A light bulb has a power rating of 40 watts. How many kilowatt-hours does it consume when used for 10 hours?

To determine the energy consumption of a light bulb rated at 40 watts when used for 10 hours, it’s essential to convert the power rating into kilowatts and then multiply it by the time in hours.

First, convert watts to kilowatts. Since there are 1000 watts in a kilowatt, a 40-watt bulb is equivalent to 0.04 kilowatts (40 watts ÷ 1000 = 0.04 kW).

Next, calculate the total energy consumed over 10 hours. This is done by multiplying the power in kilowatts (0.04 kW) by the time in hours (10 hours):

0.04 kW × 10 hours = 0.4 kilowatt-hours (kWh).

Thus, the correct answer is 0.4 kWh, reflecting the total energy consumption when the bulb operates at its rated power for the specified duration. This fundamental calculation illustrates how to measure electrical energy consumption and is applicable in many real-world scenarios, such as estimating household energy usage.