Coulombs to Joules Calculator
Convert electrical charge (Coulombs) to energy (Joules) using voltage
Calculate energy stored in a capacitor using E = ½CV²
Convert battery capacity (Ah) to energy (Joules/Wh)
| Charge (Q) | Voltage (V) | Energy (E) | Application |
|---|---|---|---|
| 1 C | 1 V | 1 J | Definition of Joule |
| 1 C | 5 V | 5 J | USB charging |
| 1 C | 12 V | 12 J | Car battery |
| 1 mC (0.001 C) | 300 V | 0.3 J | Camera flash |
| 1 μC (10⁻⁶ C) | 5 V | 5 μJ | Microcontroller circuits |
| 3600 C (1 Ah) | 3.7 V | 13,320 J (3.7 Wh) | Li-ion cell |
How to Convert Coulombs to Joules
Converting Coulombs to Joules requires knowing the voltage. A Coulomb measures electric charge, while a Joule measures energy. The relationship between them is defined by voltage—the energy per unit charge.
This formula comes from the definition of voltage: the electric potential difference is the work done per unit charge. When charge moves through a potential difference, energy is transferred. This is fundamental to understanding how capacitors store energy and how batteries deliver power.
Understanding the Units
Coulomb (C): The SI unit of electric charge. One Coulomb equals the charge of approximately 6.242 × 1018 electrons. It’s defined as the charge transported by one Ampere in one second.
Joule (J): The SI unit of energy. One Joule equals one Watt-second, or the energy transferred when one Coulomb moves through one Volt of potential difference.
Volt (V): The SI unit of electric potential. One Volt equals one Joule per Coulomb (1 V = 1 J/C), which is why voltage is the bridge between charge and energy.
Step-by-Step Calculation Examples
Example 1: USB Device Charging
A USB charger delivers 2 Coulombs of charge at 5 Volts. How much energy is transferred?
Given: Q = 2 C, V = 5 V
Formula: E = Q × V
E = 2 C × 5 V
E = 10 J
Result: 10 Joules (or 10 Watt-seconds)
Example 2: Car Battery
A 12V car battery discharges 5000 Coulombs while starting the engine. Calculate the energy used.
Given: Q = 5000 C, V = 12 V
Formula: E = Q × V
E = 5000 × 12
E = 60,000 J = 60 kJ
Result: 60 kiloJoules (≈ 16.67 Wh)
Example 3: Capacitor Discharge
A camera flash capacitor stores 0.5 milliCoulombs (mC) of charge at 300V. What energy does it release?
Given: Q = 0.5 mC = 0.0005 C, V = 300 V
Formula: E = Q × V
E = 0.0005 × 300
E = 0.15 J = 150 mJ
Result: 150 milliJoules
Coulombs to Joules Conversion Table
| Charge (C) | Voltage (V) | Energy (J) | Application |
|---|---|---|---|
| 0.001 C (1 mC) | 5 V | 0.005 J (5 mJ) | USB electronics |
| 0.01 C (10 mC) | 12 V | 0.12 J | Automotive sensors |
| 1 C | 3.7 V | 3.7 J | Li-ion battery cell |
| 1 C | 12 V | 12 J | Car battery |
| 10 C | 230 V | 2,300 J (2.3 kJ) | Household circuit |
| 100 C | 48 V | 4,800 J (4.8 kJ) | Solar power system |
| 1000 C | 400 V | 400,000 J (400 kJ) | Electric vehicle |
| 15 C | 300,000,000 V | 4.5 × 10⁹ J (4.5 GJ) | Lightning bolt (typical) |
The table shows how the same amount of charge produces vastly different energy at different voltages. This is why high-voltage systems are dangerous—even a small charge can deliver lethal energy. For related calculations, see our Volts to Joules Converter.
Frequently Asked Questions
No, you cannot. Coulombs measure electric charge, while Joules measure energy. They are different physical quantities. Voltage provides the conversion factor because it represents energy per unit charge (1 Volt = 1 Joule/Coulomb). Without knowing the voltage, there’s no way to determine how much energy the charge represents.
The relationship is defined by the equation E = Q × V:
- 1 Joule = 1 Coulomb × 1 Volt
- Voltage is defined as energy per charge (V = E/Q)
- Charge times voltage equals energy (J = C × V)
This is a fundamental definition in physics, not a derived formula.
The energy depends entirely on voltage:
- 1 Coulomb at 1V = 1 Joule
- 1 Coulomb at 5V = 5 Joules
- 1 Coulomb at 12V = 12 Joules
- 1 Coulomb at 120V = 120 Joules
- 1 Coulomb at 1000V = 1000 Joules (1 kJ)
First convert Coulombs to Joules (E = Q × V), then convert Joules to Watt-hours:
- 1 Watt-hour = 3600 Joules
- Wh = (Q × V) / 3600
- Example: 1000C at 12V = 12,000 J = 3.33 Wh
Charge and energy are fundamentally different:
- Charge (Coulombs): A property of matter that causes electric force. It’s conserved and cannot be created or destroyed.
- Energy (Joules): The ability to do work. It can be transferred between systems and converted between forms.
- Voltage connects them by defining how much energy each unit of charge carries.
