Capacitor Energy Calculator
The energy and charge stored in a capacitor at a given voltage — with the discharge time constant into a load.
How much energy a capacitor holds.
How the capacitor energy calculator works
A charged capacitor stores ½CV². The square on voltage means doubling the voltage quadruples the energy, which is why capacitor banks run as high a voltage as their dielectric allows.
Charge is simply Q = CV, and discharging through a resistance R falls exponentially with time constant τ = RC — 63% gone after one τ, 99% after five.
Formula: E = ½CV²; Q = CV; τ = RC
Worked examples
| Inputs | Stored energy | Note |
|---|---|---|
| 1,000 µF at 12 V | 0.072 J | 72 mJ |
| Twice the voltage | 0.288 J | 288 mJ — four times |
| A camera flash capacitor | 10.89 J | 10.9 J |
FAQFrequently asked questions
How much energy does a capacitor store?
Half the capacitance times the voltage squared. A 1,000 µF at 12 V holds 72 mJ — tiny next to a battery.
Why is voltage more important than capacitance?
Because energy scales with voltage squared and only linearly with capacitance. Doubling the voltage is worth four times as much as doubling the capacitance.
What is the time constant?
RC — the time to fall to 37% of the starting voltage. After five time constants less than 1% remains.
Are capacitors dangerous?
Large ones, yes. A camera flash capacitor at 330 V holds around 11 J and can deliver a serious shock long after the device is unplugged.
Why not use capacitors instead of batteries?
Energy density. A capacitor holds perhaps a hundredth of what a lithium cell of the same mass does — but delivers it far faster, which is what they are for.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.