The physics of energy


What are the physical realities that any energy system needs to deal with, regardless of how we organise its rules, ownership, and goals?

This guide aims to help us to re-imagine how our energy systems could be organised. To do that, we need to understand what is and isn’t changeable, starting with some fundamental physical realities.

Electricity is a particularly important form of energy for a sustainable energy system – sometimes when people say energy they are actually talking only about electricity.  Because of this, this guide includes a separate section on the Physics of Electricity that goes into more detail.

What is energy?

The understanding of ‘energy’ as a single concept was developed during the industrial revolution, starting from about 1680. It covers different physical phenomena including heat, motion, electricity and light. We get this energy from burning coal, wood or dung, water stored in a reservoir or flowing in a river, the wind, the sun. 

Using the concept of ‘energy‘ to include all of these phenomena is central to industrial societies.  It allows us to apply the same scientific units and equations to all forms of energy, and to understand the conversion between different forms in terms of the first and second laws of thermodynamics. This is particularly important for designing and running thermal power stations and networked energy infrastructure.

For a critical discussion of the invention of ‘energy’ as a unified concept, see this essay by teacher Robert Lehrman, or this report by the cornerhouse which puts the history of ‘energy’ in a social and political context. The flows of energy in society are so important that they are taken as an underlying basis for some important heterodox (non-mainstream) schools of economic thought, as are land and labour.

The single concept of ‘energy’ is used to include:  


The movement of electrons in a conducting material

Kinetic energy

Energy in movement, (e.g. the turning of a wind turbine)

Nuclear energy

The energy released when an atomic nucleus is split, with a rearrangement of subatomic particles

Thermal energy or heat

The vibration or movement of atoms and molecules (kinetic energy at a micro scale!)

Gravitational potential energy

The energy released if a mass that is high up is dropped (e.g. hydroelectric power from water falling from high to low)

Electromagnetic radiation

Radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma radiation

Chemical energy

The energy that is released when chemical bonds between atoms break or form (e.g. burning a fuel, storing energy in a battery, respiration in plants and animals)

The interchangeable concept of energy is illustrated by definitions of energy such as ‘the ability to heat water’ or ‘the ability to do work’, both important functions in the context of the industrial revolution.

During the industrial revolution, chemists and physicists were studying the combustion of fuels including coal, wood and gas. They wanted to compare the amount of coal burned to heat a certain amount of water from 15 degrees to 100 degrees is with the amount of wood or gas needed to heat the same volume of water by the same temperature difference.  They developed a unit to define this ‘ability to heat water’, the calorie, often abbreviated to cal.  

At the same time, physicists and engineers were studying moving objects, such as trains, pistons and levers. To compare different ways of moving objects, they created the concept of ‘the ability to do work’. This is a particular definition of ‘work’ used in physics, meaning a force causing movement of an object. The quantity of work is simply the force times the distance travelled.

Steam engines turn combustion of fuel into movement of objects, and so the two systems of understanding were brought together, and understood to be the same physical quantity: energy.

This connection of heat and movement, through the concept of energy, is called thermodynamics (meaning the study of heat – ‘thermo’ – and movement – ‘dynamics’) .

Units of energy

These different definitions have given rise to different units.

Work done = force x distance

In standard scientific units, the force needed to accelerate a mass of one kilogram by one metre-per-second is one Newton. A force of one Newton moving one metre uses one Joule (J) of energy.

The standard unit of energy considered as the ability to heat a defined mass of water a known amount is the calorie. This is the energy needed to heat one gram of water by one degree Celsius.

And we need a  conversion factor between the units of work and the units of heat energy:

1 calorie = 4.184 Joules  or 1 c = 4.184 J

Food labels often include the number of kilocalories (kcal, or 1000 calories), often referred to by the misnomer ‘calorie’.  The term ‘calorific value’ is sometimes used to mean the energy content of a fuel that is to be burned.

(We can also convert calories, or Joules, into other commonly used units of energy, such as kilowatt hours, or British Thermal Units.)

Power – energy used over time

Often, it is useful to know the rate at which energy is used or transferred  – how much electricity can a power station produce per second? How quickly can a car engine transfer enough energy from petrol to accelerate the car from zero to 60? How fast does the heat leave a warm house on a cold day? How bright is a light bulb? This is called power, a measure of the rate of transfer of energy. It is measured in a unit of Watts (W), which the number of joules transferred per second.

