Capacitor Charge and Energy Calculator

How it works

A capacitor stores charge in proportion to the voltage across it: Q = C × V, where C is the capacitance in farads, V the voltage in volts and Q the charge in coulombs. One farad holds one coulomb per volt.

Charging it takes work, because each extra bit of charge is pushed against the voltage already built up. Adding that work from zero to V gives the stored energy E = ½ × C × V² in joules, or equivalently ½ × Q × V. Energy grows with the square of voltage, so doubling the voltage on the same capacitor stores four times the energy.

To find the capacitance needed to hold a given energy at a given voltage, the energy formula is rearranged to C = 2E ÷ V². Results are shown with the SI prefix that keeps the number readable: µF, mC, mJ and so on.

Formula

Q = C × V            (coulombs)
E = ½ × C × V²       (joules)
C = 2 × E ÷ V²       (farads, for a target energy)
1 µF = 10^-6 F;  1 nF = 10^-9 F;  1 pF = 10^-12 F

Example

A 1,000 µF capacitor (0.001 F) charged to 12 V holds Q = 0.001 × 12 = 0.012 C, shown as 12 mC, and stores E = ½ × 0.001 × 12² = 0.072 J, or 72 mJ.

To store 400 J at 10,000 V, as in a heart defibrillator, the capacitance needed is C = 2 × 400 ÷ 10,000² = 8 × 10^-6 F, or 8 µF, holding 80 mC.

Assumptions and limitations

• The capacitor is ideal: capacitance does not change with voltage or temperature, and there is no leakage or internal resistance. A real part's capacitance varies with tolerance, applied voltage, temperature and age as described in its data sheet.
• Energy is what is stored, not what a circuit can extract: a converter usually cannot discharge the capacitor to zero volts, and losses in resistance reduce the energy delivered.
• The calculator does not check the capacitor's voltage rating. Stored energy at high voltage is dangerous even after power is removed.
• Results are estimates for learning and circuit sketching, not a substitute for the manufacturer's data sheet or a qualified engineer's design review.

Frequently asked questions