Watt Hour to Joule
Wh
J
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Quick Reference Table (Watt Hour to Joule)
| Watt Hour (Wh) | Joule (J) |
|---|---|
| 1 | 3,600 |
| 5 | 18,000 |
| 10 | 36,000 |
| 20 | 72,000 |
| 50 | 180,000 |
| 100 | 360,000 |
| 500 | 1,800,000 |
About Watt Hour (Wh)
A watt-hour (Wh) is the energy consumed or produced by a one-watt device operating for one hour, equal to 3,600 joules. It is widely used for small battery and energy storage capacities — smartphone batteries, power banks, and small electronic devices. A smartphone battery holds roughly 10–15 Wh; a laptop 50–100 Wh. The watt-hour is the stepping-stone unit between the joule (too small for practical appliance use) and the kilowatt-hour (the billing unit for mains electricity).
A phone charger running for an hour uses about 5–10 Wh. A 100 Wh portable power bank can charge a typical smartphone about seven times.
About Joule (J)
The joule (J) is the SI unit of energy, defined as the work done when a force of one newton displaces an object one meter in the direction of the force. It is also the energy dissipated as heat when one ampere flows through one ohm of resistance for one second. The joule underpins all other energy units in science: calories, watt-hours, and electron volts are all defined relative to it. At human scale, one joule is a small quantity — lifting a 100 g apple by one meter requires about one joule.
Lifting a medium apple (100 g) by 1 meter requires about 1 J. A typical sneeze releases roughly 1 J of kinetic energy.
Etymology: Named after English physicist James Prescott Joule (1818–1889), who experimentally established the mechanical equivalent of heat, demonstrating that work and heat are interconvertible forms of the same quantity.
Watt Hour – Frequently Asked Questions
Why are portable battery capacities listed in watt-hours instead of milliamp-hours?
Watt-hours account for both current and voltage, giving the true energy stored. A 10,000 mAh power bank at 3.7 V holds 37 Wh, but at 5 V output it delivers only about 7,400 mAh due to voltage conversion losses. Airlines use the Wh rating (max 100 Wh carry-on) because it reflects actual energy — and therefore actual fire risk — regardless of battery voltage.
How many watt-hours does a typical smartphone battery hold?
Most smartphones have batteries rated at 10–18 Wh. An iPhone 15 Pro holds about 12.7 Wh; a Samsung Galaxy S24 Ultra about 18.4 Wh. For context, fully charging an 18 Wh phone from a wall outlet costs less than 0.01 kWh — roughly one-tenth of a cent on a typical electricity bill.
What is the airline limit for lithium batteries in watt-hours?
Most airlines allow lithium-ion batteries up to 100 Wh in carry-on luggage without approval. Batteries between 100 and 160 Wh (e.g., large camera or drone batteries) require airline permission, and batteries above 160 Wh are banned from passenger flights. A standard laptop battery is 50–100 Wh; a large power tool battery can exceed 160 Wh.
Why did the electronics industry settle on watt-hours instead of joules for battery labels?
Watt-hours map directly to how consumers think about devices: a 50 Wh battery powering a 10 W laptop lasts about 5 hours — simple division. Expressing the same battery as 180,000 joules gives no intuitive sense of runtime. Airlines also adopted Wh for lithium battery safety limits (100 Wh carry-on threshold) because it communicates energy density risk in a unit engineers and passengers can both grasp.
How many watt-hours does it cost to charge a laptop?
A typical laptop battery holds 50–100 Wh, so a full charge from empty uses 50–100 Wh of energy (plus about 10–15% lost as heat in the charger). At average US electricity rates, that is roughly 1–2 cents per charge. Over a year of daily charging, a laptop costs about $4–$7 in electricity — far less than most people assume.
Joule – Frequently Asked Questions
Why is the joule named after James Prescott Joule and not after a more famous physicist?
Joule was the first to prove experimentally that heat and mechanical work are the same thing — he measured the temperature rise of water churned by a falling weight. That 1845 brewery-funded experiment settled a centuries-old debate and earned the SI energy unit his name in 1889, well before units were named after Einstein or Feynman.
How many joules are in a kilowatt-hour on an electricity bill?
Exactly 3,600,000 joules. A kilowatt-hour is simply 1,000 watts sustained for 3,600 seconds. Utilities chose kWh because quoting home energy use in megajoules (e.g., "your fridge used 129.6 MJ this month") would confuse most customers.
What everyday action uses about one joule of energy?
Lifting a medium apple one meter off the ground takes roughly 1 J. Clicking a computer mouse uses about 1.5 mJ (0.0015 J), a heartbeat expends ~1 J, and a single typed keystroke on a mechanical keyboard is around 10–40 mJ. A joule is a surprisingly tiny amount of energy at human scale.
What is the difference between a joule and a watt?
A joule measures total energy; a watt measures the rate of energy flow (power). One watt equals one joule per second. A 60 W lightbulb consumes 60 joules every second — leave it on for an hour and it uses 216,000 J (0.06 kWh). Think of joules as liters of water and watts as the flow rate of the tap.
How does the joule relate to the calorie?
One thermochemical calorie equals exactly 4.184 joules. The "calorie" on food labels is actually a kilocalorie (4,184 J). So a 2,000-Calorie daily diet supplies about 8.4 million joules — enough energy to lift a small car roughly 850 meters straight up, if your body were 100% efficient (it is not).