Power = energy transferred/time

Watts = Joules / second;

W = J / s

One watt is not very much power – a typical kettle uses around 2000-3000 W, which we can abbreviate as 2-3kW.  Prefixes kilo (k), mega (M), and giga (G) are often used to represent thousands, millions or billions of something. These and other prefixes are shown below (Table from Sustainable Energy Without the Hot Air – David MacKay – p328):

Prefix Kilo Mega Giga Tera Peta Exa

Power is often used to express the size of an engine, a power station or an electricity using device.

Here are a few examples:

  • Typical engine of a small car such as a Nissan Micra: 109 horsepower, or 81kW
  • One of the UK’s smallest coal fired power stations (Wilton) is  227MW ( 227,000kW), and the largest (Drax) is 3906MW (3,906,000 kW).
  • A typical electric kettle uses 2kW
  • Largest offshore wind turbines – 6-8MW – 6,000-8,000kW
  • The London Array offshore wind farm is 630MW, or 630,000kW, with 175 turbines each at 3.6MW.
  • Typical household solar PV array: 2kW peak (this is the maximum output at midday on a sunny summer day)

Another way of thinking about the difference between energy and power, is by analogy with distance and speed.  Achieving a task might require a certain amount of energy, and the time taken to complete the task depends on how fast that energy is deployed, or the power.  Similarly, to get from one place to another means travelling a certain distance, but the time taken to get to the destination depends on the speed you travel at.

The kWh – our main unit of energy

So far, we have talked about the basic unit of energy, the Joule, and the basic unit of power, the Watt, which is equal to one Joule per second. However, when measuring amounts of energy consumed in households, it is commonplace to use a unit of energy derived from units of power:  the kWh. kWh stands for kilowatt hour, or kW x hours. If we measured our electrical energy in Joules all the time we would have to use very large numbers! If we ran our 2kW kettle continuously for 1 hour it would then consume 2kWh of electricity. Although we wouldn’t run a kettle for an hour, the kWh is a convenient measure of the energy consumed by everyday electrical and heating appliances. 

Energy / time = power  J/s = W or 1000 J/s = 1 kW (remember, k means x 1000)

Energy = power x time 1000 W x 60s = kWh

One kWh is 1000 times one Watt times 1 hour. This is a bit like saying that a distance of 60 miles is the same as one hour times 60 mph. Of course, that assumes that we are driving consistently at 60mph. In practice, maybe the car stops at traffic lights, slows down for a roundabout, goes on a dual carriageway at 70 for a while. If we want to know how far we’ve travelled in an hour, we need to know the average speed. Many electrical appliances have a ‘rated’ power, or maximum power, but on average use much less. Fridges, for example, cycle on and off throughout the day (you may have heard the fridge get loud for a few minutes, then quieten).

The amount of energy used by some example appliances, and their peak (maximum) and average power consumption, is shown below:

Appliance power consumption hours per week energy use in in one year
EV saloon car charging 7000W 21hrs 7600 kWh
Immersion heater 3000W 28hrs 4400 kWh
Electric shower 9000W 3.5hrs 1600 kWh
LED bulb 10W 30hrs 16 kWh
Incandescent bulb 100W 30hrs 156 kWh
Fridge-freezer 300W 30hrs 450 kWh
LCD TV 200W 21hrs 220 kWh
Microwave oven 1000W 2hrsd 100 kWh
Steam iron 2000W 1.5hrs 150 kWh
Tumble dryer 2500W 7hrs 900 kWh
Smartphone charger 5W 28hrs 7 kWh
W-fi router 7W 168hrs 9 kWh

This guide uses the kWh as the main unit of energy. 

There are many different units of energy, just as there are metric and imperial systems of measuring mass and distance (kilogram vs pound, meter vs yard…).  Other units of energy that are commonly used in the context of energy systems include:

barrel of oil equivalent (boe) = 1628.2 kWh (not surprisingly, this comes from the amount of energy contained in a barrel of oil)

British Thermal Unit (btu) = 0.000293 kWh (1000btu = 0.293kWh)

kilotonnes of oil equivalent (ktoe) = 11,630,000 kWh [11.63 GWh]

You might find these units in government energy statistics.  All of these different units make comparison tricky, which can be annoying. It is easy to convert between different units with a calculator or using an online unit conversion website such as unitjuggler.

You can’t get anything for nothing: conservation of energy and the first law of thermodynamics

Energy can be transformed or converted from one form to another, but it can never be created or destroyed. Just as a rock can be smashed into sand, then compressed into sandstone, and the amount of matter remains the same at each stage (if it’s a closed system, you can’t lose any sand), energy can be converted from chemical energy in a lump of coal, to heat, to movement, and at each stage the total quantity of energy remains the same.

