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Convert voltage in volts (V) to energy in joules (J) given the charge in coulombs. Calculate total energy from voltage and charge.
Charge = Current (A) × Time (s)
Where J is energy in joules, V is voltage in volts, and C is charge in coulombs.
A 12V battery discharges 100 coulombs:
J = 12 × 100 = 1,200 joules
| Volts | 0.1 C | 0.5 C | 1 C | 5 C | 10 C |
|---|---|---|---|---|---|
| 1 V | 0.1 J | 0.5 J | 1 J | 5 J | 10 J |
| 5 V | 0.5 J | 2.5 J | 5 J | 25 J | 50 J |
| 12 V | 1.2 J | 6 J | 12 J | 60 J | 120 J |
| 24 V | 2.4 J | 12 J | 24 J | 120 J | 240 J |
| 120 V | 12 J | 60 J | 120 J | 600 J | 1200 J |
| 240 V | 24 J | 120 J | 240 J | 1200 J | 2400 J |
| 480 V | 48 J | 240 J | 480 J | 2400 J | 4800 J |
Volts to joules conversion calculates electrical energy (in joules) from voltage (in volts) and charge (in coulombs). A joule is the SI unit of energy, defined as the work done when one coulomb of electric charge passes through a potential difference of one volt. The formula J = V × C is fundamental to electrical physics and has wide practical applications in battery energy calculations, capacitor design, and electromagnetic energy analysis. For batteries, charge is often expressed in amp-hours (Ah), where 1 Ah = 3600 coulombs. For capacitors, the energy formula differs because voltage changes during charging: E = ½ × C × V², where C is capacitance in farads. Understanding this conversion is essential in fields ranging from power electronics and renewable energy storage to defibrillator design and ESD (electrostatic discharge) protection per IEC 61000-4-2 standards.
Determine the voltage of your source. Common values include 1.5V (AA battery), 3.7V (lithium cell), 12V (car battery), 120V (US mains), and 240V (industrial or international mains).
If the charge is in amp-hours (Ah), convert to coulombs by multiplying by 3600. For example, a 100 Ah battery holds 360,000 coulombs. If given milliamp-hours (mAh), divide by 1000 first, then multiply by 3600.
Apply the formula: Joules = Volts × Coulombs. For a 12V battery with 360,000 C: Energy = 12 × 360,000 = 4,320,000 joules (4.32 MJ). For capacitors, use E = ½ × C × V² instead.
For large values, convert to kilojoules (÷ 1000), watt-hours (÷ 3600), or kilowatt-hours (÷ 3,600,000). A 12V, 100 Ah battery holds 4,320,000 J = 4,320 kJ = 1,200 Wh = 1.2 kWh.
Comparing batteries with different voltages requires converting to joules (or watt-hours) for a true energy comparison. A 3.7V, 5000 mAh phone battery stores 66,600 J, while a 12V, 100 Ah car battery stores 4,320,000 J — 65 times more energy.
Capacitor energy calculations (E = ½CV²) are critical in power electronics, flash photography, and defibrillator design. A 1000 µF capacitor charged to 400V stores 80 joules — enough to deliver a significant shock.
Electrostatic discharge (ESD) energy in joules determines component damage risk. A human body at 4000V with ~100 pF capacitance can discharge about 0.8 millijoules — enough to destroy sensitive ICs. IEC 61000-4-2 defines test levels in joules.
| Battery Type | Voltage | Capacity | Energy (Joules) | Energy (Wh) |
|---|---|---|---|---|
| AA Alkaline | 1.5 V | 2,500 mAh | 13,500 J | 3.75 Wh |
| 9V Battery | 9 V | 565 mAh | 18,306 J | 5.1 Wh |
| 18650 Li-ion Cell | 3.7 V | 3,000 mAh | 39,960 J | 11.1 Wh |
| Phone Battery | 3.7 V | 5,000 mAh | 66,600 J | 18.5 Wh |
| Car Battery | 12 V | 60 Ah | 2,592,000 J | 720 Wh |
| Tesla Powerwall | 50 V (nom.) | 270 Ah | 48,600,000 J | 13,500 Wh |
| EV Battery (60 kWh) | 400 V | 150 Ah | 216,000,000 J | 60,000 Wh |
* Energy = Voltage × Capacity(Ah) × 3600. Values are approximate and vary by manufacturer and discharge rate.
12 volts alone does not specify an energy amount — you need to know the charge. With 1 coulomb of charge, 12V produces 12 joules. A 12V car battery rated at 60 Ah holds 60 × 3600 = 216,000 coulombs, storing 12 × 216,000 = 2,592,000 joules (2.59 MJ or 720 Wh). The voltage tells you the energy per unit charge; you multiply by total charge to get total energy.
A coulomb (C) is the SI unit of electric charge. One coulomb equals the charge transported by a current of one ampere flowing for one second: C = A × s. Therefore, 1 amp-hour = 1 A × 3600 s = 3600 coulombs. The coulomb is named after Charles-Augustin de Coulomb, who first measured electrostatic force in 1785.
In a battery, voltage remains approximately constant during discharge, so E = V × Q works directly. In a capacitor, voltage is proportional to stored charge (V = Q ÷ C), starting at zero and rising linearly. The average voltage during charging is V/2, leading to the formula E = ½ × C × V². This means a capacitor stores only half the energy you might naively expect from multiplying full voltage by total charge.
Convert both to joules (or watt-hours) to make a fair comparison. A 3.7V, 10,000 mAh power bank stores 3.7 × 10 × 3600 = 133,200 J (37 Wh). A 12V, 7 Ah sealed battery stores 12 × 7 × 3600 = 302,400 J (84 Wh). Despite the power bank having more amp-hours, the 12V battery stores more than twice the energy due to its higher voltage.
Common applications include: sizing battery banks for solar and off-grid systems (calculating total stored energy), designing capacitor banks for power factor correction (energy storage capacity), evaluating defibrillator energy delivery (typically 200–360 joules per shock), and calculating ESD risks for sensitive electronics per IEC 61000-4-2 (millijoule-level discharges can destroy chips).
Convert power over time to energy in joules for energy consumption calculations.
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