Practical energy usage: A device rated at 400 W runs for 30 hours. Calculate the energy consumed in kilowatt-hours (kWh) for billing purposes.

Introduction / Context:
Translating device power ratings and operating time into energy (kWh) helps estimate electricity costs and compare efficiency across appliances. This is a straightforward application of energy = power × time with proper unit conversion to kilowatts and hours.

Given Data / Assumptions:

• Power P = 400 W.
• Time t = 30 h.
• Power is assumed constant during operation.

Concept / Approach:
Energy (kWh) = (power in kW) * (time in hours). Convert 400 W to 0.4 kW, then multiply by 30 h to find the energy used.

Step-by-Step Solution:

Verification / Alternative check:
As watt-hours: 400 W * 30 h = 12,000 Wh. Divide by 1,000 → 12 kWh. Both paths give the same result.

Why Other Options Are Wrong:

• 13.3 kWh and 1.3 kWh: Result from arithmetic or decimal placement errors.
• 1.2 kWh: Off by a factor of 10; would be correct for 400 W over 3 hours, not 30 hours.

Common Pitfalls:

• Mistaking W for kW or mixing hours with minutes, leading to tenfold errors.

Final Answer:
12 kWh