Capacitor Energy Calculator
Enter capacitance and voltage to get stored energy in joules and Wh, plus charge in coulombs and mAh — the numbers behind flash circuits and supercapacitor backup.
Last updated: 2026-09-28
How the calculation works
- Energy follows E = ½·C·V²: charging a capacitor stores work equal to half the charge times the final voltage.
- Charge follows Q = C·V directly — linear in voltage, unlike the energy's square relationship.
- The mAh figure converts charge at 1 mAh = 3.6 C; it describes charge, not usable energy at a load voltage.
Formula
E = ½ × C × V² Q = C × V
| Symbol | Meaning | Unit |
|---|---|---|
E | Stored energy | J |
Q | Stored charge | C |
C | Capacitance | F |
V | Voltage across the capacitor | V |
Worked example
Interpreting the result
The V² law is the practical takeaway: doubling the voltage quadruples stored energy — and quadruples the arc-flash hazard on power-electronics bus capacitors, which stay charged long after power-off. Supercapacitors hold far less energy per kg than batteries (roughly 5-10 Wh/kg vs 100-250), but accept millions of cycles and deliver enormous power — that trade decides the application.
Assumptions
- Ideal capacitor; real ESR dissipates a few percent on fast discharge.
- Fully charged to V and discharged fully for the energy figure.
Limitations
- Capacitor voltage rating must exceed the working voltage with margin (typically 20%+).
- Series-connected capacitors share voltage unevenly without balancing resistors.
Frequently asked questions
How much energy is in a capacitor?
E = ½·C·V² joules. A 1000 µF cap at 12 V holds 0.072 J; at 300 V the same cap holds 45 J — 625× more, because voltage enters squared.
Why half of C·V²?
As the capacitor charges, its voltage rises from 0 to V, so the average voltage during charging is V/2. Energy = Q × V_avg = (C·V) × (V/2) = ½·C·V². The other half is dissipated in the charging resistance — a thermodynamic fact, not a design choice.