---
title: "Capacitance and the Energy Stored in a Capacitor"
description: "Capacitance is the charge a component holds per volt across it. Because the charge builds as the voltage rises, the stored energy is half of QV, the area under the charge-voltage line."
canonical: https://lightmysky.com/learn/science/capacitance-and-the-energy-stored-in-a-capacitor-mt_j1Eltr9z6l
source: https://lightmysky.com/learn/science/capacitance-and-the-energy-stored-in-a-capacitor-mt_j1Eltr9z6l.md
retrieved: 2026-09-12
---

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# Capacitance and the Energy Stored in a Capacitor

Capacitance is the charge a component holds per volt across it. Because the charge builds as the voltage rises, the stored energy is half of QV, the area under the charge-voltage line.

Subject: Science · Area: Energy · Ages 16 to 18
Page: https://lightmysky.com/learn/science/capacitance-and-the-energy-stored-in-a-capacitor-mt_j1Eltr9z6l

## Ready when they can

- Uses C = Q over V and works comfortably in microfarads and nanofarads
- Finds the stored energy from the area under a charge-voltage graph and from half CV squared
- Explains why the energy is not QV, arguing from the graph rather than from the formula

## Lesson: Capacitors hold charge, then pay it back

Capacitance is the charge a part holds per volt: C equals Q over V, in farads. Real labels use microfarads and nanofarads, meaning ten to the minus six and ten to the minus nine.

**Example.** A defibrillator stores hundreds of joules in a capacitor, then delivers the charge at once. The same idea keeps a calculator's memory alive and fires a camera flash.

Stored energy is half of Q V, which also reads half C V squared or Q squared over 2 C. It is not Q V, because the first charge arrives at zero volts and the last at full volts, so the average is V over 2.

**Tip.** Read the energy as the area of a triangle under the charge against voltage line, not a rectangle. Doubling the voltage quadruples the stored energy, since V is squared.

**Recap.** Capacitors hold charge per volt, and filling them banks half of Q V.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Electric Potential and the Uniform Field Between Plates](https://lightmysky.com/learn/science/electric-potential-and-the-uniform-field-between-plates-mt_LKTfeYxNde)

## Opens up

- [Discharging a Capacitor and the Time Constant](https://lightmysky.com/learn/science/discharging-a-capacitor-and-the-time-constant-mt_5sjDPudyad)
- [Dielectrics and the Polarisation of Matter](https://lightmysky.com/learn/science/dielectrics-and-the-polarisation-of-matter-mt_OYRr_k3IW6)