---
title: "Cost of electrical energy for appliances"
subject: "Basic Electrical Engineering"
chapter: 1
key: "semester_1/basic_electrical_engineering:electricity-cost-toaster-bulbs-dryer"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/electricity-cost-toaster-bulbs-dryer
site: BCT Engineering
---

# Cost of electrical energy for appliances

Basic Electrical Engineering · Chapter 1: General Electric System

- Asked in: 2068 Chaitra, 4 marks
- Video: [AQA Core Physics Lesson 18 Calculating the Cost of Electricity](https://www.youtube.com/watch?v=-jQ1Kn5Oqxw) — ATPhysics1
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/electricity-cost-toaster-bulbs-dryer
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-1

## Question

What is the total cost of using the following at Rs 7 per kilowatt hour? i) A 1200 W toaster for 30 min ii) Six 50 W bulbs for 4 hours iii) A 400 W washing machine for 45 min. iv) A 4800 W electric clothes dryer for 20 min.

## Short answer

Energy: toaster 1.2kW×0.5h=0.6kWh, bulbs 0.3kW×4h=1.2kWh, washer 0.4kW×0.75h=0.3kWh, dryer 4.8kW×1/3h=1.6kWh. Total 3.70 kWh × Rs 7 = Rs 25.90.

## Answer

Energy (kWh) = power (kW) × time (h). Cost = energy × Rs 7.

| Appliance | Power | Time | Energy |
|---|---|---|---|
| Toaster | 1.2 kW | 0.5 h | 0.60 kWh |
| Six bulbs | 6 × 50 W = 0.3 kW | 4 h | 1.20 kWh |
| Washing machine | 0.4 kW | 0.75 h | 0.30 kWh |
| Clothes dryer | 4.8 kW | 1/3 h | 1.60 kWh |

Total energy = 0.60 + 1.20 + 0.30 + 1.60 = 3.70 kWh

Total cost = 3.70 × 7 = Rs 25.90

Item costs: toaster Rs 4.20, bulbs Rs 8.40, washing machine Rs 2.10, dryer Rs 11.20.

**Answer: Total cost = Rs 25.90**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Energy in kWh is computed as:
   A. Power in kW x time in hours
   B. Power in W x time in minutes
   C. Power in kW / time in hours
   D. Voltage x current in amps
   Answer: A. kWh = kW x h. (Key point: Energy formula)

2. Six 50 W bulbs on for 4 hours use:
   A. 0.2 kWh
   B. 1.2 kWh
   C. 12 kWh
   D. 0.3 kWh
   Answer: B. 6 x 50 W = 0.3 kW; 0.3 x 4 = 1.2 kWh. (Key point: Bulbs)

3. The 4800 W dryer for 20 minutes uses:
   A. 96 kWh
   B. 0.8 kWh
   C. 1.6 kWh
   D. 16 kWh
   Answer: C. 4.8 kW x 1/3 h = 1.6 kWh. (Key point: Dryer)

4. Total energy 3.70 kWh at Rs 7 costs:
   A. Rs 7
   B. Rs 18.50
   C. Rs 37
   D. Rs 25.90
   Answer: D. 3.70 x 7 = Rs 25.90. (Key point: Total cost)

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