The energy unit used in electrical energy billing is typically kilowatt-hour. How many joules are in 1 kWh?

• A. 3.6 × 10^6 J
• B. 3.6 × 10^3 J
• C. 3.6 × 10^9 J
• D. 3600 J

Answer: (A)

When energy is billed in kilowatt-hours, you’re measuring how much energy a device at a certain power uses over time. A kilowatt is 1000 joules per second, and an hour is 3600 seconds. So 1 kilowatt-hour equals 1000 J/s × 3600 s = 3.6 × 10^6 joules. That’s why 1 kWh is 3.6 million joules.

The other numbers don’t match 1 kWh: 3.6 × 10^3 J would be the energy from a 1000 W device running for only 3.6 seconds; 3.6 × 10^9 J would require much longer or more power (for example, 1000 W for about 1,000,000 seconds). And 3600 J is the energy from a 1 W device running for one hour.