How to use it
- Type the Capacitance C, µF of the capacitor, for example 470.
- Type the Starting voltage V, V the capacitor is charged to, for example 12.
- Leave Voltage after discharge, V (optional) at 0 for a full discharge, or type the voltage it is discharged to.
- Read the Stored energy E, J, the Stored charge Q, C and the released values, then open Working, step by step.
The energy of a capacitor
The energy is stored in the electric field between the plates, and it is the work needed to charge the capacitor from zero. The energy grows with the square of the voltage, so doubling the voltage multiplies the stored energy by four. OpenStax gives it as ½ C V², and the same energy can be written as ½ Q V or Q² ÷ (2C).
E = ½ × C × V²
Q = C × V
E = ½ × Q × V = Q² ÷ (2 × C)
E energy, J
C capacitance, F
V voltage, V
Q charge, CEnergy released between two voltages
A capacitor that discharges from a voltage V1 to a voltage V2 gives out only the difference of the two energies. The energy left at V2 stays stored in the capacitor. The charge that flows is C × (V1 − V2).
Energy released = ½ × C × (V1² − V2²)
Charge released = C × (V1 − V2)What this calculator does not include
- Leakage current. A real capacitor loses its charge slowly, even with nothing connected to it.
- The equivalent series resistance and the heat it produces when a large current flows.
- The voltage rating. Choose a capacitor rated above the voltage you type, and check its datasheet.
- Discharge through a resistor over time, which is the RC time constant calculation.
Worked example: 470 µF charged to 12 V
The energy is E = ½ × 470 µF × (12 V)² = ½ × 0.00047 F × 144 V² = 0.03384 J, which is 33.84 mJ. The charge is Q = 470 µF × 12 V = 0.00564 C, which is 5.64 mC.
A full discharge to 0 V releases all of it: 33.84 mJ and 5.64 mC. The capacitor is left with no energy.
| Figure | Value |
|---|---|
| Stored energy at 12 V | 33.84 mJ |
| Stored charge at 12 V | 5.64 mC |
| Energy left at 0 V | 0 J |
| Energy released | 33.84 mJ |
| Charge released | 5.64 mC |
Frequently asked questions
Why is the energy proportional to the square of the voltage?
Each extra volt needs more charge to be pushed onto the plates, so the work grows with the voltage, and it adds up to ½ C V². That is why doubling the voltage gives four times the energy.
Why is the charge in coulombs and the energy in joules?
The charge is measured in coulombs (C), and a coulomb moved through a potential difference of one volt releases one joule. The energy is the charge times the average voltage, which is ½ Q V.
Can I use this for a supercapacitor?
The same formula applies to any ideal capacitor. Real parts have leakage and series resistance, which reduce the energy you can draw from them, so check the datasheet of the part.
What voltage should I type for a capacitor that is already discharged?
Type 0 to see the full energy that is released. Type a voltage to see the energy released when the capacitor discharges to that level, which is the energy between the two voltages.
Why does the calculator refuse a final voltage above the starting one?
A discharge takes energy out of the capacitor, so its voltage can only fall. A final voltage above the starting one would mean charging the capacitor, and this calculator does not model that.
Are my numbers sent to a server?
No. The calculation runs in your browser, and nothing you type is stored or sent to us.
Sources
Figures and text last checked on Thursday, November 12, 2026.