Capacitor Energy Calculator
A capacitor stores electrical energy as charge separated across two plates. Enter the capacitance and the voltage across it; the calculator applies the textbook E = ½ C V² and Q = C V formulas and auto-scales the energy unit (pJ / nJ / µJ / mJ / J / kJ) so the result is always readable. Handy for EE coursework, repair work, and DIY projects involving photoflash caps, supercaps, snubber networks or energy-storage banks.
Potential difference across the plates. Negative values are accepted — energy is independent of polarity.
Please enter a valid number
Stored energy E
33.84 mJ
E = ½ × C × V²
Stored charge Q
5.64 mC
Q = C × V
Summary
Formula
E = ½ × C × V² | Q = C × V | Units: capacitance in farads, voltage in volts → energy in joules, charge in coulombs.
- · Unit relations: 1 F = 1000 mF = 10⁶ µF = 10⁹ nF = 10¹² pF.
- · Energy scales with V² — doubling the voltage multiplies stored energy by 4×, while doubling the capacitance only doubles it.
- · Charge follows the sign of the voltage (polarity flip), but energy is always positive because V² is never negative.
- · Safety: a capacitor ≥ 100 µF charged above ~50 V holds enough energy to shock or arc. Always discharge through a bleeder resistor or multimeter before servicing power supplies, photoflash circuits or VFDs.
- · Real capacitors can be ±5% to ±20% off their printed value (X7R, Y5V, aluminium electrolytic), and capacitance also drifts with temperature, frequency and DC bias — this tool uses the ideal formula and ignores those effects.
- · Formulas: any standard electromagnetism textbook (e.g. Halliday-Resnick; Griffiths, Introduction to Electrodynamics, capacitor chapter).
Frequently asked
Where does the E = ½CV² formula come from?
Imagine charging the capacitor from 0 V up to V. At an instant when the voltage across the cap is v, the stored charge is q = Cv. Moving an extra dq from the 0 V plate to the positive plate takes work v dq = (q/C) dq. Integrate from q = 0 to Q = CV: W = ∫₀^Q (q/C) dq = Q² / (2C) = ½ C V². The factor of ½ shows up because the voltage rises linearly from 0 to V during charging, so the average voltage seen by the moving charge is ½V, not the full V you measure at the end.
How much energy does 470 µF at 12 V actually store? What about a camera flash?
½ × 470 µF × (12 V)² ≈ 0.034 J ≈ 34 mJ — roughly the punch of a single small LED flash. Typical camera-flash capacitors run 100–800 µF charged to 300–400 V, so they hold from ~6 J (100 µF at 350 V) up to ~58 J (800 µF at 380 V), which is why flash caps must always be bled through a resistor before servicing. AED defibrillators are scarier still: a ~150 µF capacitor charged to 1800 V stores ~240 J — exactly the life-saving jolt delivered to the patient.
Does the formula differ for electrolytic, ceramic and supercapacitors?
Same formula — E = ½CV² applies to every capacitor type. What changes is the practical capacitance range and voltage rating: ceramic caps run pF to a few µF at 6.3 V to several kV; aluminium electrolytics run µF to mF at 6.3 – 450 V; supercapacitors run 0.1 F to 3000 F at 2.5 – 5.5 V per cell. A 100 F supercap at 2.7 V stores ½ × 100 × 2.7² ≈ 365 J — enough to power a 1 W LED for over six minutes. Note: class-II ceramic (X7R, Y5V) capacitance can drop more than 50 % under DC bias, so real stored energy will be lower than the ideal calculation suggests. For accurate work, consult the C vs V curve in the datasheet.
Is a negative voltage allowed?
Yes, a negative voltage is fine in the math. Negative just means you have flipped the probe leads — the polarity of the field across the plates is reversed. Energy goes as V², so it is identical for ±V; the charge Q = CV flips sign accordingly, meaning the charge accumulates on the opposite plate. Real polarised electrolytics, however, will be destroyed if you reverse-bias them — the math here is symmetric, but the hardware is not.
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