A 100 W electric bulb is used for 10 hours a day. How many units of electrical energy are consumed by the bulb in 3 days? (1 unit = 1 kWh)

To calculate the electrical energy consumed, we use the formula: Energy (kWh) = Power (kW) × Time (h) [1]. First, the power of the bulb must be converted from watts to kilowatts. Since 1,000 W equals 1 kW, a 100 W bulb is equivalent to 0.1 kW. The bulb is used for 10 hours a day, so the daily energy consumption is 0.1 kW × 10 hours = 1 kWh. Over a period of 3 days, the total energy consumed is 1 kWh/day × 3 days = 3 kWh. In commercial terms, 1 kWh is defined as 1 'unit' of electricity [1]. Therefore, the total consumption is 3 units. This aligns with the standard conversion where energy is the product of power and time [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > 11.8 ELECTRIC POWER > p. 191
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > What you have learnt > p. 192