This is also known as the first law of thermodynamics, or the law of conservation of energy.

This rule also applies when we are converting energy in a power station. Sometimes it can seem like we ‘lose’ energy, but this is usually because some energy is converted to heat, which dissipates into the surroundings. We usually say that this energy is ‘lost’, because we can no longer use it, but that doesn’t mean it has completely disappeared! Just like when the dog eats your homework, the homework still exists, just inside the dog’s stomach.

Similarly the amount of energy arriving to the earth from the sun is normally equal to the amount leaving the earth in the form of heat and light. If it isn’t the earth will get hotter (this is why we have global warming). 

You can’t even break even: the second law of thermodynamics or the law of ever-growing mess

It takes much less effort to make a mess than to tidy it – that is why they say “a woman’s work (of cleaning) is never done”.  It is easier to produce disorder, or disorganisation, than it is to produce order, or organisation. This is quantified in chemistry and physics as ‘entropy‘ – the more mess, the higher the level of entropy.

Although the total quantity of energy in the world is always conserved, the quality, or level of organisation of the energy is always reduced when it is transformed. This means that the ‘ability to do work’ is reduced after every transformation – some of the ‘useful energy’ is lost as heat.

This is the second law of thermodynamics.

Gravitational potential energy is a highly ordered form of energy – a mass, held at a height, waiting to fall down the hill. Heat, on the other hand is very disordered – many molecules are all travelling in different directions.

For example, in a power station, a tonne of coal has a certain calorific value or energy content, measured in BTU, or Joules.  When it is burned, all of that energy content is conserved, but only some of it turns into the mechanical energy that turns the electric turbine, and only some of that turns into electricity. Energy is ‘lost’ as heat at each stage in the process. Temperature differences between different stages in the process are part of what drive the turbine, so thermal power stations need to actively get rid of heat as part of the process, either using cooling towers or water from rivers or the sea.  The energy balance – conserving the total amount of energy as per the first law of thermodynamics, is as follows:

Calorific value of coal burned (J) = waste heat (J) + electricity produced (J)

It is not possible to get useful work out of all of the primary energy that we consume. Even when what we want is heat, we want that heat to be in a particular place (e.g. in a house), and some of it will always escape.  Technology and innovation can reduce the amount of loss, or waste, but can never completely eliminate it.

Any energy conversion technology with no losses would be a perpetual motion machine – a physical impossibility.

Primary energy, energy converters, energy carriers (or vectors) and energy storage

It is helpful to distinguish between primary sources of energy (e.g. fossil fuels, solar energy, wind); energy converters (e.g. wind turbine, coal power station), energy carriers, also known as energy vectors, such as electricity, heat or fuel; and energy storage.

Primary energy sources converted
Energy system component to Energy carriers (main)
Non-renewable[nb 1] Fossil
Oil (or crude oil) Oil refinery Fuel oil
Coal or natural gas Fossil fuel power station Enthalpy, mechanical work or electricity
Natural uranium[nb 2] Nuclear power plant (thermonuclear fission) Electricity
Natural thorium Thorium breeder reactor Enthalpy or electricity
Renewable Solar energy Photovoltaic power plant (see also Solar power) Electricity
Solar power tower, solar furnace (see also Solar thermal energy) Enthalpy
Wind energy Wind farm (see also Wind power) Mechanical work or electricity
Falling and flowing water, tidal energy[6] Hydropower station, wave farm, tidal power station Mechanical work or electricity
Biomass sources Biomass power plant Enthalpy or electricity
Geothermal energy Geothermal power station Enthalpy or electricity

Table from Wikipedia

All usable energy sources come from a limited number of primary sources:  the sun’s radiation, the gravitational interaction of the sun and moon with the Earth, geothermal heating, and nuclear processes on Earth. Our main source of energy has been fossil fuels, but ultimately these came from the deposits of organic matter produced by prehistoric plants and animals, which used photosynthesis, and ultimately this is energy from the sun.  So when we burn fossil fuels, we are burning many years’ worth of buried sunshine. The fossil fuels burned in 1997 would have needed 400 times all of the biological matter produced each year now to be produced.  Wind power comes from air movement caused by the sun heating the atmosphere, and waves are caused by wind over the oceans, so when we use wind, wave or solar power we are using current sunshine – at a sustainable rate rather than tapping into many years worth of saved energy.

