Energy Defined

What is Energy? Energy is a fundamental constituent of our universe. Sometimes it’s easier to use examples. For instance, we say a car has lots of energy if it is moving fast with respect to a road. We say molecules are very energetic when they vibrate due to ‘heat’. We say pulsars emit energy as they blast cosmic rays out from their core. Energy equates with ‘something doing something’.

We also say that energy is neither created or destroyed. There is the famous equation relating energy to mass. But this is a transfer of energy rather than a loss or gain. This particular property makes it very suitable as a metric from which to assess civilizations’ past and future.

Let’s explore a bit more in the following.

Table of Contents:

  1. Energy
  2. Matter
  3. Our Sun
  4. Entropy
  5. Solar and Energy

Table of Tables:

1. Energy

The word energy gets readily bandied about. In this website, it’s meaning is reserved to be a fundamental unit of measurement. With this, energy is directly related to mass as in;

Eq. 1 Energy = Mass * (speed of light)2
– or – E = m c2

There is a choice of units for energy though each is interchangeable. This website uses a Joule. This is in honour of James Prescott Joule.

A calories is another unit of energy. With its association with food, particularly for dieters, it is perhaps better known. In a science laboratory, a calorie is the energy needed to raise the temperature of one gram of water by one degree Celsius (at standard temperature and pressure). A warning to the dieters. Their calorie which they see on food boxes is actually a thousand ‘lab’ calories. For lab work, the following relation holds;

Eq. 2 4.184 Joules = 1 calorie

This is the relationship used in this website.

Commonly, we relate energy to action. Often the action is considered to be work or heat. The following list relates energy
to action.

Table 1: Energy Usage by the Joule

Action Amount (Joules)
Yearly solar emissions 1034
Exploding volcano 1019
Energy to launch space shuttle 1013
One litre of gasoline 107
Human’s daily energy needs 107
Candy bar 106
Burning match 103
Human heartbeat 0.5

2. Matter

At one time we considered matter to be indestructible. That is, matter is neither created nor destroyed. Since then, we’ve learned about nuclear fission and fusion. Now we know that matter can be destroyed via fission as within a manmade nuclear reactor and created via fusion as within a star. But we consider matter/energy to be a constant as shown in Equation 1 above. This is significant with regard to entropy.

Energy is finite and regulated. Our Sun fuses hydrogen atoms into the bigger helium atoms and thus sends forth the energy. We see it as light, feel it as heat and watch it encourage plants to grow. This is a good thing as we eat the grown plants or other animals which themselves eat plants. Our biological energy needs are completely and solely satisfied by what we eat.

Our civilization also needs energy. As we started down the path of advanced civilizations, we simply burned wood for cooking food. Later we harnessed wind power using sails and directed water flow onto wheels to grind wheat into flour. Now, we’re splitting atoms to use the huge quantity of released energy to send electricity through our power grids. But the most ubiquitous source of energy is the fossil fuels. Petroleum, natural gas and coal are the real energy provides for
our civilization today.

These fossil fuels power electrical generating stations, push airplanes through the skies, and, propel cars along the freeways. Yet, these energy sources aren’t limitless. They all started as plants about some hundreds of millions of years ago. These plants captured the Sun’s energy, just like plants of today, however, they weren’t able to release it. Thus the energy accumulated and through a variety of processes they were transformed. Given the
hundreds of millions of years for this process, we can readily consider fossil fuels as non-renewable. Hence, we will eventually consume all that is available today.

The following numbers give an indication as to the degree of dependence upon fossil fuel in our civilization today.

3. Our Sun

The Sun is our energy source. Measurements in space have shown that a fairly steady energy flux arrives at the top of our atmosphere. Similar measurements quantify the amount of energy that reaches the land surface. We can consider this energy to be renewable. This energy limit is all that is available for all living things on Earth.

Table 2: Estimated availability

Energy flow
From sun to top of atmosphere 1387 Watts per square metre
To land surface 1000 Watts per square metre, high noon
To land surface, average 240 Watts per square metre, average over Earth
To land surface, in Joules 1.127 x 1024 J/yr

Effectively all the useful energy available to people comes from the Sun. This energy comes in a variety of shapes and forms. Because of it, the wind blows. Because of it, water evaporates from the oceans and falls onto the highlands so as to allow the water to flow back down to the oceans. Direct rays power photosynthesis in plants.

