Capacitor Energy Storage Calculator
Calculates the energy stored, charge held, or RC time constant for a capacitor given its capacitance, voltage, and series resistance. Useful for power supply design, timing circuits, and energy storage projects.
About this calculator
A capacitor stores electrical energy in an electric field between its plates. Three key quantities describe its behavior in a DC circuit. Energy stored: E = ½ × C × V², where C is capacitance in farads and V is voltage in volts; result is in joules. Charge stored: Q = C × V, where Q is in coulombs. RC time constant: τ = R × C, where R is series resistance in ohms and C is in farads; τ (tau) is the time in seconds for the capacitor to charge to about 63.2% of the supply voltage (or discharge to 36.8%). Because practical capacitors are rated in microfarads (μF), the calculator converts by multiplying by 1×10⁻⁶ before applying these formulas. These relationships are fundamental to filter design, timing circuits, camera flash units, and uninterruptible power supplies.
How to use
Example 1 — Energy: You have a 1,000 μF capacitor charged to 12 V. Select calculation_type = energy. E = 0.5 × (1000 × 10⁻⁶) × 12² = 0.5 × 0.001 × 144 = 0.072 J (72 mJ). Example 2 — Charge: Same capacitor at 12 V. Q = (1000 × 10⁻⁶) × 12 = 0.012 C (12 mC). Example 3 — Time Constant: Add a 470 Ω series resistor. τ = 470 × (1000 × 10⁻⁶) = 0.47 s. This means the capacitor charges to ~63% of 12 V (≈7.6 V) in 0.47 seconds and reaches full charge in about 5τ = 2.35 seconds.
Frequently asked questions
How much energy can a typical capacitor store compared to a battery?
Capacitors store far less energy per unit volume than batteries. A large 10,000 μF electrolytic capacitor charged to 16 V holds only about 1.28 J, while an AA alkaline battery stores roughly 9,000 J. However, capacitors can release their energy almost instantaneously, making them ideal for applications requiring high peak power in short bursts — such as camera flashes, audio amplifier power supplies, and defibrillators — where batteries would be too slow. Supercapacitors (ultracapacitors) bridge the gap, storing significantly more energy than conventional capacitors while still delivering faster discharge than batteries.
What does the RC time constant tell you about a capacitor charging circuit?
The time constant τ = R × C defines the charging and discharging speed of the capacitor. After one τ the capacitor has charged to 63.2% of the supply voltage; after 2τ it reaches 86.5%; after 5τ it is considered fully charged at 99.3%. The same curve applies in reverse during discharge. This predictable exponential behavior is exploited in timer circuits (like the 555 timer IC), low-pass and high-pass RC filters, and debounce circuits in digital electronics. Choosing R and C values to set a desired time constant is a core skill in analog circuit design.
Why does energy stored in a capacitor depend on the square of voltage?
The relationship E = ½CV² arises because as a capacitor charges, each additional increment of charge must be pushed against the growing electric field already established by the previous charge. The work done is proportional to both the charge moved and the voltage at that moment, and since voltage itself grows linearly with charge (V = Q/C), the total energy integrates to a quadratic function of voltage. A practical consequence is that doubling the voltage quadruples the stored energy — making high-voltage capacitors extremely energy-dense but also more dangerous to handle, as the discharge current can be lethal.