Capacitor Energy Calculator
Calculate the energy stored in a capacitor using its capacitance and voltage. Switch to reverse mode to find the required capacitance or voltage for a target energy level.
How to Use This Capacitor Energy Calculator
Select what you want to solve for using the toggle at the top, then fill in the known values:
Energy mode — enter capacitance (choose your unit from the dropdown) and the voltage across the capacitor. The stored energy is shown instantly in joules, millijoules, or microjoules.
Capacitance mode — enter the target energy in joules and the supply voltage. The calculator returns the required capacitance in a human-readable unit.
Voltage mode — enter the target energy in joules and the available capacitance. The calculator returns the voltage needed to store that energy.
⚠ Always ensure the capacitor's voltage rating exceeds the applied voltage by a comfortable safety margin — typically 20 % or more.
Capacitor Energy Formula
The energy stored in a capacitor is given by:
Rearranged to find capacitance:
Rearranged to find voltage:
Where E is energy in joules (J), C is capacitance in farads (F), and V is voltage in volts (V).
Example
Given: C = 1000 µF, V = 12 V
Step 1 — Convert capacitance:
1000 µF = 0.001 F
Step 2 — Apply formula:
E = 0.5 × 0.001 × 12²
Step 3 — Square the voltage:
12² = 144
Step 4 — Multiply:
E = 0.5 × 0.001 × 144
E = 0.072 J (72 mJ)
Frequently Asked Questions
What is energy stored in a capacitor?
When a capacitor is charged, it stores electrical energy in the electric field between its plates. This energy can be released back into a circuit when needed — making capacitors useful in flash circuits, power supplies, and energy harvesting applications.
Why does energy scale with voltage squared?
Each additional volt of charge requires more work because it must push against an already-existing electric field. The quadratic relationship means doubling the voltage quadruples the stored energy, which is why high-voltage capacitors store enormous energy in small packages.
What unit is capacitor energy measured in?
Joules (J). Because capacitor values are often small, results are commonly expressed in millijoules (mJ, 10⁻³ J) or microjoules (µJ, 10⁻⁶ J). One joule equals one watt-second of energy.
Can I use this for supercapacitors?
Yes. Simply enter the capacitance in farads (select F from the unit dropdown) and the operating voltage. Supercapacitors can reach hundreds or even thousands of farads, storing significant energy at low voltages.
Why must the capacitor voltage rating exceed the applied voltage?
Exceeding a capacitor's rated voltage can break down the dielectric layer, causing permanent damage or catastrophic failure. A 20 % derating margin is a common rule of thumb — for example, use a 25 V-rated capacitor for a 20 V circuit.