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Convert power in watts (W) to energy in joules (J) given time. Calculate how much energy is used over a period.
Where J is energy in joules, W is power in watts, and s is time in seconds.
A 100-watt light bulb running for 1 minute:
J = 100 × 60 = 6,000 joules
| Power (W) | 1 sec | 1 min | 1 hour |
|---|---|---|---|
| 1 W | 1 J | 60 J | 3,600 J |
| 10 W | 10 J | 600 J | 36,000 J |
| 60 W | 60 J | 3,600 J | 216,000 J |
| 100 W | 100 J | 6,000 J | 360,000 J |
| 500 W | 500 J | 30,000 J | 1,800,000 J |
| 1,000 W | 1,000 J | 60,000 J | 3,600,000 J |
| 2,000 W | 2,000 J | 120,000 J | 7,200,000 J |
| 5,000 W | 5,000 J | 300,000 J | 18,000,000 J |
Watts to joules conversion calculates total energy (joules) from a known power level (watts) over a given duration (seconds). The watt, named after James Watt, is the SI unit of power — the rate at which energy is transferred or used. The joule, named after James Prescott Joule, is the SI unit of energy. Their relationship is fundamental: 1 watt = 1 joule per second, so energy in joules equals power in watts multiplied by time in seconds (J = W × s). This conversion bridges the gap between instantaneous power ratings on equipment nameplates and the total energy consumed or delivered over time. It is used across electrical engineering, mechanical engineering, thermodynamics, and physics to quantify energy in heating systems, electrical devices, chemical reactions, and even food calorie calculations (1 food calorie = 4184 joules).
Check the nameplate rating of your device or measure power using a watt meter. Common values include 10W (LED bulb), 100W (incandescent bulb), 1000W (microwave), and 1500W (space heater).
Convert the operating time to seconds. If given in minutes, multiply by 60. If in hours, multiply by 3600. For example, 5 minutes = 300 seconds, 2 hours = 7200 seconds.
Apply the formula: J = W × s. For example, a 1500W heater running for 30 minutes: J = 1500 × 1800 = 2,700,000 joules (2.7 MJ). This represents the total thermal and electrical energy consumed.
Convert as needed: divide by 1000 for kilojoules (kJ), by 3600 for watt-hours (Wh), by 3,600,000 for kilowatt-hours (kWh), or by 4184 for food calories (kcal). These conversions help compare energy across different contexts.
While watts tell you how fast energy is used, joules tell you the total amount. A 2000W kettle running for 3 minutes uses 360,000 J (360 kJ) — the actual energy needed to heat the water from the first law of thermodynamics.
The joule is the standard SI energy unit across all scientific disciplines. Researchers use watts-to-joules conversions in laser energy calculations, chemical reaction energetics, metabolic rate studies, and thermal analysis in accordance with ISO standards.
Converting watts to joules (and then to kWh) lets you calculate the exact cost of running any electrical device. A 100W lamp for 10 hours uses 3,600,000 J = 1 kWh, which costs about $0.10–$0.30 depending on your utility rate.
| Unit | Equivalent in Joules | Conversion Formula | Common Use |
|---|---|---|---|
| 1 Watt-second (Ws) | 1 J | J = Ws × 1 | Photography flash energy |
| 1 Watt-minute (Wm) | 60 J | J = Wm × 60 | Short process cycles |
| 1 Watt-hour (Wh) | 3,600 J | J = Wh × 3,600 | Battery capacity |
| 1 Kilowatt-hour (kWh) | 3,600,000 J | J = kWh × 3,600,000 | Electricity billing |
| 1 Calorie (cal) | 4.184 J | J = cal × 4.184 | Chemistry, physics |
| 1 Food Calorie (kcal) | 4,184 J | J = kcal × 4,184 | Nutrition labeling |
| 1 BTU | 1,055 J | J = BTU × 1,055 | HVAC and heating |
| 1 Electronvolt (eV) | 1.602 × 10⁻¹⁹ J | J = eV × 1.602e-19 | Particle physics |
A 100-watt light bulb running for one hour consumes 100 × 3600 = 360,000 joules (360 kJ). This equals 100 watt-hours or 0.1 kilowatt-hours. At a typical electricity rate of $0.15/kWh, that hour of lighting costs about $0.015 (1.5 cents). A 10W LED producing the same brightness would use only 36,000 joules — 90% less energy.
Watts measure the rate of energy transfer (power), while joules measure the total amount of energy. Think of it like a water analogy: watts are the flow rate (liters per second), and joules are the total volume (liters). A 100W device uses energy at a rate of 100 joules per second. Running it for 60 seconds consumes 6,000 joules total. The distinction is critical for energy planning — you pay for joules (kWh), not watts.
Divide joules by 3,600,000 (since 1 kWh = 1000 W × 3600 s = 3,600,000 J). For example, 7,200,000 J ÷ 3,600,000 = 2 kWh. For large values, it may be easier to first divide by 3600 to get Wh, then divide by 1000 to get kWh.
The average US household uses about 30 kWh per day, which equals 30 × 3,600,000 = 108,000,000 joules (108 MJ). This varies widely: efficient homes may use 15 kWh/day (54 MJ) while large homes with electric heating can exceed 60 kWh/day (216 MJ). The Department of Energy tracks these statistics for energy efficiency programs.
In everyday terms, a joule is quite small. Lifting an apple (about 1 newton of force) one meter high requires approximately 1 joule. Boiling a cup of water from room temperature requires about 330,000 joules (330 kJ). Running a typical house for one day requires about 108 million joules (108 MJ). This is why electrical energy is more commonly expressed in kilowatt-hours for practical billing.