Table 3: Transfer to Living Things

Solar power incident on earth, rate 1 x 1017 Watts
Solar power incident on earth, yearly total 3.2 x 1024 J/yr
Solar power incident on land 2.2 x 1024 J/yr
Maximum solar power taken by net primary producers on land 2.2 x 1023 J/yr
Maximum solar power taken by herbivores on land 1.1 x 1022 J/yr
Maximum solar power taken by carnivores 5.5 x 1020 J/yr
Global human energy consumption 4 x 1020 J/yr

The above table quantifies the theoretical maximum energy available. It assumes a 5% efficiency rate in transferring energy from plants to nimals and from animals into other animals. From this, in theory, humans are now capturing for themselves almost all the available energy or carnivores. True, the energy source is now mainly fossil fuels but this serves to highlight that without fossil fuels, we’d have difficulties meeting our energy needs.

Table 4: Energy Consumption

Human Energy Consumption (2002)
Global human energy consumption 4 x 1020 J/yr
Primary energy consumption 9405.0 mtoe
Primary energy consumption 3.95 x 1020 J/yr
Primary energy consumption – oil 1.48 x 1020 J/yr
Primary energy consumption – natural gas 0.96 x 1020 J/yr
Primary energy consumption – coal 1.0 x 1020 J/yr
Primary energy consumption – nuclear 0.26 x 1020 J/yr
Primary energy consumption – hydro 0.25 x 1020 J/yr

The above table shows where people get their energy. It is interesting to note that the above value for the primary energy consumption doesn’t even include biomass. That is, the amount of energy we derive from wood and other vegetation is so small that it isn’t worth including.

The units of mtoe are million tons of oil equivalent. It’s a huge amount of energy and is regularly used in annual summaries for national consumption. One mtoe has about 4 x 1016 joules of energy.

Table 5: People’s Biological Needs

Minimum Energy Intake
Average human male 2500 kCalories/day
Average human male 10.5 megajoules/day
Earth’s human population (2005) 6 413 956 859
Base living requirements 2.452 x 1019J/yr

But is all this energy really used by our civilization? Surely we must be using energy just to keep our bodies functioning? This is true, our bodies do need energy. But, we are using much more as the table above shows. With over 6 billion people alive today, and assuming each needs the average adult male energy requirements, our bodies use a little over 2 x 1019joules of energy each year. This is a small fraction of the amount of the primary energy consumption of our civilization which we saw in the preceding table.

With the above, we see that now our question gets a bit more complicated. A lot of people live on the Earth’s surface. They all need to eat to live. Many want the comforts of an advanced civilization which requires lots of energy. The Earth is finite. Eventually we will reach a maximum number of people and a maximum energy consumption. At least this is true if we rely solely upon renewable energy sources. However, if we rely upon non-renewable energy sources and the supply of energy runs out, what then?

Table 6: Our Earth

Earth Parameters (area in km2)
Area of Earth’s land surface 148 847 000
Snow land – 20% 29 769 400
Dry land – 20% 29 769 400
Mountains – 20% 29 769 400
Farmable land – 30% 44 654 100
Land with no topsoil – 10% 14 884 700
Area of Earth’s water surface – 70% 361 254 000

The Earth is magnificent and has the necessary features to support life. Yet, even with its fantastic facilities, it isn’t perfect. Only a small fraction of the Earth’s surface is suitable for farming. Farms are where we get our energy needs. Or, at least our biological energy needs. The question remains as to whether the farms can support today’s current population and the level of civilization to which they are accustomed. The reference text looks at this question and considers a future without the energy we derive from fossil fuels.

4. Entropy

Entropy is a simple concept with far reaching importance. Entropy tells us that energy, though indestructible, becomes less able. This is our experience on Earth and, except for black holes of outer space, seems to hold for everywhere in the universe. Less capable means that energy, once used, can’t do as much work or create as much heat.

Let’s picture this idea. Imagine a lake filled with water. At one end of the lake, the water empties through a river and descends over a waterfall. At the base of the waterfall is a mill. The mill uses the water to spin stones that grind the grain to make flour. After the water spins the stones, it flows to another lower lake. In this closed example, water is neither created nor destroyed. The water that starts at the top lake has the ability to do work, which it does when it descends through the waterfall and turns the wheel. The water at the bottom lake is the same to all appearances as the water in the top lake but it can’t do work. It’s ability has gone. That’s the picture.

