What is energy?

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Energy is vital for our lives: our bodies run on the energy we get from the food we eat, which requires energy to farm, process, transport and cook. Our industries, commerce, education, health-care, communications and recreation all consume energy. Some of this energy comes from the sun, such as our food, and biomass (such as firewood), which are produced by plants converting solar energy into chemical energy via photosynthesis. Water power driving water wheels, and turbines to drive mills and produce hydroelectricity are also powered by solar energy which evaporates water from seas to produce rain which falls from higher altitudes. However most of the energy we use now comes from burning fossil fuels: coal, oil and gas. These are also derived from energy from the sun, but energy that has been built up over millions of years, and which we are burning millions of times faster than it was stored. Our food, biomass and fossil fuels all release Carbon Dioxide (CO
) into the atmosphere, which causes global warming, and dissolves into water causing ocean acidification. However when we burn food and biomass they release CO
at about the same rate that it was removed from the atmosphere through photosynthesis, whereas burning fossil fuels quickly releases Carbon that has been locked up for 300 million years or more.

Energy and Power

The terms Energy and Power tend to be used interchangeably to refer to the electricity, oil, gas etc we all consume, and we talk about "wind energy" and "solar power" in the same context. However the terms have specific - and different - meanings: energy is the ability to do a certain amount of work e.g. boil a particular quantity of water or move a car a certain distance, and power is the rate at which work is done - how quickly the water is heated or the car travels. Energy and work can be thought of as cause and effect: putting a certain amount of energy into a kettle or a car results in an equivalent amount of heating or movement happening in it.

Units of measurement


There are various different units for measuring energy (or work) and power. For power probably the most common units are watts (W), kilowatts (KW), megawatts (MW), gigawatts (GW) and even terawatts (TW). For energy common units are watt hours (or more commonly kilowatt hours (KWh)). The "unit" of electricity (used on British electricity meters and bills) is the same as one KWh. Sometimes a quantity of energy (e.g. the amount supplied by a solar panel over a certain period) is wrongly (and confusingly) stated as so many kilowatts rather than kilowatt hours.

kWh/y and Joules

Another unit of power based on watts is kilowatt hours per year. Since there are 8,766 hours in a year a power of 1KW (roughly the consumption of a 1 bar electric fire) is 8,766 KWh/y. A unit of energy more often used in scientific work is the Joule, which is one watt second, so 3,600 (60 times 60) joules are a watt hour, and 3.6 megajoules are 1KWh (or Unit).

Horespower, BTUs and TOEs

An older unit of power is the horsepower, which is about 746 Watts. The British Thermal Unit (BTU or BThU) is another old unit of energy, which is mostly obsolete in the UK (though still used in the US). A common measure of energy, usually used on a large, even national, scale, is the Tonne of Oil Equivalent and its multiples such as the mega-tonne of oil equivalent (MTOE).

Cubic Miles of Oil

A Mega-Tonne of Oil is not an easy quantity to visualise and global scales of energy are measured in daunting numbers of MTOEs, so the unit of a Cubic Mile of Oil (a "CMO") has been suggested. (Globally we consume about one Cubic Mile of Oil plus the equivalent of another CMO-worth of energy from coal, about 3/4 of a CMO-worth of natural gas, and 1/4 CMO-worth each of hydro, nuclear, and wood. Solar, wind, and biofuels are less than 1/10th of a CMO-worth. Total energy consumption is the equivalent of about 3.5 Cubic Miles of Oil.)

Homes (and other units)

Publicity material and news articles about energy projects often talk about the number of homes they can power. One figure for the amount of power this represents is given by DUKES[1] of 4370 KWh per household per annum. This equates to about 0.5 kW or 500 Watts, so dividing the number of "homes" by 2 gives the equivalent kilowatts. According to David MacKay[2] the British Wind Energy Association uses the figure 4700 kWh per year, equivalent to 0.54 kW or 540 Watts, and other organisations use 4000 kWh/y per household -- 0.46 kW or 460 Watts.

North American usage is higher: Canada's Ontario Power Generation cites "monthly domestic usage of 972 kWh per home" or 1.33 kW.[3]

MacKay also points out that the “home” unit only covers average domestic electricity consumption of a household, not gas or oil used for home heating, cooking and hot water, the energy that occupants use in their workplaces and for transport, or all the other energy-consuming things that society does for them; all of which add up to roughly 24 times more than a "home".

MacKay also discusses other units including "power stations", "cars taken off the road", "calories", barrels, gallons, tons, BTUs, quads, cups of tea, double decker buses, Albert Halls and Wembley Stadiums. (Not to be confused with The Size of Wales which is a unit of area.)

MacKay also provided this chart for translating power units.

SEWTHA powerChart.png

The IEA provides an online energy units conversion calculator.

Primary, secondary, and final energy, and thermal equivalence

Fossil fuels such as coal, oil, and natural gas, and biomass, contain hydrocarbons which embody certain amounts of chemical energy which can be released by oxidation, e.g. by burning them. Water in a reservoir has potential energy which can be released by letting it descend to a lower height. Flowing water and wind has kinetic energy which can be tapped by slowing it. The light and heat of the sun are forms of energy embodied in electromagnetic radiation, and certain isotopes of Uranium and some other elements can release energy when their atoms split. These are forms of primary energy.

When we burn fossil fuels or biomass, fission Uranium in a nuclear reactor, or focus sunlight on a target in a solar thermal power station, we generally produce steam, as secondary energy, which we then use to drive turbines to drive generators which produce electricity which is delivered to consumers as final energy. The distinction between secondary and final energy depends on what parts of a system one is analysing: for example electricity is never consumed directly, but is always converted to other forms of energy such as heat in a toaster, kettle or electric arc furnace, to light in lamps, or to mechanical work in electric vehicles (possibly via chemical energy in batteries). However for the purposes of planning sustainable energy systems, and factoring in CO
emissions, it is generally useful to consider electricity as a final energy form.

