Calculator KB-27
Capacitor and supercapacitor energy calculator
Useful for backup power with supercapacitors and for assessing the hazards of a charged capacitor.
Result
In brief: Calculate the energy and charge of a capacitor or supercapacitor, and the runtime of a load during discharge at constant current or power, with a voltage graph. Example: Energy delivered: 9.375 J.
Calculation example
| Capacity | 1 F |
|---|---|
| Initial voltage | 5 V |
| Final voltage | 2.5 V |
| Load | DC |
| Current, A or power, W | 0.1 |
| Energy delivered | 9.375 J |
|---|---|
| Also in mWh | 2.6 mWh |
| Charge | 2.5 C |
| Runtime | 25 s |
| Used from stored energy | 75 % |
How it is calculated
t = C · ΔU / I or t = E / P
Energy scales with the square of the voltage, so discharging to half the voltage uses up three quarters of the stored energy. At constant current, voltage drops linearly; at constant power, it drops faster and faster toward the end.
What to look for
- Large capacitors are dangerous even when switched off. Discharge them through a resistor, not by shorting them with a screwdriver.
- Supercapacitors are rated for about 2.7 V per cell. They need balancing when connected in a pack.
Frequently asked questions
How do you calculate a capacitor's energy?
E = C · U² / 2. A 1000 µF capacitor at 400 V stores 80 J, while a 1 F supercapacitor discharged from 5 to 2.5 V delivers 9.375 J.
How long will a supercapacitor last?
With a constant current, t = C · ΔU / I. A 1 F supercapacitor discharged from 5 to 2.5 V at 0.1 A will last 25 seconds.
Is a charged capacitor dangerous?
Yes. A 400 V mains capacitor can hold its charge long after being disconnected, and 80 J is enough to cause a severe shock and burns. Discharge it through a resistor and check the voltage with a multimeter.
