Calculate Energy Stored in a Capacitor
Use this calculator to find how much electrical energy a capacitor stores from its capacitance and voltage.
What Is Capacitance Energy?
A capacitor stores energy in an electric field when a voltage is applied across its terminals. This stored energy is often called capacitance energy or electrostatic energy. The amount of energy depends on two things: the capacitor’s capacitance (how much charge it can hold per volt) and the applied voltage.
In practical electronics, this matters for power supply smoothing, pulse discharge circuits, backup power, motor drives, flash photography, and many other applications where short-term energy storage is useful.
The Core Formula
E = 1/2 × C × V²
This is the standard formula used in the calculator:
- E = energy in joules (J)
- C = capacitance in farads (F)
- V = voltage in volts (V)
Because voltage is squared, a small increase in voltage can significantly increase stored energy. Doubling voltage raises energy by a factor of four.
How to Use This Calculator
Step-by-step
- Enter the capacitor value (for example, 1000).
- Select the correct capacitance unit (for example, µF).
- Enter the voltage value (for example, 25).
- Select the voltage unit and click Calculate Energy.
The tool returns the energy in joules, an auto-scaled energy unit (like mJ or kJ), watt-hours for comparison, and stored charge using Q = C × V.
Worked Examples
Example 1: Small decoupling capacitor
Suppose C = 10 µF and V = 5 V. Convert capacitance: 10 µF = 10 × 10-6 F = 0.00001 F. Then E = 1/2 × 0.00001 × 5² = 0.000125 J, or 0.125 mJ.
Example 2: Electrolytic capacitor in a power circuit
If C = 470 µF and V = 12 V: E = 1/2 × 470×10-6 × 12² ≈ 0.03384 J (33.84 mJ).
Example 3: High-energy capacitor bank
For C = 0.1 F and V = 400 V: E = 1/2 × 0.1 × 400² = 8,000 J (8 kJ). This is substantial energy and requires proper safety design.
Common Unit Conversions
- 1 mF = 10-3 F
- 1 µF = 10-6 F
- 1 nF = 10-9 F
- 1 pF = 10-12 F
- 1 kV = 1000 V
- 1 mV = 0.001 V
Engineering Notes and Safety
Voltage rating matters
Never operate a capacitor above its rated voltage. Overvoltage can cause dielectric breakdown, overheating, leakage, venting, or catastrophic failure.
Discharge procedures
Capacitors can remain charged after power is removed. Use properly rated discharge resistors, insulated tools, and lockout practices in high-voltage systems.
Real-world losses
Ideal formulas assume no losses. In reality, equivalent series resistance (ESR), leakage current, dielectric absorption, and temperature effects reduce usable energy in dynamic circuits.
Quick FAQ
Why does energy increase so fast with voltage?
Because voltage appears as V² in the equation. Increasing voltage has a quadratic effect on stored energy.
Can this calculator be used for supercapacitors?
Yes. Enter capacitance in farads and voltage in volts. For supercapacitors, keep in mind that usable energy in systems may be less than ideal due to balancing circuits and voltage limits.
Is watt-hour output useful?
Yes. Joules are standard for physics, while watt-hours are often easier for battery and power comparisons. The conversion is 1 Wh = 3600 J.