Find how much energy a capacitor stores from its capacitance and voltage. You can also solve for the voltage you need, or the capacitance you need, if you know the energy.
How to use the Capacitor Energy Calculator
This calculator helps you find one missing value for a capacitor using the formula:
E = 0.5 x C x V^2
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Pick what you want to solve for:
At the top, choose one:
- Solve for energy (find E)
- Solve for voltage (find V)
- Solve for capacitance (find C)
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Fill in the two values you already know:
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Capacitance (C): Type the number, then pick the unit (pF, nF, uF, mF, or F).
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Voltage (V): Type the voltage in volts (V).
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Stored energy (E): Type the energy, then pick the unit (uJ, mJ, J, or kJ).
If you picked “solve for energy,” you do not need to enter energy. If you picked “solve for voltage,” you do not need to enter voltage. If you picked “solve for capacitance,” you do not need to enter capacitance.
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Optional: change the output unit:
Open Advanced options (optional) and choose how you want energy results displayed. “Auto” picks a unit that is easier to read.
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Click “Calculate”:
You will see your result and the inputs the calculator used.
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Copy your result summary (optional):
Click Copy summary to copy a short text summary you can paste somewhere else.
Tips:
- Use positive numbers for capacitance. Do not use 0 for capacitance.
- Voltage and energy should be 0 or higher.
- If you solve for capacitance and you enter V = 0 and E = 0, the calculator will say “Any value” because many capacitances fit that case.
Methodology and sources
This calculator uses the ideal capacitor energy relationship. It converts your inputs into base SI units, applies the relevant rearranged equation, then formats the result in a readable unit.
Core equations
Stored energy in a capacitor:
E = 0.5 x C x V^2
Rearrangements used by the other modes:
- Voltage from energy and capacitance: V = sqrt(2E / C)
- Capacitance from energy and voltage: C = 2E / V^2
The calculator also shows charge when both capacitance and voltage are known:
Q = C x V
Unit handling
- Capacitance is converted to farads (F): pF = 1e-12 F, nF = 1e-9 F, uF = 1e-6 F, mF = 1e-3 F, F = 1 F.
- Energy is converted to joules (J): uJ = 1e-6 J, mJ = 1e-3 J, J = 1 J, kJ = 1e3 J.
- Voltage is treated as volts (V).
Display rules used in the results
- If “Result energy display” is set to Auto, energy is shown as:
- kJ when |E| >= 1000 J
- J when |E| >= 1 J
- mJ when |E| >= 1e-3 J
- uJ otherwise
- Charge (Q) is shown in C, mC, uC, or nC using similar size-based scaling.
Input rules and edge cases
- Capacitance must be greater than 0 when it is an input.
- Voltage is treated as 0 or greater.
- Energy is treated as 0 or greater when it is an input.
- When solving for capacitance:
- If V = 0 and E = 0, any capacitance value fits E = 0. The calculator reports “Any value”.
- If V = 0 and E > 0, there is no real solution. The calculator shows an error.
What this does not model
- Leakage, ESR, dielectric absorption, temperature effects, and capacitor nonlinearity.
- Voltage rating and safety margins. A capacitor can fail if used above its rated voltage even if the energy calculation is correct.