About

A capacitor stores energy in its electric field proportional to capacitance and the square of applied voltage. The governing relation E = 12CV² means energy scales quadratically with voltage. Doubling voltage quadruples stored energy. Miscalculating this value leads to undersized discharge circuits, premature dielectric breakdown, or dangerously energetic fault currents in power electronics. This tool computes stored energy in Joules and watt-hours, plus accumulated charge Q = CV, across the full SI prefix range from picofarads to farads and millivolts to kilovolts.

Results assume an ideal linear capacitor with constant capacitance independent of voltage. Real electrolytic and ceramic (Class II) capacitors exhibit voltage-dependent capacitance loss of 20 - 80% at rated voltage. For energy-critical designs, derate the nominal capacitance or measure effective capacitance at operating bias. Pro tip: supercapacitors specify capacitance at low voltage; actual energy at full charge is often 25% below the ideal calculation.

Formulas

The energy stored in an ideal linear capacitor is derived from integrating the work done to move charge q across a voltage v = qC. The three equivalent forms are:

E = 12 C V² = Q²2C = Q V2

The stored charge is:

Q = C ⋅ V

Conversion to watt-hours:

E_Wh = E_J3600

Where E is stored energy in J (joules), C is capacitance in F (farads), V is voltage in V (volts), and Q is charge in C (coulombs). The 12 factor arises because average voltage during charging is half the final voltage.

Reference Data

Capacitor Type	Typical Range	Max Voltage	ESR	Typical Application
Ceramic (C0G/NP0)	0.5 pF - 100 nF	50 V	< 0.1 Ω	RF filters, timing
Ceramic (X7R)	1 nF - 100 μF	6.3 - 100 V	0.01 - 1 Ω	Decoupling, bypass
Ceramic (Y5V)	10 nF - 47 μF	6.3 - 50 V	0.05 - 5 Ω	Non-critical bypass
Aluminum Electrolytic	0.1 μF - 1 F	6.3 - 500 V	0.01 - 10 Ω	Power supply filtering
Tantalum	0.1 - 1000 μF	4 - 50 V	0.05 - 5 Ω	Embedded, medical
Film (Polyester/PET)	1 nF - 100 μF	50 - 2000 V	< 0.01 Ω	Audio crossover, snubber
Film (Polypropylene)	100 pF - 10 μF	100 - 3000 V	< 0.005 Ω	PFC, resonant circuits
Mica	1 pF - 10 nF	100 - 1500 V	< 0.01 Ω	Precision RF, transmitters
Supercapacitor (EDLC)	0.1 - 3000 F	2.5 - 2.7 V	0.1 - 100 mΩ	Energy harvesting, backup
Supercapacitor (Hybrid)	1 - 500 F	3.8 - 4.0 V	1 - 50 mΩ	UPS, automotive start
Vacuum	1 - 5000 pF	5 - 60 kV	< 0.001 Ω	High-power RF transmitters
Glass	10 - 1000 pF	up to 7500 V	< 0.01 Ω	Military, aerospace
Silicon (Integrated)	0.1 - 50 pF	1.8 - 5 V	0.1 - 10 Ω	On-die decoupling
Paper (Oil-Impregnated)	1 nF - 50 μF	200 - 100000 V	0.01 - 1 Ω	High-voltage power systems
Polymer Aluminum	10 - 1000 μF	2 - 25 V	5 - 30 mΩ	VRM, CPU power

Frequently Asked Questions

Energy is the integral of power over time. As charge accumulates on a capacitor, the voltage across it rises linearly with charge (V = Q/C). Each successive increment of charge must be pushed against a higher voltage, so the work (voltage × charge) grows quadratically. This is why doubling the voltage stores four times the energy, while doubling capacitance only doubles it.

Class II ceramics (X5R, X7R, Y5V) lose effective capacitance under DC bias. An X7R 10 μF rated at 25 V may retain only 2 - 4 μF at 20 V bias. The actual stored energy can be 50 - 80% lower than the ideal calculation. Always consult the manufacturer's DC bias curve and use the derated capacitance value in this calculator for accurate results.

The commonly cited threshold is 1 J at voltages above 50 V DC or 25 V AC. IEC 62368-1 defines hazardous energy as exceeding 20 J when the circuit can deliver it in under 1 s. Large electrolytic banks in power supplies regularly store 10 - 100 J. Supercapacitors at 100 F and 2.7 V store 364.5 J, enough to cause burns or weld contacts. Always use bleeder resistors.

Hold-up time t = C(V_max² − V_min²)2P, where P is the load power in watts. Use this calculator to find E at V_max and V_min, subtract them, then divide by load power. Note that DC-DC converter efficiency (typically 85 - 92%) must be factored in.

For C0G/NP0 ceramics and film capacitors, temperature coefficients are under ±30 ppm/°C, so the effect is negligible. For X7R ceramics, capacitance varies ±15% over −55 to +125 °C. Aluminum electrolytics can lose 40 - 50% capacitance at −40 °C. For temperature-critical applications, use the worst-case capacitance value in this calculator.

Yes, but first compute the equivalent capacitance. For parallel: C_eq = C₁ + C₂ + … For series: 1C_eq = 1C₁ + 1C₂ + … Enter the equivalent value and total applied voltage to get the system energy.