Part of Power and energy calculators
Capacitor Energy Calculator
Calculate stored energy and charge from capacitance and voltage.
Inputs
Select which capacitor value to solve, then enter the other two values and their tolerances.
Results
Stored energy rises with the square of voltage. Large capacitors and high-voltage banks can retain hazardous energy after power is removed.
Use capacitor energy checks for hold-up, discharge, and stored-energy risk
Capacitors can store enough energy to affect brownout behaviour, load transients, inrush, discharge timing, and safety. Use this calculator to solve stored energy, capacitance, or voltage, and to see how tolerance changes the result.
Hold-up estimates
Estimate how much stored energy is available before checking the real load profile and voltage limits.
Discharge awareness
Quantify stored energy before designing bleeders, discharge switches, or service procedures.
Tolerance range
Check minimum and maximum energy from capacitance and voltage tolerances.
Equations and model
The calculator uses the ideal stored-energy relationship and applies tolerance ranges to the selected values. Treat the capacitance value as effective capacitance under operating conditions.
Stored energy
Energy stored in an ideal capacitor at voltage V.
Stored charge
Charge stored by the capacitor at the specified voltage.
Voltage sensitivity
Energy changes with the square of voltage, so voltage tolerance has a large effect.
E - Stored energy
Unit: joules (J)
Energy available in the charged capacitor before losses and discharge-path limits.
C - Capacitance
Unit: farads (F)
The effective capacitance under the actual voltage, temperature, tolerance, and ageing conditions.
V - Capacitor voltage
Unit: volts (V)
Voltage across the capacitor. Energy rises with the square of this value.
Q - Stored charge
Unit: coulombs (C)
Charge stored in the capacitor at the selected voltage.
Worked example
This example is covered by the stored-energy test suite, including tolerance corners.
Design question: A 1000 µF capacitor is charged to 12 V. How much energy and charge are stored?
Inputs: C = 1000 µF, V = 12 V.
Energy: E = 1/2 × 1000 µF × 12² = 0.072 J.
Charge: Q = 1000 µF × 12 V = 0.012 C.
Tolerance case: with ±20% capacitance and ±5% voltage, the minimum and maximum stored energy move significantly because voltage is squared.
Nominal capacitance versus effective capacitance
For energy and hold-up calculations, the number printed on the capacitor may not be the capacitance the circuit actually gets.
Capacitance can fall
- MLCC capacitance can reduce under DC bias.
- Temperature and ageing can move the real value.
- Wide tolerance parts may have much less energy at worst case.
Other limits can dominate
- ESR and ripple current can limit usable performance.
- Leakage current affects long hold-up and discharge timing.
- Voltage rating and derating must be checked from the datasheet.
Assumptions and limitations
Ideal energy equation
The calculation does not model ESR, ESL, leakage, dielectric absorption, ripple heating, or converter efficiency.
Not a safety certification
Hazardous-energy thresholds, touch safety, and service discharge requirements need standards-based review.
Discharge path is separate
Use an RC discharge calculation to size bleed resistors and verify discharge time, resistor power, and voltage rating.
Related calculators and next checks
Follow the next check based on whether the capacitor is used for stored energy, timing, coupling, or unit conversion.
RC time constant calculator
Use to check capacitor charge, discharge, bleed, and hold-up timing.
Engineering conversion calculator
Convert µF, mF, joules, millijoules, and SI-prefixed values.
AC coupling capacitor calculator
Use when the capacitor is intended for coupling rather than energy storage.
Power and energy hub
Follow related stored-energy and power workflows.
Analogue and filter hub
Follow RC timing, coupling, and filter workflows.
FAQ
Why does voltage matter so much?
Capacitor energy is proportional to voltage squared. Doubling the voltage gives four times the stored energy for the same capacitance.
Is the labelled capacitance always the effective capacitance?
No. Ceramic capacitors can lose capacitance with DC bias, temperature, ageing, and package choice. Electrolytics and film capacitors also have tolerance, leakage, ESR, and ripple-current limits.
Does this calculator design the discharge path?
No. It calculates stored energy and charge. Discharge resistor value, power, voltage rating, safe touch time, and failure modes need a separate RC and safety review.
Engineering reference
Equations, assumptions, and design guidance
Solves capacitor stored energy, capacitance, or voltage and reports tolerance-derived minimum and maximum values.
Equations and variables
E = 0.5 * C * V^2Q = C * V- E
- Stored energy (J)
- C
- Capacitance (F)
- V
- Capacitor voltage (V)
Assumptions and limitations
Assumptions
- Capacitance is linear at the entered voltage.
- Voltage is non-negative and DC.
Limitations
- Voltage derating, DC bias capacitance loss, ESR, ripple current, leakage, and discharge path safety are not modelled.
Worked example and design use
1000 uF at 12 V
Inputs: C = 1000 uF, V = 12 V
Outputs: E = 72 mJ, Q = 12 mC
Design guidance
- Stored energy can remain after power is removed; provide a safe discharge path where needed.
- Use real capacitance under DC bias for ceramic capacitors.