Capacitor Charge & Energy Calculator
Compute the stored charge (Q = CV) and energy (E = ½ CV²) of a charged capacitor, with intuitive real-world comparisons.
Interactive tool
Presets
Inputs
Engineering notation: 1000u = 1000 µF, 2.2n = 2.2 nF.
Result
What is the Capacitor Charge & Energy Calculator?
A charged capacitor stores energy in its electric field. Two numbers describe this: the charge Q = C × V (in coulombs) and the energy E = ½ C V² (in joules). This tool computes both for any capacitance and voltage, and translates the joule value into intuitive comparisons, because "0.5 J" rarely tells you whether the cap can light an LED for a second or for an hour.
How to Use the Calculator
- 1Enter the capacitance C in farads (use engineering notation: 100n, 10u, 1000u, 1)
- 2Enter the terminal voltage V in volts
- 3Read off the stored charge Q, the stored energy E, and the intuitive comparisons
- 4Use the presets to see typical ceramic, electrolytic, and supercapacitor values at a glance
Key features
Charge and energy
Q in coulombs (and equivalent Ah/mAh), E in joules (and equivalent Wh/mWh)
Intuitive comparisons
"Enough to power a 1/4 W LED for X seconds": puts joules in real-world terms
Engineering notation
Inputs accept SI prefixes; outputs auto-scale (µC, mC, J, mJ, Wh)
Useful presets
Ceramic decoupling caps, electrolytics, PSU bulk caps, and supercaps
Live evaluation
No submit button; results update as you type
100% private
All math runs in your browser; no data is uploaded
Why a Capacitor Energy Calculator?
Capacitor energy density is easy to under- or over-estimate. A 100 µF / 50 V electrolytic stores 0.125 J: enough to draw a respectable spark but not enough to hold up a microcontroller through a power blip. A 100 F supercapacitor at 2.7 V holds 364 J: enough to back up RAM for minutes. Translating the formula E = ½ CV² into something you can act on is what this tool does.
Common use cases
- Size a bulk power-supply capacitor for the hold-up time you need
- Estimate the spark hazard of a charged capacitor in industrial equipment
- Compare supercaps and small Li-Po cells for energy-harvesting designs
- Calculate the discharge energy when fault current flows through a defibrillator cap
- Teach E = ½ CV² to students with hands-on real values
- Plan a flash capacitor for a camera or strobe
Formulas
- Q = C × V (coulombs)
- E = ½ × C × V² (joules)
- Energy scales with the square of voltage: doubling V quadruples E
- 1 J = 0.278 mWh; 1 Wh = 3600 J
Energy density by type
- Ceramic (X7R, 100 nF / 50 V): ~125 µJ
- Electrolytic (1000 µF / 25 V): ~0.31 J
- Tantalum (10 µF / 16 V): ~1.3 mJ
- Supercapacitor (1 F / 2.7 V): ~3.6 J per farad-volt-squared
- Li-Po cell (1000 mAh / 3.7 V): ~13.3 kJ, about four orders of magnitude denser than a supercap
Tips & best practices
Voltage rating matters
Always charge to less than the rated voltage. Most aluminium electrolytics are happy at 80% of rated; running closer shortens life significantly.
Polarized caps care about polarity
Reverse-biasing an electrolytic releases the stored energy as a small explosion. Mind the polarity stripe.
Stored vs delivered
A real cap can’t deliver all of E at full voltage: voltage falls as it discharges. Useful energy down to a brown-out threshold is less than E.
Privacy & security
All math runs in your browser; no values leave your device.
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Frequently Asked Questions
What is the formula for capacitor energy?
E = ½ × C × V², where C is capacitance in farads and V is voltage in volts. The result is in joules. Stored charge is Q = C × V in coulombs.
Why does doubling the voltage quadruple the energy?
Because energy scales with V squared. Going from 5 V to 10 V on the same capacitor stores 4× the energy. This is why high-voltage caps (PSU bulk, defibrillators, photoflashes) hold so much: it’s the voltage doing the work, not the capacitance.
How does a supercapacitor compare to a Li-Po battery?
Energy density differs by roughly 1000×1 to 10000×. A 100 F / 2.7 V supercap holds about 365 J. A 1000 mAh / 3.7 V Li-Po holds about 13,300 J. Supercaps win on cycle life and peak power; batteries win by orders of magnitude on energy.
Can I get all the stored energy out?
No. As you draw current, the capacitor voltage drops. The useful energy is the difference between the starting energy and the energy at the brown-out threshold of your load. Going from 5 V to 3 V on a 1 F cap delivers only 8 J of the original 12.5 J.
Is high-energy capacitor discharge dangerous?
Yes: above a few joules, an instantaneous discharge can cause burns, vaporise metal, or trigger arc-flash. Always bleed high-voltage caps through a resistor before handling, and respect the energy rating of your equipment.
Is anything sent to a server?
No. All computation happens in your browser. Nothing is uploaded.