About the Capacitor Energy Calculator
The Capacitor Energy and Charge calculator finds how much energy is stored in a capacitor and how much electric charge it holds at a given voltage. Electronics hobbyists, students, and engineers use it when sizing capacitors for power smoothing, energy storage, or timing circuits, since both energy and charge scale directly with the capacitor's rated capacitance and the voltage applied to it.
How It Works
Enter the capacitance in microfarads and the voltage across the capacitor. The calculator converts capacitance to farads, then computes stored energy from the standard capacitor energy formula and stored charge from the basic charge-voltage relationship, presenting both results in the more practical millijoule and millicoulomb units for typical small capacitors.
Formula & Methodology
To calculate by hand, first convert the capacitance from microfarads to farads by dividing by one million. Square the voltage, multiply by the capacitance, and take half of that product to get the energy in joules; multiply farads by volts directly, with no squaring, to get the charge in coulombs. Multiplying each result by 1000 converts joules to millijoules and coulombs to millicoulombs for more readable numbers.
Examples
A Standard Filter Capacitor
A 1000 microfarad capacitor charged to 12 volts stores E = 0.5 x 0.001 x 12^2 = 0.072 joules (72 mJ) and holds Q = 0.001 x 12 = 0.012 coulombs (12 mC) of charge.
A Smaller Electrolytic Capacitor at Higher Voltage
A 470 microfarad capacitor charged to 25 volts stores E = 0.5 x 0.00047 x 25^2 = 0.146875 joules, about 146.9 mJ, and holds Q = 0.00047 x 25 = 0.01175 coulombs (11.75 mC), noticeably more energy than the first example despite the smaller capacitance, because the higher voltage matters more due to the squared term.
Advantages
- Calculates both energy and charge together from the same two inputs, saving a separate lookup or formula for each quantity.
- Reports results in millijoules and millicoulombs, the practical scale for the small capacitors used in most everyday electronics.
- Makes the nonlinear relationship between voltage and stored energy visible by comparing results across different voltage inputs.
Common Mistakes
- Forgetting to convert microfarads to farads when doing the calculation by hand, since the formula requires capacitance in base SI units.
- Assuming stored energy scales the same way as stored charge with voltage, when in fact energy scales with the square of voltage and charge scales linearly.
- Ignoring a capacitor's rated working voltage, which this calculator does not check, and which if exceeded can damage or rupture the component regardless of what the theoretical energy calculation shows.
Edge Cases to Watch For
- Because energy depends on voltage squared while charge depends on voltage only linearly, doubling the voltage on the same capacitor quadruples the stored energy but only doubles the stored charge, a distinction that matters when comparing capacitors for energy storage versus charge delivery.
- The calculator assumes an ideal capacitor with no leakage current and no equivalent series resistance, so it reports the theoretical maximum energy and charge; real capacitors, especially electrolytics, lose some charge over time and have practical voltage limits that must not be exceeded.
- Entering a voltage above the capacitor's rated working voltage isn't flagged by the calculator itself, since it has no rated-voltage input, so that check has to be done separately against the component's datasheet.
Common Use Cases
- Electronics hobbyists sizing capacitors for power supply filtering or energy storage in a circuit.
- Students verifying capacitor energy and charge formulas from a physics or electronics course.
- Engineers doing a quick sanity check on how much energy a capacitor bank could deliver in a pulsed-power or flash application.