How it works
A watt is one joule per second, so energy is simply power multiplied by the time it flows: E = P × t. Run a 60 W bulb for 60 seconds and it consumes 60 × 60 = 3,600 joules. The joule is the SI unit of energy; the watt is the rate at which that energy is delivered or used.
This makes converting between the two direct: joules from watts needs a time, and watts from joules needs a duration. A device's wattage tells you nothing about total energy until you say for how long — which is exactly why electricity is billed in kilowatt-hours (a power multiplied by a time), not in watts.
Worked example
Calculate the joules
A 2,000 W kettle running for 90 seconds uses E = 2,000 × 90 = 180,000 J (180 kJ), which is 180,000 / 3.6×10⁶ = 0.05 kWh. A 5 W phone charger left on for an hour (3,600 s) uses 5 × 3,600 = 18,000 J, or exactly 0.005 kWh.
Use it in real life
Energy bills: your meter counts kilowatt-hours — power × time. Reading a joule total back as kWh (divide by 3.6 million) turns an abstract physics number into money.
Battery sizing: a 100 Wh power bank stores 360,000 J. Divide by a device's wattage to estimate runtime — a 5 W device runs about 20 hours.
Cooking and heating: a 1,000 W microwave for 3 minutes delivers 1,000 × 180 = 180,000 J of energy into your food — the same joules a 100 W bulb would emit over half an hour.
Frequently asked questions
How do you convert watts to joules?
Multiply the power in watts by the time in seconds: joules = watts × seconds. A watt is defined as one joule per second, so 100 W for 10 s is 1,000 J. Watts alone can't give joules — you always need a time.
How many joules are in a kilowatt-hour?
Exactly 3,600,000 J (3.6 megajoules). A kilowatt-hour is 1,000 watts sustained for 3,600 seconds, and 1,000 × 3,600 = 3.6 million joules.
What's the difference between a watt and a joule?
A joule is an amount of energy; a watt is a rate — one joule per second. Watts tell you how fast energy is used, joules how much in total. Time links them: energy = power × time.