Capacitor EnergyFree online science calculator — instant results, no sign-up
Calculate the energy stored in a capacitor (E = ½CV²).
About the Capacitor Energy
The Capacitor Energy calculator calculates the electrical energy stored in a capacitor using its capacitance in microfarads and the voltage across its terminals. It applies E = ½CV² and reports the result in joules and millijoules. With the default inputs of 1000 µF and 12 V, the stored energy is 0.072 J, or 72 mJ.
- Capacitance (µF)
- The capacitor's capacitance, entered in microfarads (µF), indicates its ability to store charge per unit voltage and defaults to 1000 µF.
- Voltage (V)
- The potential difference across the capacitor's terminals, entered in volts (V), defaults to 12 V.
Formula and method
C_F = C_µF × 10⁻⁶; E_J = ½ × C_F × V²; E_mJ = E_J × 1000.
- C_µFCapacitance entered in microfarads (µF).
- C_FCapacitance converted to farads (F).
- VVoltage across the capacitor, in volts (V).
- E_JStored electrical energy in joules (J).
- E_mJThe same stored electrical energy expressed in millijoules (mJ).
Convert the capacitance from microfarads to farads, then multiply it by the square of the voltage and by one-half. For the default inputs, 1000 µF × 10⁻⁶ = 0.001 F, so E = ½ × 0.001 × 12² = 0.072 J. Multiply by 1000 to express this as 72 mJ.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Capacitance1000 µF
- Voltage12 V
Results
- Stored energy0.072 J
- In millijoules72 mJ
How to use the Capacitor Energy
- 1Enter the capacitance in microfarads; keep 1000 µF to reproduce the default example.
- 2Enter the voltage actually across the capacitor; keep 12 V for the default example.
- 3Calculate the energy using E = ½CV², with capacitance converted to farads.
- 4Read the stored-energy output in joules: the default example gives 0.072 J.
- 5Read the millijoule output for the same energy on a smaller unit scale: the default example gives 72 mJ.
Reading your result
The 'Stored energy' row gives the capacitor's electrical energy in joules; with 1000 µF at 12 V, it reads 0.072 J. The 'In millijoules' row expresses exactly the same energy as 72 mJ, since 1 J = 1000 mJ. These are energy values, not power ratings or estimates of how long the capacitor can operate a load.
Limitations
The formula assumes an ideal capacitor with constant capacitance. Real capacitance can vary with tolerance, temperature, and applied voltage, and leakage and internal resistance reduce energy delivered to a load. The result does not predict discharge time, peak current, or charging losses. Use nonnegative capacitance and the actual terminal voltage; the capacitor's voltage rating is a limit, not necessarily its operating voltage. This calculation does not establish electrical safety: observe voltage and polarity ratings and use appropriate discharge procedures.
Common questions
Why must 1000 µF be converted before using E = ½CV²?+
The formula gives energy in joules when capacitance is in farads and voltage is in volts. Therefore, 1000 µF becomes 0.001 F before calculating ½ × 0.001 × 12² = 0.072 J.
Should I enter the capacitor's rated voltage or its measured voltage?+
Enter the voltage actually across its terminals. The rated voltage is the manufacturer's operating limit; using it would estimate energy at that voltage rather than the energy currently stored.
How does voltage affect the stored energy compared with capacitance?+
Energy is proportional to capacitance but to the square of voltage. At unchanged voltage, doubling capacitance doubles energy; at unchanged capacitance, doubling voltage quadruples energy, provided the capacitor remains within its ratings.
Can a load receive all 72 mJ from the default capacitor?+
The 72 mJ result is the ideal stored energy at 1000 µF and 12 V. A real load usually receives less because of losses and any energy left when discharge stops. For constant capacitance, the ideal energy released between initial and final voltages is ½C(V_initial² − V_final²), with C in farads.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.