Primary energy is converted by energy converters such as wind turbines, solar PV panels, or power stations into directly useful forms or transportable forms, including electricity, heat or hydrogen.

Energy carriers or vectors enable energy to be transported from one place to another. Electricity and hydrogen are energy vectors rather than primary sources of energy.  Some primary sources of energy are also energy vectors as they can be directly transported (e.g. shipping coal or pumping oil or gas through pipelines).

We need more energy at certain times than others, and some forms of primary energy are only available at certain times. This means that energy needs to be stored. Electricity can only  be stored on a small scale, in a device known as a capacitor. Use of batteries for storing electricity actually means converting electricity to chemical energy and back again (see physics of electricity page). Fuels such as wood, coal or oil can readily be stored.  Heat can be stored to some extent e.g. in insulated tanks of hot water, but some is lost over time.


Given that there is always a loss in useful energy in any conversion process, it is useful to be able to quantify how good a process is at converting energy from the form that we have, to a form that we want.   Efficiency is the measure of how much we get of what we wanted, relative to what we put in. In a 100% efficient process, we would get out the same effort as we put in, but we’ve already seen this is physically impossible. In general, energy efficiency can be measured as follows:

Energy efficiency = useful energy out / total energy in

And remembering the law of conservation of energy, there is an ‘energy balance’:

total energy in = useful energy out + non-useful energy out

When we burn coal in a power station, the useful energy out is the electricity produced, and the energy in is the calorific value of the coal. The overall efficiency of the power station is therefore defined as follows:

Efficiency of power station = electricity produced (J) / calorific value of coal burned (J)

Where the energy balance is:

Calorific value of coal burned = Electricity produced + waste heat

Efficiency is a ratio of one thing relative to another, which can be expressed as a percentage. The two things must have the same unit – both electricity and calorific value of coal are measured in Joules.

Similarly when we switch on an electric light bulb, electricity is converted into useful light, and waste heat.  

Efficiency of light bulb = light produced (J) / electricity consumed (J)

There are physical limits to how much electricity can be obtained from a thermal power station. However, we can increase the ‘efficiency’ beyond this point by making use of some of the waste heat produced, e.g. for heating houses or for industrial processes in a combined heat and power plants.  The energy balance then changes:

Calorific value of coal (J) = electricity (J) + useful heat (J) + waste heat (J)

The overall efficiency could then be measured as:

Overall efficiency = (electricity (J) + useful heat (J) ) / calorific value of coal (J)

Often, it is useful to break this down into two different efficiencies:

Thermal efficiency = useful heat (J) / calorific value of coal (J)

Electrical efficiency = electricity (J) / calorific value of coal (J)

There may be trade-offs between the thermal efficiency and the electrical efficiency, and designing the combined heat and power plant to produce more heat, or to produce more electricity, may affect the overall efficiency.

Economists like to use the term ‘efficiency’ as though this was an absolute thing with no ambiguity. However, the term efficiency is meaningless if we do not define what the ‘useful’ outcomes are, and what the ‘total inputs’ are. The useful outputs measured by one person might be different to those measured by another. Similarly, the inputs measured by one person might be different to those measured by another (is unpaid care work including cooking and cleaning included in the ‘cost of labour’?).

Increasing energy efficiency is sometimes assumed to automatically lead to a reduction in demand. However, it can lead to using more energy in total, as discussed in more detail here.

Illustration by Liz Snook

Storage of electricity or heat also involves the conversion of energy from one form to another, and leads to losses.   Some efficiencies for energy transformation processes and energy storage are listed in the table below.

