Solve capacitor stored energy, required capacitance, or required voltage with unit conversions, charge context, and common-voltage reference rows.
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Stored energy rises with voltage squared A capacitor’s stored energy scales linearly with capacitance and with the square of the
charging voltage, which is why a modest voltage change can move stored joules quickly.
Capacitor energy calculator: solve stored joules, required capacitance
A capacitor energy calculator is most useful when it does more than print one joule value. This version can solve stored energy directly from capacitance and voltage, back-solve the capacitance required for an energy target, or find the charging voltage needed to reach a target energy. It also keeps the supporting circuit context visible by reporting charge and showing how the same capacitor’s stored energy changes at common voltages.
What this capacitor energy calculator is measuring
A capacitor stores energy in an electric field. The stored energy depends on two things: the capacitance and the applied voltage. More capacitance means more charge can be stored per volt, while more voltage increases stored energy much faster because voltage enters the formula as a squared term.
That squared-voltage behaviour is why capacitor energy can change so quickly as charging voltage rises. Doubling capacitance doubles stored energy, but doubling voltage multiplies stored energy by four. A useful capacitor energy calculator therefore needs to keep both relationships visible instead of treating the result as a black-box number.
E = 1/2 × C × V²
Stored energy equals one half times capacitance times the square of the applied voltage.
Q = C × V
Stored charge gives supporting context for the same operating point.
Why the three solve modes matter
Energy mode is the direct bench-style calculation: enter capacitance and voltage and the calculator reports stored energy in joules, millijoules, and microjoules. This is the mode you use when the component value and charging voltage are already known.
Capacitance mode is useful when the design requirement is stated as an energy target at a known voltage. Rearranging the same formula gives the capacitance required to reach that target. Voltage mode does the opposite: it tells you how much voltage would be needed to store a stated amount of energy in a known capacitor.
That makes the page useful for more than classroom algebra. It supports first-pass component sizing, quick checks on low-energy storage stages, and intuition about how aggressively energy rises when voltage increases.
Worked example: 100 uF charged to 12 V
Suppose a 100 uF capacitor is charged to 12 V. Converting 100 uF into farads gives 100 × 10^-6 F, or 0.0001 F. Plugging that into the energy formula gives 0.0072 J because 1/2 × 0.0001 × 12² equals 0.0072. That same result can also be written as 7.2 mJ or 7,200 uJ depending on the scale that is most useful for the circuit discussion.
The same operating point stores 0.0012 C of charge because Q = C × V = 0.0001 × 12. If you kept the same capacitor but charged it to 24 V instead, the stored energy would jump to 0.0288 J, which is four times higher than the 12 V case. That is the voltage-squared effect in practical form.
Worked examples like this are why a capacitor energy worksheet should show more than a headline value. The charge, converted units, and comparison voltages all help you interpret whether the number is trivial, useful, or potentially hazardous in context.
This page uses the ideal capacitor energy relationship. It does not model ESR, leakage, dielectric absorption, discharge-path resistance, temperature effects, or the pulse-current limits that often matter in real designs. It also does not tell you whether the capacitor is rated safely for the voltage you are considering.
Use the result as a planning and educational reference. If the stored energy is part of a safety-critical circuit, a flash discharge, a snubber network, or a high-voltage design, you still need to confirm component ratings, fault behaviour, discharge time, and physical layout against the real design requirements.
Further reading
NIST — SI units — NIST reference for SI base and derived units, including the joule and farad used in capacitor-energy calculations.
BIPM — SI Brochure (9th edition) — Primary SI reference for unit definitions and notation relevant to joules, coulombs, volts, and farads.
Wikipedia — Capacitor — General reference article covering capacitor behaviour, charge, voltage, and stored energy context.
Frequently asked questions
Why does doubling voltage increase capacitor energy by four times?
Because voltage is squared in the formula E = 1/2 × C × V². If capacitance stays the same and voltage doubles, V² becomes four times larger, so stored energy becomes four times larger as well.
Is capacitor energy the same as stored charge?
No. Stored charge is measured in coulombs and follows Q = C × V. Stored energy is measured in joules and follows E = 1/2 × C × V². They are related, but they are not the same quantity.
Can I use this to size a capacitor for a pulse or backup-energy job?
Yes as a first-pass estimate. The calculator helps you see the basic capacitance, voltage, and energy relationship, but real sizing still depends on ESR, voltage rating, discharge time, allowable sag, temperature, and the exact load profile.
Does a larger capacitor always mean a dangerous amount of energy?
Not automatically. Risk depends on both capacitance and voltage, and on how that energy can be released. A very large capacitor at a low voltage may store less energy than a smaller capacitor charged to a much higher voltage. Always look at the actual joule value and the real discharge path.
