Capacitor Energy Calculator
A capacitor stores electrical energy in an electric field between its plates. This calculator computes stored energy in Joules from capacitance and voltage, and shows charge stored and energy in millijoules.
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Formula
E = ½CV²
Energy stored E = ½ × C × V², where C is capacitance in Farads and V is voltage in Volts. Charge stored Q = C × V (Coulombs). The factor of ½ arises because voltage builds up as charge is added — the average voltage over the charging process is V/2. Note: capacitance is entered in microfarads (μF); the formula converts to Farads by dividing by 1,000,000.
How to use the Capacitor Energy Calculator
- 1
Enter your capacitance
Value should be in μF.
- 2
Enter your voltage
Value should be in V.
- 3
Read your results instantly
Results update in real time as you type.
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How capacitors store energy
A capacitor consists of two conducting plates separated by an insulating dielectric. When voltage is applied, positive charge accumulates on one plate and negative on the other, creating an electric field in the dielectric. This field is where the energy is stored — not in a chemical reaction like a battery.
Because charging is instantaneous (in theory) and discharge can be nearly so, capacitors can deliver very high power bursts. A camera flash capacitor (typically 100-1,000 μF charged to 300-400V) stores 4.5-80 J and discharges in milliseconds — peak power of thousands of Watts, far beyond what the AA batteries could directly supply.
Capacitors vs. batteries
Batteries store energy electrochemically — typically 1-3 MJ/kg (around 200 Wh/kg for lithium-ion). Capacitors store energy in an electric field — typically 1-10 J/kg for conventional electrolytic capacitors, or up to 10 kJ/kg for supercapacitors (also called ultracapacitors or EDLCs).
Capacitors charge and discharge much faster than batteries (seconds vs. hours) but store much less energy. Supercapacitors bridge the gap: they charge in minutes and store more energy than conventional caps, used for regenerative braking, UPS systems, and engine starting. A 3,000 F supercapacitor at 2.7V stores 10,935 J = 3 Wh — about 1/30th of a AA battery but capable of delivering far more current.
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RC time constant and applications
When a capacitor charges or discharges through a resistor, the time constant τ = RC (seconds). After one time constant, a charging capacitor reaches 63% of final voltage; after 5τ, it's 99.3% charged. This RC relationship is fundamental to filters, timers, and oscillators.
Applications of capacitors: power supply filtering (smoothing ripple voltage), motor starting (phase shift), audio crossovers (frequency filtering), timing circuits (RC oscillators), energy storage (flash, defibrillators), and decoupling (bypassing high-frequency noise on power rails in digital circuits).
Tips & Insights
Voltage rating is critical
Never exceed a capacitor's voltage rating. Electrolytic capacitors fail catastrophically (sometimes explosively) when over-voltaged. Always use a capacitor rated at least 20% above the maximum circuit voltage.
Energy scales as voltage squared
Doubling voltage quadruples stored energy. A capacitor at 24V stores 4× the energy as the same capacitor at 12V.
Discharge can be dangerous
Large capacitors charged to high voltage can deliver lethal shocks. Always discharge capacitors safely before handling. A 1,000 μF capacitor at 400V stores 80 J — enough to cause cardiac arrest.
Worked Examples
Camera flash capacitor
E = ½ × 0.001F × 300² = 45 J. Charge = 0.001 × 300 = 0.3 C. Discharged in 1ms: peak power = 45,000 W.
Power supply filter cap
E = ½ × 0.0047F × 256 = 0.602 J = 602 mJ. Charge = 0.0752 C.
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Frequently Asked Questions
How does a capacitor store energy?
By accumulating charge on two separated conducting plates, creating an electric field in the dielectric between them. Energy = ½CV².
What is capacitance?
Capacitance (C) measures how much charge a capacitor stores per volt: C = Q/V. Unit: Farads (F). 1 μF = 10⁻⁶ F.
What is the difference between a capacitor and a battery?
Batteries store energy chemically (much more energy per kg). Capacitors store energy electrically (much faster charge/discharge). Supercapacitors partially bridge the gap.
What is the RC time constant?
τ = R×C seconds. After τ, a charging capacitor reaches 63% of final voltage. After 5τ, it is effectively fully charged (99.3%).
Why is the energy formula ½CV²?
As charge accumulates, voltage increases proportionally. Average voltage during charging is V/2. Energy = average voltage × charge = (V/2) × CV = ½CV².
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