Skip to main content
ElectraCore
LearnCalculatorsCircuit DesignerReferences
Saved
Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Electrical Fundamentals/Power & Energy

lesson · 8min · Lesson 20 of 37

Electrical power: watts and horsepower

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 · 8min
POWER TRIANGLEφP: real (W)Q (VAr)S (VA)PF = P / S= cos φS² = P² + Q²
Lesson contents 0%

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
0% read
ElectraCore lesson handout

Electrical power: watts and horsepower

Electrical Fundamentals · Lesson 20 · Beginner

BeginnerReview: professional review pending

Purpose

Calculate electrical power and distinguish watts from energy and mechanical horsepower.

Before you beginVoltage · Current · Ohm's law

Learning objectives

  • Use P=VI
  • Derive resistive power forms
  • Distinguish power from energy
  • Convert horsepower only with a stated convention

Power is the rate of energy transfer. Watts describe how fast energy changes form; watt-hours and joules describe how much energy is transferred.

POWER: ENERGY RATEPOWER: ENERGY RATEinputoutputP = VIunits carry meaning

Core theory

Electrical power is p=vi; for steady DC or a purely resistive operating point, P=VI.

Combining Ohm's law gives P=I²R and P=V²/R for resistive loads.

Mechanical horsepower is a power unit, not efficiency. Motor input electrical power exceeds useful shaft output because of losses.

Terms, symbols, and units
TermMeaningSymbolUnit
PowerRate of energy transferPwatt (W)
HorsepowerNon-SI mechanical power unithpState convention; mechanical hp ≈ 746 W
EfficiencyUseful output divided by inputηratio or %
P = VI = I²R = V²/R

The alternate forms assume resistance describes the operating point.

W = V·A = J/s
Worked exampleA 24 V heater draws 3 A. Find power and energy transferred in 10 min.

Assumptions: Steady DC values.

  1. Power: P = 24 × 3 = 72 W.
  2. Time: 10 min = 600 s.
  3. Energy: E = Pt = 72 × 600 = 43,200 J = 43.2 kJ.

Power is 72 W; energy in 10 min is 43.2 kJ.

Reasonableness check: At 72 joules each second, 600 seconds transfers 43,200 joules.

Common mistakes
  • Calling kWh a power unit
  • Applying V²/R to a non-ohmic load without qualification
  • Equating motor input watts directly to shaft horsepower

Where this appears in practice

Power determines heat, energy use, source loading, and equipment ratings.

SafetyCalculated normal power does not quantify fault energy. Confirm overload, short-circuit, thermal, enclosure, and disconnection requirements separately.
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

A 12 V resistor carries 2 A. What power does it dissipate?

24 W. P = VI = 12 × 2.

Answer: 24 W. P = VI = 12 × 2.

Practical exercise

Calculate power three ways for a 10 Ω resistor at 5 V and confirm agreement.

Summary

  • Power is energy per time
  • Watts are not watt-hours
  • Resistive power forms derive from Ohm's law

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.

Personal study record

Lesson notes

Ready

Notes are stored on this device unless you export them.