Energy is like the water in this imaginary scenario. It exists. It starts with the ability to do work or perform an action which it naturally will do. Once it does so, it remains but its ability is gone. The entropy relates to where energy exists in this imaginary scenario. Energy that is akin to water in the top lake has the greatest amount of energy. Energy that is akin to water in the bottom lake has the least amount of energy. And, like water being unable to flow uphill, entropy of a system will not naturally improve.
What does entropy mean for us on Earth? Well, consider our solar system as a closed system. It has a given entropy. This entropy will naturally increase (capability decrease). Though there may be localized areas of the solar system where entropy decreases, on the whole it increases. The same is true for any system on Earth. Small locations may see drops in entropy especially with man-made works such as electrical generating station. But expand the boundaries of these small locations for the system to include the complete thermal cycle and the natural tendency to increase entropy remains.

The implication of entropy for humanity is that energy capability is naturally continuing to decrease. As we burn wood in furnaces, capture photons with photo-voltaic cells or split atoms in reactors, we make a brief local increase in energy ability which we use. But overall we increase entropy. In effect, our action increases the size of the topmost river in our imaginary scenario. Our actions allow more water to descend over the falls and driver more mills. But this means the water more quickly loses its capability. And we know it never recovers this.

This one-way nature of energy should be a warning to humanity. There is lots of energy about us but its entropy is continually increasing. Would a wise person be sure to make the best utility of the available energy? Do we make the best choices? Take a look at the consumption patterns to see where we are using energy today.

One question may still perplex you. Where does energy’s ability go to? Simply, it dissipates as heat. Cold areas get warmer. Cold areas don’t get colder. As heat energy radiates out from stars or planets, the energy transfers to the atoms spread throughout the universe. If this were the only force, eventually all material objects of the universe will all have the same temperature. But that temperature would still be very darn cold so don’t expect any paradise.

5. Summary

Energy is fundamental. And energy is a great metric with which to measure civilizations. Let’s continue on and consider the most obvious source of energy for humankind. It’s our nearest start, the Sun!


6. The Sun

Our Sun is one big fusion reactor. Hydrogen atoms fling about throughout its volume. They sometimes hit each other, fuse, and, make Helium. Helium makes Oxygen. Oxygen fuses into Carbon. When two get together to make a third, the third has a little bit less mass. This mass loss appears as electromagnetic energy. The energy spreads somewhat unevenly across the infinite spectrum. But, the majority of it exists as visible light.
Solar spectral irradiance

(from wikipedia/commons)

7. Sunshine to Earth

Because of the fusion of atoms on the Sun, light energy, is thrown in all directions. Energy aimed toward the Sun gets absorbed by the Sun. Energy aimed away from the Sun continues on until it meets something sold. When we see stars at night, our eyes are stopping the light energy or photons that came from another fusion reactor in space. As our Sun is so much closer than other stars then much more light energy reaches us. This energy, known as the solar wind, acts very much like the wind that flows at the surface of the Earth. At the top of Earth’s atmosphere, the Sun delivers 342 watts per square metre, non stop, and with very little variation.

8. The Magnetosphere

Next to nothing inhibits the Sun’s energy from reaching the top of Earth’s atmosphere. But, once the solar wind comes into the influence of the Earth, things begin happening. First, the magnetosphere directs some energy and particles away as shown in the following.

from NASA

Though the magnetosphere greatly decreases the energy in the solar wind, some continues on toward Earth. But on encountering the Earth’s atmosphere, again more energy leaks out of the wind. The following gives an idea of the average energy disbursement.

from NASA

9. Insolation

No matter where we stand on Earth, we always say that the Sun is up in the sky. Just look up and the great yellow star will be shining down upon you. But, the amount of annual light or light energy is different for every different location on Earth. Locations on the equator are closer to the Sun and thus will have great values. Locations at either poles will have much smaller values as the Sun is never directly overhead and, for weeks at a time, can be completely absent.

For the United States, the following shows the average annual solar radiation at ground level in kWh/m2/day.


As to be expected, Alaska being farther from the equator, has low values of about 3 kWh/m2/day (avg of 11MW/day). However, places along the countries southern border which are also at high altitudes have the highest annual radiation of about 7 kWh/m2/day (avg of 25 MW/day).

