Fast Facts About
An energy system converts primary energy resources like fossil fuels or wind into energy services. Energy services are what humans care about, like hot showers and cold beverages. There are energy losses each time we convert energy from one form to another. Energy systems are most efficient when we can closely match the resource with the service (e.g., using sunlight for illumination). The earth is an open energy system that is always getting new energy from the sun.
Energy cannot be created or destroyed, but we can theoretically run out of certain forms of energy like fossil fuels. Fossil fuels are a stock resource (we have a set amount on earth) that can meet the world’s energy needs many times over. However, we need to stop using fossil fuels due to their significant environmental and climate change impacts.
Energy is a conserved quantity that can be accumulated and is transferred as heat, work, and in matter.
Modern civilization is possible because people have learned how to change energy from one form to another and then use it to do work.
Energy is like the amount of water a bucket can hold.
Typical Energy Units
British thermal unit ( Btu)
Power is the instantaneous rate of energy use or flow.
Power is like the rate at which water pours out of the bucket.
Typical Power Units
Energy = Power x Time
Laws of Thermodynamics
Law 1: Energy can neither be created nor destroyed.
However, energy can be converted into different forms to provide energy services. For example, a space heater converts electrical energy to heat.
Law 2: Heat flows from hot to cold, and there are losses when converting from heat to work.
Using heat for anything but heat is inherently inefficient (e.g., heat engines in cars).
Law 3: The entropy of a system approaches a constant value when its temperature approaches absolute zero.
Key Energy Terms
Energy forms that exist naturally.
Examples: fossil fuels, solar, nuclear
Created from primary energy to provide useful services.
Examples: electricity, gasoline
(end uses of energy)
Things people want and care about.
Examples: heating, cooking, lighting, transportation
Enables use of energy at a later time.
Examples: batteries, ice/steam
Not all energy is equal.
Energy sources are not always easily subbed for one another.
Matching the resource to the desired service is key.
A cat knows how to do it. Humans make it more complicated.
Excerpt from The Solar Cat Book, Copyright 2004, Jim Augustyn
Energy Efficiency is providing the same or better service using less energy.
Conversion Efficiency measures how well energy is converted from one form to another as shown in the energy system examples below.
Conversion Efficiency = Useful Energy Output / Energy Input
Efficiency of System = Conversion Efficiency 1 x Conversion Efficiency 2….
A Human-Made Energy System Example: Only 1% Efficient (35% x 90% x 3% ≈ 1%)
Power Station and Grid
~3% efficient (incandescent lightbulb)
A More Efficient Human-Made Energy System Example: 16% Efficient (60% x 90% x 30% ≈ 16%)
State-of-the-Art Combined Cycle Natural Gas Power Plant
~30% efficient (ultra-efficient LED)
Key Attributes of Primary Energy Resources
Form of Energy
|Stock (Depletable) /
Updated January 2024
Before You Watch Our Lecture on
We assign videos and readings to our Stanford students as pre-work for each lecture to help contextualize the lecture content. We strongly encourage you to watch the Essential video below before watching our lecture on Energy Basics. Include selections from the Optional and Useful list based on your interests and available time.
- The Essentials of Energy. Be Smart. May 6, 2015. (5 min)
A tour of the essential principles behind the energy machine that puts fuel in our tanks and brings electricity to our homes.
Optional and Useful
- Introduction to Energy. NEED.org. 2023. (4 pages)
Provides a comprehensive introduction to key energy terms and concepts.
- Part 1: The Story of Energy. Wait But Why. June 2, 2015. (14 pages)
An entertaining and informative overview of key concepts for energy, fossil fuels, and climate change.
- Worksheet 1: Unit Conversion. Stanford Understand Energy. August 9, 2016. (5 pages)
Explains key energy units and how to convert and keep track of them.
- Worksheet 2: Energy vs Power. Stanford Understand Energy. August 9, 2016. (3 pages)
Describes the differences between energy and power.
Our Lecture on
This is our Stanford University Understand Energy course lecture that introduces the topic of energy, including key energy terms and basic energy properties. We strongly encourage you to watch the full lecture to gain foundational energy knowledge that will be helpful as you explore other topics on our site. For a complete learning experience, we also encourage you to watch the Essential video we assign to our students before watching the lecture.
Presented by: Diana Gragg, PhD; Core Lecturer, Civil and Environmental Engineering, Stanford University; Explore Energy Managing Director, Precourt Institute for Energy
Recorded on: March 23, 2022 Duration: 33 minutes
Table of Contents
(Clicking on a timestamp will take you to YouTube.)
01:04 Energy and Power Defined
03:18 Laws of Thermodynamics Simplified
06:10 Energy Quality
08:04 Origins and Forms of Energy
09:57 Conversion of Energy Resources to Energy Services
15:28 Matching Energy Resources to the Use
26:36 Conversion Efficiency
29:53 Wrap up: Example Conversion Efficiency Limits
Lecture slides available upon request.