Capacitor Energy & Charge Calculator
Stored energy, charge, and time constant
Required Parameters
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Quick Answer
Energy stored: E = ½CV² (joules). Charge stored: Q = CV (coulombs). A 1000µF cap at 50V stores 1.25 joules.
Design Notes
Capacitor energy scales with voltage squared — doubling voltage quadruples energy. This is critical for safety: a 400V bus capacitor in a motor drive stores lethal energy. Always include bleeder resistors for discharge. For energy harvesting, supercapacitors (1-100F) bridge the gap between batteries and ceramic caps.
Common Mistakes
- 1
Assuming a capacitor can deliver its stored energy at constant voltage — voltage drops as energy is extracted.
- 2
Ignoring ESR losses during rapid charge/discharge cycles.
- 3
Confusing capacitor voltage rating with energy — a 100µF/50V cap stores 62× more energy than a 100µF/6.3V cap.
Knowledge Base
How much energy does a capacitor store?
Energy E = ½CV², where C is capacitance in farads and V is voltage. A 1000µF capacitor charged to 50V stores E = ½ × 0.001 × 2500 = 1.25 joules. Energy scales with voltage squared — doubling voltage quadruples stored energy.
What is the charge stored in a capacitor?
Charge Q = C × V, measured in coulombs. A 100µF capacitor at 5V stores Q = 100×10⁻⁶ × 5 = 500 microcoulombs. One coulomb equals one ampere flowing for one second, so this capacitor could supply 500µA for one second (in theory).
How long does it take to charge a capacitor?
Through a resistor, charging follows an exponential curve. Time constant τ = R × C. After 1τ: 63.2% charged. After 3τ: 95%. After 5τ: 99.3% (considered fully charged). A 100µF cap through 10kΩ: τ = 1 second, fully charged in ~5 seconds.
Can a capacitor kill you?
Yes. Capacitors storing more than 1 joule at voltages above 50V are potentially lethal. A 450V 470µF electrolytic in a power supply stores ½ × 470µF × 450² = 47.6 joules — enough to cause cardiac arrest. Always discharge capacitors safely with a bleeder resistor before servicing equipment.
How do supercapacitors compare to batteries?
Supercapacitors (1-3000F) bridge the gap. Energy density: batteries store 100-250 Wh/kg, supercaps store 5-10 Wh/kg. But supercaps charge in seconds (vs hours), last 500K+ cycles (vs 500-1000), and work from -40°C to +65°C. Best for: energy harvesting, backup power, regenerative braking.
Why does energy scale with voltage squared?
Because work is done against an increasing electric field. As charge accumulates on the plates, the voltage rises, and each additional unit of charge requires more work to push against the existing field. Integrating W = ∫V·dQ = ∫(Q/C)·dQ from 0 to Q gives E = Q²/2C = ½CV².
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