Capacitor Energy Calculator
Unit
V
Stored Energy0.007200 J
Stored Charge0.001200 C
A charged capacitor stores energy in the electric field between its plates, and this calculator quantifies both that energy and the charge it holds. Enter the capacitance (in farads down to picofarads) and the voltage across the terminals, and it returns the stored energy in joules along with the accumulated charge in coulombs. These figures are essential for sizing energy-storage banks, flash circuits, and power-supply hold-up capacitors.
Formula
E = ½ · C · V² and Q = C · V
- E
- Stored energy (joules)
- Q
- Stored charge (coulombs)
- C
- Capacitance (farads)
- V
- Voltage across the capacitor (volts)
How it works
- Enter the capacitance value and pick its unit (F, mF, µF, nF, or pF); the calculator converts it to farads internally.
- Enter the voltage across the capacitor in volts.
- It applies E = ½·C·V² to get the stored energy in joules and Q = C·V to get the stored charge in coulombs, both reported to six decimal places.
Worked example
A 100 µF capacitor charged to 12 V in a power-supply rail.
- Convert capacitance: 100 µF = 0.0001 F.
- Energy = ½ × 0.0001 × 12² = ½ × 0.0001 × 144 = 0.0072 J.
- Charge = 0.0001 × 12 = 0.0012 C.
The capacitor stores 0.0072 J of energy and 0.0012 C of charge.
Frequently asked questions
- Why does stored energy depend on the square of the voltage?
- Energy accumulates as charge is pushed onto the plates against a rising voltage, so the work done integrates to ½·C·V². Doubling the voltage therefore quadruples the stored energy while only doubling the charge.
- What is the difference between stored charge and stored energy?
- Charge (Q = C·V) is the quantity of separated electrons measured in coulombs, while energy (½·C·V²) is the work stored in the field measured in joules. A capacitor can hold a large charge yet modest energy if the voltage is low.
- Does this account for capacitor leakage or ESR?
- No. The calculator gives the ideal energy and charge at the stated voltage. Real capacitors lose energy slowly through leakage current and dissipate some in equivalent series resistance during charge and discharge.
- How do I size a capacitor for a given energy?
- Rearrange the energy formula to C = 2·E / V². Choose the operating voltage first, then solve for the capacitance needed, and pick a part rated comfortably above that voltage.