit, along with farads, millifarads, microfarads, and stored charge. Stored charge is calculated as Q = C × V in coulombs.
This is an ideal calculation. Real capacitor banks may need additional design margin for voltage rating, ESR, leakage, temperature, aging, ripple current, tolerances, and safety requirements.
Example calculation
Using the default values, enter 10 J for stored energy and 12 V for voltage. The capacitance is C = 2 × 10 / 12² = 20 / 144 = 0.1389 F, or about 138.9 mF. The stored charge is Q = 0.1389 × 12 = 1.6667 C.
Frequently asked questions
Why does capacitance decrease when voltage increases?
For the same stored energy, capacitance is inversely proportional to voltage squared. Doubling the voltage reduces the required capacitance to one quarter.
Does this calculator account for capacitor losses?
No. It assumes an ideal capacitor and does not include ESR, leakage, dielectric losses, temperature effects, or aging.
What voltage should I enter for a capacitor bank?
Enter the voltage at which the specified energy is stored. For usable energy over a voltage range, a different calculation using the difference between maximum and minimum voltage energy is needed.
How is stored charge related to the result?
After finding capacitance, the calculator computes charge using Q = C × V, giving the stored charge in coulombs.
Can I use watt-hours or kilowatt-hours as the energy input?
Yes. The calculator converts Wh and kWh to joules before applying the capacitance formula.