Example 1
C = 1e-6 F, U = 12 V -> E = 7.2e-5 J.
Capacitor energy calculator
Type C [F] and U [V]. The calculator computes E = ½ C U²: 1e-6 F at 12 V is 72 µJ, and 1e-4 F at 5 V is 1.25 mJ. Twice U is four times E.
Short alias: capacitor stored energy. Capacitance: C = Q/U.
Result
Capacitance C (F) and Voltage U (V). The result shows up here.
How it works
Energy in a capacitor is E = ½ C U². At 1 µF (1e-6 F) and 12 V: E = 0.5 × 1e-6 × 144 = 7.2e-5 J = 72 µJ. At 100 µF and 5 V: E = 0.5 × 1e-4 × 25 = 1.25 mJ. U is squared: 24 V at 1 µF is 288 µJ, four times 12 V.
The form has two fields: C in farads and U in volts. The result is in joules. Type 1 µF as 1e-6, not as 1. A comma, a period, and 1e-6 mean the same C. 0.000001 also parses.
Typed 0 in C or U gives E = 0: no capacitance or no voltage. Both fields need a number before 72 µJ appears. A negative U is squared and gives the same E as |U|.
C = Q/U is on the neighbouring page. Here energy, there capacitance from charge and voltage. The short card of the same formula is stored energy: fewer examples, same result.
The joules are the same unit as in Ek = ½ m v². Different formula: C and U here, mass and speed there. Q = I t says where plate charge came from, not E.
Type 1e-6 and 12, click Calculate, and check 72 µJ. Then 24 V at the same C and see 288 µJ. The calculator computes ½ C U²; it does not charge the plates.
How to use
- In the first field enter C in farads, for example 1e-6. 1 µF is 1e-6, not 1.
- In the second field enter U in volts, for example 12.
- Click Calculate. The calculator computes ½ C U². 1e-6 and 12 give 72 µJ. 1e-4 and 5 give 1.25 mJ.
- Typed 0 gives E = 0. Both fields need a number. Twice U is four times E.
- For C = Q/U, open capacitance. The short page of this formula is stored energy.
Formula
E = ½ · C · U2
Units: J, F, V.
E, C, and U on the full energy card
The full page multiplies E = ½·C·U². 1e-6 F at 12 V is 72 µJ. 1e-4 F at 5 V is 1.25 mJ. Twice U is four times E.
- E
- Stored energy [J]. ½ × 1e-6 × 144 = 72e-6 J, which is 72 µJ.
- C
- Capacitance [F]. 1 µF is 1e-6. It enters linearly, not as Q/U in this calculator.
- U
- Voltage [V] on the capacitor. Squared: 12 V and 24 V give an energy ratio 1:4.
Real-life examples
Example 2
C = 1e-4 F, U = 5 V -> E = 1.25e-3 J.
Example 3
Small C, high U: E ~ 12.4 J.
Example 4
Typical filter capacitor.
Example 5
C = 2.2e-5 F, U = 16 V.
Example 6
High voltage, small C.
Example 7
9 V battery, E = 2.03e-3 J.
Example 8
Buffer capacitor.
Ways to use this calculator
- You compute 72 µJ from 1 µF at 12 V.
- You compare 12 V and 24 V at the same C.
Frequently asked questions
How much E at 1 µF and 12 V?
Energy is 72 µJ. You compute ½ × 1e-6 × 144, which is 7.2×10⁻⁵ J, then write it in microjoules.
How much at 100 µF and 5 V?
You get 1.25 mJ. Half times 1e-4 times 25 is 1.25×10⁻³ J, the millijoule from the second example.
Which units do I type?
Capacitance C [F], voltage U [V]. Result E [J]. Type 1 µF as 1e-6, not as 1.
Why is twice U four times E?
U is squared. 24 V at 1 µF is 288 µJ, four times 72 µJ, not twice.
How is this different from C = Q/U?
Here you compute energy from capacitance and voltage. There you solve for C from charge and voltage. Different unknown, same capacitor.
Does a comma in 0.000001 work?
Yes. 0.000001, 0,000001 and 1e-6 are the same C [F]. 1e-6 is easier.
Where is the short page?
On capacitor stored energy. Same ½ C U² formula, shorter card, fewer examples.
Is ½ C U² the same J as Ek?
Yes, result E [J] is the same joule as in Ek = ½ m v². Different formula: here C and U, there mass and speed.
What if C or U is 0?
Energy is 0 J. No capacitance or no voltage means an empty capacitor. Both fields still need a number.
How much at 1 µF and 24 V?
You get 288 µJ. That is four times the 72 µJ from twelve volts, because U is squared.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.