Source: Wikipedia

Conversion processConversion typeEnergy efficiency
Electricity generation
Gas turbineChemical to electricalup to 40%
Gas turbine plus steam turbine (combined cycle)Chemical/thermal to electricalup to 60%
Water turbineGravitational to electricalup to 90% (practically achieved)
Wind turbineKinetic to electricalup to 59% (theoretical limit)
Solar cellRadiative to electrical6–40% (technology-dependent, 15-20% most often, 85–90% theoretical limit)
Fuel cellChemical to electricalup to 85%
World Electricity generation 2008Gross output 39%Net output 33%[8]
Electricity storage
Lithium-ion batteryChemical to electrical/reversible80–90% [9]
Nickel-metal hydride batteryChemical to electrical/reversible66% [10]
Lead-acid batteryChemical to electrical/reversible50–95% [11]
Combustion engineChemical to kinetic10–50%[12]
Electric motorElectrical to kinetic70–99.99% (> 200 W); 50–90% (10–200 W); 30–60% (< 10 W)
TurbofanChemical to kinetic20-40%[13]
Natural process
PhotosynthesisRadiative to chemicalup to 6%[14]
MuscleChemical to kinetic14–27%
Household refrigeratorElectrical to thermallow-end systems ~ 20%; high-end systems ~ 40–50%
Incandescent light bulbElectrical to radiative0.7–5.1%,[15] 5–10%[citation needed]
Light-emitting diode (LED)Electrical to radiative4.2–53% [16]
Fluorescent lampElectrical to radiative8.0–15.6%,[15] 28%[17]
Low-pressure sodium lampElectrical to radiative15.0–29.0%,[15] 40.5%[17]
Metal-halide lampElectrical to radiative9.5–17.0%,[15] 24%[17]
Switched-mode power supplyElectrical to electricalcurrently up to 96% practically
Electric showerElectrical to thermal90–95% (multiply with the energy efficiency of electricity generation for comparison with other water-heating systems)
Electric heaterElectrical to thermal~100% (essentially all energy is converted into heat, multiply with the energy efficiency of electricity generation for comparison with other heating systems)
FirearmChemical to kinetic~30% (.300 Hawk ammunition)
Electrolysis of waterElectrical to chemical50–70% (80–94% theoretical maximum)

Primary energy sources and Energy Return on Energy Invested

UK energy statistics are often presented in terms of primary energy and final energy, where primary energy is the raw fuel arriving into the system, and final energy is the useful energy used by households and industry.  The difference is the energy ‘lost’ in the energy industry itself, e.g. heat rejected in cooling towers in thermal power stations or heat lost in electricity transmission cables.

A good way of depicting the losses in the energy system, from ‘primary’ energy sources to the useful energy provided to consumers, is a sankey diagram, or flow diagram.  The sankey diagram for the UK energy system in 2017 is shown below:

Gas Stocks Coal Stocks Product Stocks Crude Stocks 40.0 45.1 1.9 6.5 NA TURAL GAS 85.2 COAL 8.5 ELECTRICITY 22.5 IMPORTS 3.5 5.8 HYDRO, WIND & SOLAR 15.1 PETROLEUM 146.1 NUCLEAR 36.7 50.9 58.5 75.4 10.8 0.4 24.6 5.5 0.4 42.6 25.9 1.8 1.4 0.9 32.6 30.2 2.6 Primary Supply 200.1 Primary Demand 199.9 4.3 3.1 0.5 0.1 71.4 7.5 4.3 28.0 42.0 Coal Gas OIL REFINERIES POWER STATIONS OTHER TRANSFORMATION 22.2 10.2 6.1 10.2 22.2 22.2 CONVERSION LOSSES 35.8 ENERGY INDUSTRY USE AND DISTRIBUTION LOSSES 15.0 NON-ENERGY USE 8.0 EXPORTS AND MARINE BUNKERS 81.9 Primary demand 76.5 IRON & STEEL 0.9 OTHER INDUSTRY 23.2 TRANSPORT 56.5 DOMESTIC 40.1 8.3 4.3 0.9 0.7 55.1 0.4 25.5 0 . 4 2.5 0 . 4 2.2 8.0 2.0 OTHER FINAL CONSUMERS 20.5 0.3 0.3 Primary supply 76.4 2.6 FOOTNOTES: 1. Coal imports and exports include manufactured fuels. 3. Includes heat sold. 2. Bioenergy is renewable energy made from material of recent biological origin derived from plant or animal matter. 4. Includes non-energy use. This flowchart has been produced using the style of balance and figures in the 2018 Digest of UK Energy Statistics, Table 1.1. (gross calorific values basis) 0.2 0.3 Electricity 0.3 BIOENERGY 16.4 1.6 12.9 16.0 16.0 0.4 Bioenergy 1.2 1.0 1.8 6.1 9.4 Gas IMPORTS Coal IMPORTS 1 Electricity HYDRO, WIND & SECONDARY ELECTRICITY Manufactured Fuels 3 Crude Oil and NGL IMPORTS Refined Products IMPORTS Petroleum IMPORTS Bioenergy 2 Energy Flow Chart 2017 (million tonnes of oil equivalend) INDIGENOUS PRODUCTION AND IMPORTS 278.6 T O T A L FINA L CONSUMPTION 4 149.1 7.7 9.1 8.4 75.0 0.2 SOLAR, IMPORTS &

On the left, primary energy inputs enter the energy system. On the right, final energy uses are shown. Energy lost is shown exiting the UK system at the bottom of the diagram. The thickness of each line of flow represents the amount of energy. According to the first law of thermodynamics, total energy should be conserved – i.e. the sum of the final energy on the right, and the lost energy at the bottom, should equal the incoming energy on the left.