10. Power Generation

We need energy to power our bodies and our mechanical devices. We can’t eat solar energy but we can collect it and have it do our bidding. Many different ways exist to do this. But, the largest solar powered electrical uses the well known process of using the energy from the solar wind to heat a fluid. The heat is transferred to water which becomes steam. The steam enters a turbine which in response rotates. The rotation generates the electricity as in the following.


Though the Sun can feel hot, it’s not enough to power this system. So, mirrors focus the Sun’s energy onto a fluid, in this case oil, and the heated oil supplies the energy to heat the water. This is the technique used at a solar power electrical generating station in the Mojave desert of the Unites States as seen in the following.

from ReflecTech from Sandia

The following table has the value of the captured energy.

SEGSLocation in Mojave desertArea
(h)Gross annual solar production
of electricity (MWh)

I Daggett 8.3 16500
II Daggett 16.5 32500
III Kramer Junction 23 68555
IV Kramer Junction 23 68278
V Kramer Junction 23.3 72879
VI Kramer Junction 18.8 67758
VII Kramer Junction 19.4 65048
VIII Harper Lake 46.4 137990
IX Harper Lake 48.4 125036
Total 654544

In the above table, the gross annual solar is the four year average from 1998 to 2000 (see here).

The nine solar collection sites produce an annual 654544 MWh from fields covering 890 hectares. Or, the sunshine on the ground annually provides 2715 kWh/m2 of which we capture 73 kWh/m2, about a 2.7% efficiency. Afterward, some electricity is lost through conversion, delivery and the final consumption by us, the consumer. Hence, we will never realize the full potential of the Sun but we can still benefit.

11. Summary

The Sun provides 342 watts /m2 or 1.1e10 joules/m2 per year at the top of the atmosphere. But, the atmosphere greatly affects the solar wind so only a small fraction reaches the Earth’s surface. As well, the Earth spins so only part of it receives solar energy at any given time. The better locations on Earth’s land surface have an insolation of 7 kWh/m2/day or on average 25.2e7 Joules/m2/day or about 9e9 Joules/m2 annually. Using today’s advanced technology, we can capture the Sun’s energy and generate about 73 kWh/m2 annually. A typical refrigerator uses over 700 kWh per year. The people of the United States consumed over 3e13 kWh in the year 2007. The United States would need 4.2e7 hectares of equivalent solar collectors if the Sun was the primary source for all their power needs. This area is more than half the area of the complete state of Texas.

12. Energy Values

Energy is neither created nor destroyed. Rather, it loses its potential to do work. Humans lower stores of potential when they want work done. A wood burning fire is an example. Fissioning a nucleus is another. Energy loss is usually associated with the outflow of heat.

Energy stores come in many shapes and sizes. As well, we have different methods of applying the energy’s potential. when it comes to providing energy, we aim for a method that provides more than the effort in making its provisions. We can represent this as a ratio. The following table shows some ratios.

Source R3 Energy Ratio.


Input % of
lifetime output
Hydro Uchiyama 1996 50 2.0
Held et al 1977 43 2.3
Quebec Gagnon et al 2002 205 0.5
Nuclear (centrifuge enrichment) see table 1. 59 1.7
PWR/BWR Kivisto 2000 59 1.7
PWR Inst. Policy Science 1977* 46 2.2
BWR Inst. Policy Science 1977* 43 2.3
BWR Uchiyama et al 1991* 47 2.1
Nuclear (diffusion enrichment) see table 1. 21 4.8
PWR/ BWR Held et al 1977 20 5.0
PWR/BWR Kivisto 2000 17 5.8
Uchiyama 1996 24 4.2
PWR Oak Ridge Assoc.Univ. 1976* 15.4 6.5
BWR Oak Ridge Assoc.Univ. 1976* 16.4 6.1
BWR Uchiyama et al 1991* 10.5 9.5
Coal Kivisto 2000 29 3.5
Uchiyama 1996 17 5.9
Uchiyama et al 1991* 16.8 6.0
unscrubbed Gagnon et al 2002 7 14
Kivisto 2000 34 2.9
Natural gas – piped Kivisto 2000 26 3.8
Natural gas – piped 2000 km Gagnon et al 2002 5 20
LNG Uchiyama et al 1991* 5.6 17.9
LNG (57% capacity factor) Uchiyama 1996 6 16.7
Solar Held et al 1997 10.6 9.4
Solar PV rooftop Alsema 2003 12-10 8-10
ground Alsema 2003 7.5 13
amorphous silicon Kivisto 2000 3.7 27
Wind Resource Research Inst.1983* 12 8.3
Uchiyama 1996 6 16.7
Kivisto 2000 34 2.9
Gagnon et al 2002 80 1.3
Aust Wind Energy Assn 2004 50 2.0

* In IAEA 1994, TecDoc 753.
Source is here.
All these ratios come with lots of assumptions and considerations. Compare the above to the following table’s values to see this.
Heinberg in his book “The Party’s Over” provides similar ratios which he defines as net energy. His values (which he attributes to others) follow.

