Energy stored, charge held, and the RC charge-and-discharge timing of a capacitor — plus the voltage at any moment as it charges or discharges through a resistor.
What a Capacitor Holds, and How Fast
A capacitor stores energy in an electric field. Two numbers describe how much: the charge it holds and the energy in it. A third — the time constant — tells you how quickly it fills or empties through a resistor. Together they cover most of what you need for timing circuits, filters, power-hold-up and energy storage.
Energy rises with the square of voltage, so doubling the voltage quadruples the stored energy — which is why voltage rating matters so much. Charge is simply capacitance times voltage. And the product of resistance and capacitance sets the timing.
The 63% Rule
Charge a capacitor through a resistor and it does not jump to full — it follows a curve. After one time constant τ = RC it reaches about 63% of the supply voltage; after two, 86%; after five, over 99%, which engineers treat as fully charged. Discharging mirrors it: down to 37% after one τ, near zero after five. That single curve underlies RC timers, debounce circuits and filter roll-off.
Frequently Asked Questions
Why five time constants for full charge?
The approach is exponential, so it never mathematically reaches 100%. At 5 RC it is at 99.3%, close enough for practical purposes – hence the rule of thumb.
Charge or energy – which matters?
Depends on the job. Timing and current draw care about charge (Q = C V); how much work the capacitor can deliver, or how much it dumps in a fault, depends on energy (half C V squared).
What sets the inrush current?
At the instant of connection the capacitor looks like a short, so the peak current is just V divided by R. A small series resistor limits that inrush.
Related calculators
- AC Impedance Calculator — capacitive reactance and resonance in AC circuits.
- Ohm’s Law Calculator — voltage, current and resistance.
- Power Converter — watts, horsepower and more.
- Engineering Unit Converter — electrical and other units.
Worked example
Capacitors in parallel add: 10 µF + 22 µF = 32 µF. Two 10 µF capacitors in series give 5 µF. A 10 kΩ resistor with a 100 µF capacitor has a time constant of 10,000 × 0.0001 = 1 second.
Common mistakes
Check the voltage rating of every capacitor and stay well below it. Electrolytic capacitors are polarized and can fail if connected backwards, and real capacitors have tolerances that can be 20% or more.
Related calculators
Try the Ohm’s Law Calculator.