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Courses/Electrical Fundamentals/Power & Energy

lesson · 7min · Lesson 21 of 37

Energy: kilowatt-hours and joules

Course syllabusCourse overview
01Atoms, Electrons & Electric Charge
  1. ReadWhat is electricity?: atomic model
  2. ReadElectric charge and the coulomb
  3. ReadConductors, insulators, and semiconductors
  4. ReadConventional current vs electron flow
  5. quizModule quiz
02Voltage, Current & Resistance
  1. ReadPotential difference: the driving force
  2. ReadCurrent: the rate of charge flow
  3. ReadResistance and resistivity
  4. ReadOhm's Law: derivation and examples
  5. exerciseWorked examples: Ohm's Law problems
03Series & Parallel Circuits
  1. ReadSeries circuits: characteristics and rules
  2. ReadParallel circuits: characteristics and rules
  3. ReadCombined series-parallel networks
  4. ReadVoltage dividers and current dividers
  5. exerciseCircuit analysis practice
04Kirchhoff's Laws
  1. ReadKCL: Kirchhoff's Current Law
  2. ReadKVL: Kirchhoff's Voltage Law
  3. ReadMesh and nodal analysis
  4. exerciseKirchhoff's law problems set
05Power & Energy
  1. ReadElectrical power: watts and horsepower
  2. ReadEnergy: kilowatt-hours and joules
  3. ReadEfficiency and power loss in cables
  4. quizPower quiz
06Alternating Current Fundamentals
  1. ReadAC vs DC: why AC won
  2. ReadSinusoidal waveforms: peak, RMS, average
  3. ReadFrequency and period
  4. ReadPhase relationships
  5. ReadAC circuit analysis introduction
07Capacitors & Inductors
  1. ReadCapacitor construction and capacitance
  2. ReadCapacitors in AC circuits: reactance
  3. ReadInductor construction and inductance
  4. ReadInductors in AC circuits: reactance
  5. ReadRC and RL circuits: time constants
08Measurement & Instruments
  1. ReadMultimeters: AC/DC voltage and current
  2. ReadClamp meters and measuring live current
  3. ReadOscilloscopes: reading waveforms
  4. quizFinal assessment
Lesson · 7min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Energy: kilowatt-hours and joules

Electrical Fundamentals · Lesson 21 · Beginner

BeginnerReview: professional review pending

Purpose

Relate joules, watt-hours, and kilowatt-hours without confusing energy and power.

Before you beginElectrical power · Time conversion

Learning objectives

  • Use E=Pt
  • Convert Wh to J
  • Calculate kWh consumption
  • State assumptions for variable loads

Energy billing and battery use depend on power accumulated over time. A high-power load used briefly can consume less energy than a smaller load used all day.

ENERGY OVER TIMEENERGY OVER TIMEinputoutputE = Ptunits carry meaning

Core theory

One joule is one watt-second. One watt-hour is 3600 joules, and one kilowatt-hour is 3.6 megajoules.

For constant power, E=Pt. For varying power, energy is the integral or a sum of power over time intervals.

A device rating is not necessarily its actual average power; duty cycle and control behaviour matter.

Terms, symbols, and units
TermMeaningSymbolUnit
EnergyAccumulated capacity or transferEJ, Wh, or kWh
Kilowatt-hourEnergy from one kilowatt for one hourNot applicablekWh
Duty cycleFraction of time a load operatesNot applicableNot applicable
E = P t; 1 kWh = 3.6 MJ

Match the power and time units to the desired energy unit.

J=W·s; Wh=W·h
Worked exampleA 1.5 kW heater runs 40 min. Find energy in kWh and MJ.

Assumptions: Constant input power.

  1. Hours: 40 min = 2/3 h.
  2. kWh: E = 1.5 × 2/3 = 1.0 kWh.
  3. Joules: 1.0 kWh = 3.6 MJ.

The heater uses 1.0 kWh, equal to 3.6 MJ.

Reasonableness check: Running 1.5 kW for two-thirds of an hour gives one kilowatt-hour.

Common mistakes
  • Writing kW/h instead of kWh
  • Using minutes as hours
  • Assuming nameplate power equals measured average consumption

Where this appears in practice

Energy calculations support bills, batteries, generators, solar yield, and efficiency comparisons.

SafetyEnergy-cost calculations do not validate circuit capacity or appliance condition. A high-energy load requires suitable conductors, connections, and protection.
Local code checkThese analysis laws are universal. Installation design, conductor selection, protection, testing, and permitted work must follow current local standards and manufacturer data.

Knowledge check

How many megajoules are in 2 kWh?

7.2 MJ. Multiply kWh by 3.6 MJ/kWh.

Answer: 7.2 MJ. Multiply kWh by 3.6 MJ/kWh.

Practical exercise

Compare daily energy for a 2 kW kettle used 12 min and a 100 W device used 8 h.

Summary

  • Energy equals power times time
  • 1 kWh is 3.6 MJ
  • Use duty cycle for intermittent loads

Sources and review

  • University Physics Volume 2: Kirchhoff's Rules: OpenStax; Current web edition; Physics reference
  • SI Units: Electric Current: NIST; Current web edition; United States / SI reference
  • The International System of Units (SI Brochure): BIPM; 9th edition, updated 2026; International

Editorial review date: 2026-08-21. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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