Sizing a capacitor for an energy store starts from how much energy is needed and at what voltage. Doubling the voltage cuts the required capacitance by four.
How to use the Capacitor Size Calculator
- Required energy
- Voltage
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the required energy and the working voltage.
Inputs explained
| Field | What to enter |
|---|---|
| Required energy | Units: J, mJ, kJ, Wh |
| Voltage | Units: V, mV, kV, µV, MV |
What each result means
- Required capacitance = 2×[Required energy]÷[Voltage]^2
- Charge = 2×[Required energy]÷[Voltage]
Example values
How the main result changes when Required energy changes and everything else stays at the example values above:
| Required energy (J) | Required capacitance (µF) |
|---|---|
| 25 | 945.1796 |
| 37.5 | 1,417.7694 |
| 50 | 1,890.3592 |
| 62.5 | 2,362.949 |
| 75 | 2,835.5388 |
| 100 | 3,780.7183 |
Formula
C = 2E / V²
Worked example
Inputs — Required energy: 50 J; Voltage: 230 V.
Results — Required capacitance = 1,890.35917 µF; Charge = 434.78261 mC.
Key terms
- Energy storage
- Capacitor energy is ½CV².
Frequently asked questions
Why does voltage matter so much?
Energy grows with V², so higher voltages need much less capacitance.
Should I add a margin?
Yes – capacitance tolerance and ageing can be 10–20%.
What voltage rating should I choose?
At least 20–50% above the working voltage.
Can this size a motor start capacitor?
As a first estimate from the start-up energy; see also the motor capacitor calculator.