Convert Joules (J) to Watt-hours (Wh)
1 joule equals 0.000277778 watt-hours.
Joule to Watt-hour Converter
What Is a Joule?
The joule (J) is the SI unit of energy, defined as the work done when a force of 1 newton moves an object 1 meter (1 J = 1 N·m = 1 kg·m²/s²). Named after James Prescott Joule, it is used in physics, chemistry, thermodynamics, and nutrition (1 calorie = 4.184 joules).
What Is a Watt-hour?
The watt-hour (Wh) is a practical unit of energy equal to 3,600 joules. It represents the energy consumed by a 1-watt device running for 1 hour. Electricity bills use kilowatt-hours (kWh = 1,000 Wh), and battery capacity is often stated in Wh or mAh.
Joules to Watt-hours Formula
The conversion is exact by definition:
watt-hours = joules ÷ 3,600 exact
Derived from: 1 Wh = 1 W × 1 hour = 1 J/s × 3,600 s = 3,600 J.
Worked Example
Convert 18,000 joules (approximate energy in a AA battery) to watt-hours:
- Divide: 18,000 ÷ 3,600 = 5 Wh
A standard AA battery stores about 5 Wh (1.5V × 3,000 mAh × discharge efficiency).
Quick Estimation
÷ 3,600 — or divide by 4,000 and add 10%. Example: 7,200 J ÷ 4,000 = 1.8, +10% ≈ 2.0 Wh (exact: 2.0 Wh). For kJ to kWh: divide by 3,600 (same factor).
Common Joules to Watt-hours Conversions
| Joules | Watt-hours | Context |
|---|---|---|
| 1 J | 0.000278 Wh | Lifting a small apple 1 meter |
| 360 J | 0.1 Wh | Very small battery (coin cell) |
| 1,000 J | 0.278 Wh | 1 kJ — roughly 1 food calorie / 4 |
| 3,600 J | 1 Wh | Exact — by definition |
| 10,000 J | 2.78 Wh | Small phone battery (older model) |
| 18,000 J | 5 Wh | AA battery (approximate) |
| 36,000 J | 10 Wh | Small power bank |
| 100,000 J | 27.8 Wh | Smartphone battery (modern) |
| 360,000 J | 100 Wh | TSA battery limit for flights |
| 1,000,000 J | 278 Wh | 1 MJ — large laptop battery |
| 3,600,000 J | 1,000 Wh | 1 kWh — electricity billing unit |
Joules vs Watt-hours — Why Both Exist
The joule is the SI standard, used in physics and science. The watt-hour is a practical convenience for electricity: saying "a device uses 500 Wh per day" is more intuitive than "1,800,000 joules per day." The relationship is simple — 1 Wh = 3,600 J — because an hour has 3,600 seconds. In practice: physics uses joules, power engineering uses watt-hours (or kWh), and nutrition uses calories (1 cal = 4.184 J).
When You Need This Conversion
- Battery comparison: Some specs list capacity in joules (scientific), others in Wh (practical). A 100,000 J battery = 27.8 Wh — below the 100 Wh TSA carry-on limit for lithium batteries.
- Physics homework: Textbook problems use joules, but real-world appliances use kWh. Converting helps connect theory to practice: running a 100W bulb for 1 hour = 360,000 J = 100 Wh = 0.1 kWh.
- Energy comparison: Comparing fuels in joules with electricity in kWh. 1 liter of gasoline ≈ 34.2 MJ ≈ 9.5 kWh of thermal energy.
Frequently Asked Questions
How many joules in a watt-hour?
Exactly 3,600 joules. This is exact because 1 Wh = 1 J/s × 3,600 s = 3,600 J.
Why use Wh instead of joules?
Convenience. A household uses about 30 kWh/day, which is 108,000,000 joules. Writing "30 kWh" is much more practical. The watt-hour aligns with how we measure power (watts) over time (hours).
How do I convert kJ to kWh?
Divide by 3,600 (same factor as J to Wh). Example: 10,000 kJ ÷ 3,600 = 2.78 kWh.
How many joules in a AA battery?
About 10,000-18,000 joules (2.8-5 Wh), depending on discharge rate and chemistry. A typical alkaline AA at 1.5V × 2,500 mAh ≈ 3.75 Wh ≈ 13,500 J under ideal conditions.
Common Joule to Watt-hour Conversions
| Joule | Watt-hour | Actions |
|---|---|---|
| 1 joule | 0.000277778 watt-hours | |
| 100 Joules | 0.0277778 watt-hours | |
| 500 Joules | 0.138889 watt-hours |
| Joule | Watt-hour | Actions |
|---|---|---|
| 1000 Joules | 0.277778 watt-hours | |
| 3600 Joules | 1 watt-hour | |
| 10000 Joules | 2.77778 watt-hours |
Joules to watt-hours Conversion Table
Reference table with common joules to watt-hours conversions. All values calculated with high precision.
Joules to Watt-hours Table
10000 to 1 × 109
Watt-hours to Joules Table
1 to 100000
Looking for the reverse? Convert Watt-hour to Joule
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- Watts to Kilowatts — kW measures power; kWh measures energy (power × time)
