Capacitor Energy Calculator
Capacitance & voltage
Stored charge & energy
| Stored charge (Q) | — |
|---|---|
| Stored energy (E) | — |
How to Use This Capacitor Energy Calculator
Lab worksheets ask for joules stored on a cap, but the part label lists microfarads and the bench supply reads volts—so you end up juggling prefixes on scrap paper until something looks off by a factor of ten. In my experience breadboarding filter stages and camera flash triggers, the mistake is almost always unit conversion, not the physics.
To get started, enter capacitance and the voltage across the plates. The tool converts your cap value to farads internally, then reports charge Q and energy E with sensible SI prefixes. Results refresh as you type—handy when you are comparing a 20 V bank versus a derated 16 V supply.
- Capacitance: Type the value printed on the component and pick μF, nF, or pF from the dropdown. Default example: 300 μF.
- Voltage: Enter the potential difference between plates in volts—the supply you charge to, not the ripple peak unless that is what you mean to model.
- Read Q and E: Charge shows in C, mC, or μC as appropriate; energy in J, mJ, or μJ. Use the calculation box to audit arithmetic before a lab submission.
For circuit current and power at the same voltage, continue with the Ohm's Law Calculator or the Electricity Calculator. For other stored-energy forms, see the Elastic Potential Energy Calculator and Work Calculator.
Capacitor Energy Calculator Formulas and Practical Applications
A capacitor is a pair of conductors separated by an insulator. Charge piles on the plates until the electric field between them balances the battery pushing electrons around the loop. That stored field energy is electrostatic potential energy—not the same as kinetic energy, but it can be dumped into a load in microseconds when you close the switch.
E = ½ × C × V²
Q = C × V
Equivalent forms use C = Q / V: E = ½ × Q² / C and E = ½ × Q × V. They are the same physics; pick whichever matches the givens on your homework page.
Worked example (300 μF at 20 V)
Convert capacitance: C = 300 μF = 3 × 10⁻⁴ F. Then Q = 3 × 10⁻⁴ × 20 = 6 × 10⁻³ C = 6 mC. Energy E = ½ × 3 × 10⁻⁴ × 20² = 6 × 10⁻² J = 60 mJ. That is the same order as a small LED flash capacitor—enough to sting a finger if you discharge through skin, which is why bench rules say discharge caps before you touch nodes.
LC tanks and energy swapping
In an LC resonant circuit, energy sloshes between the capacitor’s electric field and the inductor’s magnetic field like water between two cups—electrostatic at one instant, mostly magnetic a quarter-cycle later. Real boards add resistance, so the oscillation dies out, but the capacitor energy formula still tells you the peak stored in the cap at the top of each swing. Radio front-ends and signal filters lean on that behavior.
Multiple capacitors in one network
If several caps sit in series or parallel, find the single equivalent capacitance first, then run E = ½ C V² for the network voltage across the combination. Mixed values and unequal voltages in series need a full network analysis—this page assumes one capacitance and one plate voltage, which matches the standard intro-lab problem.
Standard Units and Conversion Tables
Capacitance prefixes
| Unit | Symbol | Farads (SI) |
|---|---|---|
| Farad | F | 1 |
| Millifarad | mF | 10⁻³ |
| Microfarad | μF | 10⁻⁶ |
| Nanofarad | nF | 10⁻⁹ |
| Picofarad | pF | 10⁻¹² |
Charge and energy at a glance
- Charge (Q): coulombs (C); 1 C = 1 A·s
- Energy (E): joules (J); 1 J = 1 W·s
- Voltage (V): volts; must match the potential across the capacitor you are modeling
Frequently Asked Questions
How does a capacitor store energy?
Separated charge on the plates creates an electric field between them. Maintaining that field against the opposite charges takes work from the charging source—that work stays stored as electrostatic potential energy until the cap discharges.
How do I calculate the energy stored by a capacitor?
Multiply capacitance in farads by voltage squared, then divide by two: E = ½ C V². Charge is Q = C V. Enter both inputs here and the calculator handles prefix conversion.
What is the energy in a 120 pF capacitor at 1.5 V?
E = ½ × 120 × 10⁻¹² × 1.5² ≈ 1.35 × 10⁻¹⁰ J (about 135 picojoules). Charge Q ≈ 1.8 × 10⁻¹⁰ C. Tiny caps in RF matching networks store small energies, but the voltage rating still matters for breakdown.
Why is capacitor energy divided by two?
While charging, voltage ramps from 0 to V, so the average voltage during the process is V/2. Work integrates to ½ Q V, not Q V as if full voltage applied the whole time—that is why the factor ½ appears in ½ C V².
Do I need capacitance in farads?
SI formulas use farads. Marked values are usually μF, nF, or pF; pick the matching unit in the dropdown and the calculator converts before applying Q = C V and E = ½ C V².