Capacitor Energy: E = ½CV²
Energy stored in a capacitor grows with the square of the voltage. Doubling V quadruples the energy. The diagram shows how energy scales across common capacitor types.
Above ~0.25 J and ~50 V — a charged capacitor becomes a shock and burn hazard. High-voltage capacitors (300–400 V) store dangerous energy even at small capacitance values.
Capacitor Energy Calculator
A triangle solver for E = ½CV². Enter any two of capacitance, voltage, or energy — the calculator solves for the third. Work forwards (“how much energy does this capacitor store?”) or backwards (“what capacitance do I need for 1 joule at 12 V?”). Also shows charge stored, watt-hour conversion, and a safety hazard assessment.
The Three Formulas
C = 2E / V² — find capacitance from energy and voltage
V = √(2E / C) — find voltage from energy and capacitance
Leave one field empty. The calculator fills it. The solved value is highlighted in green.
Forward Solve — Find Energy
1000 µF Electrolytic at 25 V
Q = 1000×10−6 × 25 = 25 mC
Wh = 0.3125 / 3600 = 0.0868 mWh
0.31 J at 25 V. Just above the 0.25 J caution threshold but below 50 V — yellow safety status. Enough energy to feel a tingle but not dangerous.
330 µF Camera Flash at 300 V
Q = 330×10−6 × 300 = 99 mC
Wh = 14.85 / 3600 = 4.125 mWh
14.85 J at 300 V. Red safety status — both voltage above 50 V and energy above 0.25 J. This is a lethal shock and burn hazard. The 330 µF capacitor is only 3.3× the capacitance of the 100 µF electrolytic above, but at 300 V it stores 47× the energy. Voltage matters far more than capacitance for energy.
10 F Supercapacitor at 2.7 V
Q = 10 × 2.7 = 27 C
Wh = 36.45 / 3600 = 10.1 mWh
36.45 J — more than the camera flash capacitor — but only 2.7 V, so green safety status (no shock hazard). The watt-hour figure (10.1 mWh) puts this in perspective: a small 500 mAh lithium cell stores ~1850 mWh, or 183× more. This is why supercapacitors handle short-term backup (seconds to minutes) while batteries handle long-duration power.
Reverse Solve — Find Capacitance
“I Need 1 Joule at 12 V”
About 13890 µF — a large electrolytic or a small supercapacitor. This is the reverse solve that makes energy-first design possible: start with your energy budget, set the voltage, and find the capacitance.
“I Need 100 J for a Pulse Discharge at 400 V”
1250 µF at 400 V. A single 1500 µF / 450 V electrolytic or a bank of smaller capacitors in parallel. Red safety status — 100 J at 400 V is extremely dangerous.
Reverse Solve — Find Voltage
“A 470 µF Capacitor Stores 5 J — What Voltage?”
~146 V. Red safety status. This solve direction is useful for safety assessment: you know the capacitance and measured energy (or calculated it from a discharge pulse) and need to confirm the voltage the capacitor reached.
Why Voltage Matters More Than Capacitance
Energy scales with V² but only linearly with C. Practical comparison:
100 µF at 25 V → E = 31.3 mJ (25000× more energy)
330 µF at 300 V → E = 14.85 J (475000× more than the 100 nF)
1000 µF at 400 V → E = 80 J (the highest bar in the chart)
The 330 µF at 300 V stores 475× more energy than the 100 µF at 25 V — despite only 3.3× the capacitance. The 12× higher voltage contributes 144× to the energy (12² = 144). This V² relationship is the most important thing to understand about capacitor energy.
Safety Assessment
Yellow — either voltage above 50 V OR energy above 0.25 J. One condition alone warrants caution — possible shock or localised burn.
Red — voltage above 50 V AND energy above 0.25 J. Lethal shock and burn hazard. Discharge through a bleeder resistor before handling. Follow lockout/tagout procedures.
The 0.25 J / 50 V thresholds come from safety standards (IEC 62368-1, IEC 60950). Below 50 V, current cannot drive enough through skin resistance to cause fibrillation. Below 0.25 J, even at higher voltages, the energy dissipates before causing significant tissue damage. Above both thresholds, the combination is dangerous.
Watt-Hour Conversion
Divide joules by 3600 to get watt-hours. This puts capacitor energy in the same units used for batteries:
36.45 J supercap → 10.1 mWh
500 mAh lithium cell at 3.7 V → 1850 mWh
Ratio: battery stores 183× more
Capacitors deliver energy fast (microseconds to seconds) but store very little compared to batteries. Batteries store a lot but deliver slowly (minutes to hours). The watt-hour comparison makes this trade-off concrete.
Common Applications
Energy Storage Sizing
Start with the energy your system needs to ride through a power interruption. Use C = 2E/V² to find the capacitance. Factor in that you can only use the energy between Vmax and Vmin (not all the way to 0 V), so the usable energy is E = ½C(Vmax² − Vmin²).
Pulse Power
Camera flashes, defibrillators, pulsed lasers, and electromagnetic forming all need a burst of energy delivered in milliseconds. Size the capacitor for the required energy, then verify the peak current and discharge time with the Capacitor Charge Calculator.
Safety Audits
Before servicing any equipment with capacitors above 50 V, calculate E = ½CV² to assess the hazard. If the result is above 0.25 J, the capacitor must be discharged and verified before contact.
Capacitor Comparison
Comparing capacitors by capacitance alone is misleading. A 10 F supercapacitor at 2.7 V stores 36.45 J. A 1000 µF electrolytic at 400 V stores 80 J — with 10000× less capacitance. Always compare by energy (joules) or energy density (J per volume or weight).
Frequently Asked Questions
Last updated: March 2026