Capacitor Energy & RC Time Calculator

Use E = 1/2 × C × V², with capacitance in farads and voltage in volts. A 470 µF capacitor charged to 48 V stores 0.5 × 0.00047 × 48², which is about 0.54 J. Because the voltage term is squared, doubling the voltage quadruples the stored energy, which is why capacitors in higher voltage equipment deserve more respect than their physical size suggests.

The time constant τ = R × C tells you how quickly a capacitor charges or discharges through a resistance. After one time constant the capacitor reaches 63.2% of the applied voltage, and after five time constants it is 99.3% charged, which is treated as fully charged in practice. A 10 kΩ resistor with a 47 µF capacitor gives τ = 0.47 s, so full charge takes roughly 2.4 seconds.

The stored energy E = 1/2 × C × V² stays in the capacitor until something discharges it. Large electrolytic capacitors in variable speed drives, UPS systems and power factor correction equipment on commercial sites can hold a hazardous charge for minutes after isolation. Always follow the manufacturer's stated discharge time, prove dead, and discharge through a suitable resistor before working on capacitor banks.

A simple RC low-pass filter attenuates signals above its cutoff frequency f = 1 / (2 × π × R × C). At that frequency the output has fallen to 70.7% of the input. The RC tab of this calculator reports the cutoff alongside the time constant: 1 kΩ with 100 µF gives a time constant of 100 ms and a cutoff near 1.6 Hz, useful for smoothing slow sensor signals.

Charge Q = C × V is measured in coulombs (this calculator shows microcoulombs) and describes how much electric charge the plates hold. Energy E = 1/2 × C × V² is measured in joules and describes the work the capacitor can deliver. Charge scales linearly with voltage while energy scales with its square, so a capacitor at twice the voltage holds twice the charge but four times the energy.