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Energy stored in a capacitor calculator

A-level Electricity

The energy stored in a charged capacitor.

E = ½ C V²

This free calculator is for A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

EEnergy stored to find 0.0360 J

CCapacitance

VPotential difference

Significant figures

Working

Write the equation
E = ½ C V²
Substitute the values
E = ½ × (5 × 10⁻⁴) × 12²
Calculate
E = 0.036 J
Give the answer to 3 significant figures
E = 0.0360 J

Worked example

Find E, the energy stored. Given: capacitance C = 5.00 × 10⁻⁴ F; potential difference V = 12.0 V.

Write the equation
E = ½ C V²
Substitute the values
E = ½ × (5 × 10⁻⁴) × 12²
Calculate
E = 0.036 J
Give the answer to 3 significant figures
E = 0.0360 J

Symbols and units

Symbol	Quantity	SI unit
E	Energy stored	J
C	Capacitance	F
V	Potential difference	V

Energy stored in a capacitor: rearranged

To find energy stored: E = ½ C V²
To find capacitance: C = 2E / V²
To find potential difference: V = √(2E / C)

Exam tip

Equivalent forms are ½QV and Q²/2C. Use whichever matches the data you are given.

Common questions

What is the formula for energy stored in a capacitor?

The equation is E = ½ C V², where E is the energy stored (J); C is the capacitance (F); V is the potential difference (V).

What units should I use for energy stored in a capacitor?

In SI units: energy stored in J, capacitance in F, potential difference in V. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange E = ½ C V²?

Rearranging gives these forms: to find energy stored: E = ½ C V². to find capacitance: C = 2E / V². to find potential difference: V = √(2E / C).

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Read the full explanation on the Energy stored in a capacitor formula page. More electricity equations: Charge, current and time, Ohm’s law, Electrical power, Electrical power, Energy transferred, Resistors in series, Resistors in parallel, Resistivity. See the full list on the a-level physics equation calculator page.