Process Energy Profit Ratio
Nonrenewable Oil and gas (domestic wellhead, 1940s) Discoveries > 100
Oil and gas (domestic wellhead, 1970s) Production 23
Coal(mine mouth) 1950s 80
Coal(mine mouth) 1970s 30
Oil shale 0.7 to 13.3
Coal liquefication 0.5 to 8.2
Geopressured gas 1 to 5
Renewable Ethanol (sugarcane) 0.8 to 1.7
Methanol (wood) 2.6
Solar space heat, flat-plate collector 1.9
Electricity Production Coal, US averagetd> 9
Hydropower 11.2
Nuclear (light-water reactor) 4.0
Solar, power tower 4.2

However, these ratios only consider the application of energy by humans to obtain energy for humans. Precious little consideration or value gets considered for any loss of energy capture by autotrophs. A mine lays waste thousands of hectares and may take hundreds to thousands of years to return to a pristine state. The ratios don’t include this consideration.

Energy By the Numbers

13. Food

From USDA Home and Garden Bulletin Number 72.

Food Mass(g) Energy(MegaJoules) Density(kJ/g)
Cheddar Cheese 28 0.477 17
Milk 2% 244 0.507 2.07
Halibut 85 0.498 5.86
Rice, wild 164 0.694 4.24
Apple 138 0.339 2.46

14. Fuel

From ORNL, these sources of energy are physical heat rather than edible food. These liquid fuels assume standard temperature and pressure.

Fuel EnergyDensity Units
Ethanol HHV 23.4 MJ/litre
Propane 25.5 MJ/litre
Automotive Gasoline 34.8 MJ/litre
Jet Fuel (naptha) 35.5 MJ/litre
Crude Petroleum 38.5 MJ/litre
Diesel Motor Fuel 38.7 MJ/litre

We can also use natural solid material to provide (heat) energy. The following values also come from ORNL.

Fuel EnergyDensity Units
Switchgrass 17.1 MJ/kilogram
Wood HHV, bone dry 22 MJ/kilogram
Coal Anthracite 25.2 MJ/kilogram

HHV higher heating value

15. Needs

Human’s Annual Energy Needs, with drastic assumptions about activity levels.

HumanTime of Life Joulesx109
Infant 1.1456
Child 2.75
Male 3.36
Female 3.05


16. Activities

In a society grounded upon electricity, we’ve come to rely upon its utility. Even if wonderfully optimized, we use lots. The following associates a common household activity with its energy consumption.

Activity Energy (kWh) Joules
Clothes Dryer (1 load) 1.89 6792453
Clothes Washer (1 load/hot wash) 6.42 23094000
Clothes Washer (1 load/cold wash) 0.94 3396226
Electric Stove (1 family meal) 4.15 14943396
Dishwasher (1 load) 3.02 10867924
AC Central 20 degrees (1 hour) 2.64 9509433


17. Embodied Energy

Objects can release energy via chemical reactions as with fire. Usually, humans put much more energy into fabricating a product than what can ever come out. This emplaced energy or embodied energy, as seen in the following table, gives an indication of this great disparity.

Embodied Energy

MJ/kg MJ/m
Aggregate 0.10 150
Stone (local) 0.79 2030
0.94 2350
Concrete (30Mpa) 1.30 3180
Concrete precast 2.00

Brick 2.50 5170
Cellulose insulation 3.30 112
Steel (recycled) 8.90 37210
Steel 32.00 251200
Plywood 10.40 5720
Glass 15.90 37550
Fibreglass insulation 30.30 970
Zinc 51.00 371280
Brass 62.00 519560
PVC 70.00 93620
Copper 70.60 631164
Paint 93.30 117500
Linoleum 116.00

Polystyrene Insulation

117.00 3770
Carpet (synthetic) 148.00 84900
Aluminium (recycled) 8.10 21870
Aluminium 227.00 515700