Capacitor charge and energy calculator
Charge, voltage or capacitance of a capacitor, plus the stored energy.
Fill in the fields and the result will appear here automatically.
Solve for charge, voltage or positive capacitance of one ideal linear capacitor. Q is the signed charge on the chosen plate; U is that plate's potential relative to the other plate. Capacitance in µF and charge in µC are consistent because µF·V = µC. Component markings normally give capacitance and voltage rating, rather than the current stored charge.
How it works
Formula and logic
Q = C·U; U = Q/C; C = Q/U. These micro-units need no charge conversion factor. Energy E = C·U²/(2·10⁶) J converts µF into F. Reversing U reverses Q but leaves energy nonnegative. At U = 0 with known C, Q = E = 0; the pair Q = U = 0 cannot determine C.
Example
100 µF at 12 V gives Q = 1 200 µC and E = 0.0072 J. At −12 V, charge is −1 200 µC and energy is unchanged. At 24 V, charge is 2 400 µC and energy is 0.0288 J, four times larger.
Fields and units
- What to find — list option
- Capacitance — µF
- Voltage — V
- Charge — µC
How to use
- — Choose the unknown and fill the two visible known fields; read the answer in the result.
- — Use C in µF, U in V and Q in µC, with positive C.
- — Keep the same plate and voltage orientation. Solving C requires nonzero U and Q with matching signs.
- — Energy is in J. Check voltage rating and discharge procedures separately in the component documentation.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- OpenStax: charge, potential difference and the farad OpenStax: field energy E = CU²/2
- Limitation
- Ideal linear capacitor. The calculation does not establish touch safety, discharge time or permissible operating conditions.
FAQ
How do farads differ from amp-hours?
A farad is charge per voltage: F = C/V. An amp-hour is charge, with 1 Ah = 3 600 C. Charge can be compared at a given voltage, but capacitance in F cannot be equated directly to Ah.
Why does doubling voltage quadruple energy?
Q depends linearly on U, whereas E = C·U²/2 is quadratic. This assumes constant capacitance; real voltage and temperature dependence are absent.
Can charge and voltage be negative?
Yes, with a consistent plate and voltage orientation. Positive C requires matching Q and U signs. The ideal plates carry opposite charges; their combined charge is not Q.
Does this check voltage ratings or connections?
No: this is one ideal capacitor. Voltage rating, polarity, ESR and safe discharge need actual component data. Capacitor groups have a separate tool.