There are also energy losses that take place before the energy enters the sankey diagram – the fuel burned in ships and mining machinery, cooling and compression of natural gas for transport as liquefied natural gas or LNG, exploration, drilling and pumping of oil, manufacture of solar panels and wind turbines.  We can calculate a ratio of the final energy available to the energy used to make it available – this is called the ‘energy return on energy invested’, or EROI. If a form of energy has an EROI of 1, that would mean that all of the energy produced is used in production – i.e. it is completely useless. If it has an EROI of 30, that means that 30 times the energy is produced as is used in production, so it’s providing a lot of energy value to society at low energy cost.

All economic processes use energy, so the availability of abundant energy at low energy cost supports high levels of economic growth. In addition to measuring the EROI of particular energy sources, one can measure the ‘societal EROI’ – the average EROI of a society.  Studies show strong correlation between EROI and economic growth. Fossil fuels have a very high EROI, of the order of 30:1, compared to biomass and wood of the order of 4:1. Wind power has an EROI of about 18:1.

The use of fossil fuels during the 19th and 20th centuries enabled high levels of economic growth.  The EROI of unconventional fossil fuels, e.g. shale oil or shale gas, is much lower than conventional fossil fuels (around 4:1 for Canadian tar sands), and even the EROI of conventional fossil fuels has reduced dramatically over time (global oil and gas in 1999 EROI 35, in 2006 was 18), as the best resources were used first.

Additionally, renewable energy sources from wind and sun are reliant on energy being stored. If the energy needed to create the storage is also included, this reduces the EROI of those forms of energy.  The EROI of various fuels and renewable energy sources, with and without storage is shown in the image below.

0 10 20 30 40 50 60 70 80 with energy storage without energy storage Nuclear Hydro Coal Gas CCGT Solar CSP Wind Biomas Solar PV 7 with energy storage Energy Returned On Investment Relative to the breakeven value of 1

The exact methodology used for measuring EROI is important, particularly as the boundary of what is included can be set at different points (e.g. do you count the energy used to build the roads that the oil tankers drive on). Additionally, some EROI calculations take into account the quality of the final energy. If we measure the energy needed to produce electricity from different primary energy sources, we get a very different picture.

The image below shows the EROI for the thermal energy available from different fuels

0 20 40 60 80 100 EROI Diesel from Biomass Ethanol from Biomass Oil Shale Tar Sands Oil and Gas (World) Coal

The following image shows the EROI for electric energy available. The high losses in converting coal to electricity means that coal and gas have a lower EROI for electricity than wind and solar PV. This does not take into account the energy used for storage.

0 20 40 60 80 100 EROI Solar (PV) Wind Geothermal Hydroelectric Nuclear Gas Coal

Things to note about EROI

  • The assumptions and boundaries of what is included make a big difference to the outcomes
  • It is good to think about how much energy is needed to produce energy, rather than just how much it costs financially (although these are often correlated)
  • If future societal EROI is going to be lower than EROI in the last 2 centuries, this has potentially serious implications for economic growth. This is being explored by the Transition Towns movement, the degrowth movement, and studies of prosperity without growth.
  • EROI is just one measure – the amount of land used to produce energy is also extremely important, as are other issues (including climate change, biodiversity, social impact etc). Some of these may be correlated with EROI.
Illustration by Liz Snook


There are many forms of energy, including heat, motion, electricity and stored energy in fuels, and energy can be converted from one form to another.  Every time energy is converted from one form to another, transported, or stored, some of it is ‘lost’ to waste heat. This means that however efficient we get, there will be limits to how much useful energy we can get per unit of primary energy.  Efficiency is the amount of useful energy you get out divided by the total amount of energy that you put in. All energy available originates from the sun, from gravity, or from nuclear reactions, and fossil fuels use many millions of years’ worth of buried sunshine. Energy and power are different – by analogy with water flowing into a bucket, energy is like the amount of water in the bucket, and power is the rate at which the water flows into the bucket. Or energy is like a distance travelled, and power is like the speed at which you travel.

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