What is a Kilowatt Hour? Understanding Watts, Kilowatts, Joules and Electricity Bills

Overview

This video from the Engineering Mindset explains what a kilowatt hour (kWh) is and how it relates to watts (W) and kilowatts (kW). It shows how to convert between watts and kilowatts, how to compute energy usage (energy = power × time), and how electricity tariffs convert kWh consumption into a monthly bill. Practical examples illustrate energy calculations for devices such as heaters and light bulbs, and guidance is given on estimating monthly consumption across a building.

Introduction to Kilowatt Hours and Power

The video opens by defining a kilowatt hour as a unit of energy, not power. It explains the fundamental relation energy = power × time, and clarifies that kilowatt hours are the billing unit used by electricity providers. A watt is defined as one joule per second, and a kilowatt hour corresponds to 3.6 megajoules of energy. The audience learns that electrical products are rated in watts or kilowatts, representing the rate at which they consume energy.

Key Units and Conversions

The presenter reviews the hierarchy of units: watts (W), kilowatts (kW), and kilowatt hours (kWh). He notes that 1 kilowatt equals 1000 watts and that to convert energy, time must be in hours for billing. The video includes practical conversion examples: 1 kW = 1000 W and 100 W = 0.1 kW. It also introduces joules as a measure of energy and reiterates that 1 kWh equals 3.6 MJ, helping viewers connect electrical energy to mechanical and thermal contexts.

Calculating Energy Use

Using the formula E = P × t, the video demonstrates how to determine how much energy a 2 kW heater uses if it runs for 3 hours (6 kWh). It then shows a scenario where the heater runs for 30 seconds, converting the time to hours (30 s / 3600 s per hour = 0.0083 h) to obtain about 0.0167 kWh. For smaller devices rated in watts, such as a 60 W light bulb, the energy is computed by first converting to kilowatts (0.060 kW) and then multiplying by the usage time (10 hours) to obtain 0.6 kWh.

Cost Calculation and Tariffs

The cost is obtained by multiplying energy use by the tariff. An example shows a 2 kW heater used for 3 hours consuming 6 kWh, billed at $0.10 per kWh, yielding $0.60. The approach scales to monthly usage by summing energy for all devices in a table and applying the per-kWh charge, with consideration for taxes or additional fees as needed.

Monthly Electricity Planning

To estimate monthly consumption, the video recommends creating a table of all electrical items, recording their power in kilowatts, counting the quantity of each item, and estimating the hours each item is switched on per month. The energy for each item is computed with E = P × t, then all item energies are summed to obtain total kilowatt-hour usage. This total can then be multiplied by the tariff to estimate the monthly bill. The video also suggests using plug-in energy monitors to measure real-time volts, amps, watts, and kWh for accuracy, and mentions adding taxes and fees separately as needed.

Practical Tips and Wrap-Up

Throughout the video, emphasis is placed on practical calculation techniques, the importance of using kWh for billing, and the usefulness of simple tools to verify power consumption. Viewers are encouraged to apply these methods to their own homes or buildings to understand energy use and control costs more effectively. The speaker closes by inviting questions and directing viewers to the Engineering Mindset website for more resources.