Energy Stored in a Capacitor Calculator

Example data table

Formula used

The stored energy in an ideal capacitor equals the work required to move charge onto its plates. Depending on which quantities you know, use any equivalent expression below.

• E = ½ C V² (capacitance and voltage known)
• E = ½ Q V (charge and voltage known)
• E = Q² / (2C) (charge and capacitance known)

For a simple parallel-plate model, capacitance is: C = ε₀ εᵣ A / d, where ε₀ is vacuum permittivity, εᵣ is relative permittivity, A is plate area, and d is separation.

How to use this calculator

1. Select an input mode that matches the values you have.
2. Enter numbers and choose units for each visible field.
3. Press Calculate to show energy above the form.
4. Use Download CSV or Download PDF for reports.
5. Switch modes to cross-check results using another expression.

Energy Stored in a Capacitor: Practical Guide

1) What stored energy means

A capacitor stores energy in an electric field between conductors. When voltage rises from 0 to V, the field strengthens and work is done to move charge onto the plates. This calculator reports that work as energy in joules, plus quick conversions to watt-hours and smaller scales.

2) The core energy relationship

For an ideal capacitor, energy depends on capacitance and voltage as E = ½ C V². Because energy scales with V², doubling voltage quadruples stored energy. This is why voltage ratings matter as much as capacitance when comparing designs.

3) Equivalent forms using charge

If you know charge, you can compute energy with E = ½ Q V, or E = Q²/(2C). The calculator derives the missing variable (Q = C V or V = Q/C) so the final energy is consistent across modes.

4) Units and magnitude checks

Capacitors often use microfarads or nanofarads, while charges are commonly microcoulombs. Small values can still produce meaningful energy for pulse loads. Use the displayed mJ, µJ, and nJ conversions to sanity-check whether a result is reasonable for your component size.

5) Geometry option for parallel plates

When capacitance is not directly known, the geometry mode estimates it using C = ε₀ εᵣ A / d. Increasing plate area increases capacitance linearly, while increasing separation reduces capacitance. The dielectric constant εᵣ can multiply capacitance several times compared to air.

6) Real-world limits and safety

Stored energy is not the same as deliverable energy in all circuits. Internal resistance, leakage, and switching losses reduce what reaches the load. Always stay within the voltage rating and consider surge conditions, because energy rises rapidly with voltage.

7) Common applications

Engineers use capacitor energy for camera flashes, hold-up time in power supplies, snubber networks, pulse forming, and defibrillator-style energy estimates at a conceptual level. Comparing multiple capacitors in joules helps you pick a part that matches required pulse energy.

8) Interpreting outputs and exports

The result panel shows capacitance, voltage, charge, and energy in SI units for clarity. Export CSV to archive runs, or generate a PDF summary to share calculations. If you switch input modes and get the same energy, your inputs are internally consistent.

FAQs

1) Which formula should I use?
Use the mode that matches your known values. All three energy expressions are equivalent when inputs are consistent, so switching modes is a good verification step.

2) Why does energy increase so fast with voltage?
Energy follows E = ½ C V², so it grows with the square of voltage. A small increase in voltage can significantly raise stored energy.

3) What if I only know capacitance from geometry?
Choose the geometry mode, enter area, separation, and relative permittivity, then add voltage. The calculator estimates capacitance using the parallel-plate model and computes energy.

4) Can energy be negative?
The stored energy magnitude is non‑negative. If you enter a negative voltage or charge, the calculator still reports positive energy because energy depends on squared or paired terms.

5) How accurate is the geometry model?
It is idealized. Edge fringing, multilayer dielectrics, and real electrode shapes can change capacitance. Use it for estimates or learning, and prefer measured capacitance for precision work.

6) What does watt-hour mean here?
Watt-hour is an energy unit: 1 Wh = 3600 J. The calculator converts joules to Wh to compare capacitor energy with battery-like energy quantities.

7) Does a higher capacitance always mean higher energy?
At the same voltage, yes, because energy is proportional to capacitance. But practical limits include voltage rating, size, internal resistance, and leakage, which affect usable performance.