Where this really matters is when we want to compare fuels for producing electricity. Power sources such as solar photovoltaic, wind, hydroelectric, wave and tide produce electricity directly, whereas coal, oil, gas, biomass, and nuclear produce heat. Electricity can be converted to heat, if that's what we want (for example for heating buildings, or in industrial processes), with practically 100% efficiency, but converting heat energy to electricity is, at best, only around 60% efficient, and often only half that.

So if we know that a country consumes, say, a certain amount of coal, oil, or gas, resulting in a corresponding amount of CO
emissions, how much emission could be saved by replacing the fossil fuel with carbon free electricity depends on what the fuel is being used for. If it is used for heating then every unit of clean electricity will replace approximately the same amount of primary energy (since combustion fuels can be converted into heat very efficiently). But if the fossil fuel is being used to generate electricity then a carbon-free alternative will replace far more fossil fuel primary energy (and emissions) - how much more depending on the conversion efficiency. For example with a 33.3% conversion efficiency one unit of clean energy will replace 3 units of fossil fuel.

In some statistical publications this sort of conversion is assumed, and factored in to allow meaningful comparison of, say, how much fossil fuel is saved by a given amount of hydro, wind or nuclear. This conversion is particularly likely to have been used when "tonnes of oil equivalent" ("toe"s) and their multiples (Mtoes, Gtoes) are quoted, for example in the BP Statistical Review (which uses a conversion factor of 38% - "the average for OECD thermal power generation").

See also Primary energy on Wikipedia

Energy density and population density

Different parts of the world have different densities of population, and those people use energy at different rates (so in cities in the developed, "first" world more energy is consumed in a given area than in rural areas in the developing world).

Different sources of energy also have different densities: a 1GW coal, gas or nuclear power station may require a few tens of square kilometres (including the mines or wells needed to supply it) whereas to generate the same power from say, biomass, requires thousands of square kilometres to grow energy crops. When considering what sources of energy could power a given country or region we can compare the energy densities of supply and demand; David MacKay's Map of the World provides a convenient way of doing so visually for various countries and sources of low-carbon energy.

MacKay PPPersonVsPDen2WA.eps.png

Capacity Factor

Generators - whether fossil fuelled, nuclear, hydro, wind, solar etc - don't produce electricity (or other forms of useful energy) continuously, 24*7. All mechanical plants can break down, and most are stopped from time to time for inspection and maintenance. Most present day nuclear power stations have to be shut down for refuelling. And hydro, wind, solar, wave and tidal generators can only produce energy when there is the water, wind, sun, wave and tidal conditions they need. The percentage of a generator's maximum ("nameplate") output (or "installed capacity") which it achieves over a representative period in practice is known as its "capacity factor"; this is typically in the range 10-30% for solar, 20-40% for wind and around 90% for nuclear and other thermal power stations.

Because of the significant difference in capacity factors between - in particular - solar and wind, and nuclear, it is misleading to compare installed capacities without compensating for capacity factors. (See e.g. this and this).

Energy Return On Energy Invested

Energy is needed to build, operate, maintain, and eventually decommission any power generating plant. As long as the plant generates more energy in its lifetime than it consumes it will provide a surplus for useful purposes. The greater the surplus the more it allows society to provide other goods and services – to grow food, to build, heat and cool homes and workplaces, run water and sewerage services, provide education, health care, transport, recreation etc. The ratio of energy generated to energy consumed is known as Energy Return On Energy Invested (EROEI, or sometimes EROI) and it is estimated that for a modern civilisation to function the EROEI of its energy supplies in aggregate must be in the region of 10 or more.

Even an EROEI less than unity – a system that consumes more energy than it produces – may be useful if it provides services such as storing energy for use when and where it is needed but unavailable, such as to supply energy when demand temporarily exceeds supply, or to convey energy from fixed generators to electric vehicles. However the overall, aggregate, EROEI of the system of generators and storage systems must still be high enough to allow for a functioning civilisation.

For more see Energy Return On Energy Invested

Carbon Intensity

Also known as emission intensity, life-cycle greenhouse-gas emissions or levelized CO
, the carbon intensity of an energy source is the amount of CO
and equivalent greenhouse gases (e.g. Methane) it emits for each unit of energy (usually electricity) it produces. It is important that the emissions incurred in building and decommissioning generating plant, and mining, processing and transporting fuel, etc, are included, not just the emissions from burning fuel itself. Thus non-fossil fuel sources like wind, solar and nuclear do not have zero emissions (although their emissions are very low). Emissions from these non-generating processes may be subject to a virtuous circle where these emissions become smaller as the carbon intensity of the electricity (and other fuels) they use decreases.


As with carbon intensity, the safety of an energy source must include all deaths and injuries involved in the technology e.g. from mining accidents, from air pollution produced by burning fuels, etc.

This "Visual Capitalist" page discusses the relative safety of different sources of energy, which it presents in this graphic:


The Visual Capitalist site actually specialises in graphical visualisations of all sorts, from the rise of online dating to the raw materials demand for wind, solar and electric vehicles.

See also Safety of energy sources article.


Unsubsidized Levelized Cost of Energy Comparison Lazard

Stanford scientists calculate the carbon footprint of grid-scale battery technologies

Stanford scientists have developed a novel way to calculate the energetic cost of building large batteries and other storage technologies for the electrical grid.

Solar energy in the context of energy use, energy transportation and energy storage David MacKay; Philosophical Transactions of the Royal Society

In a decarbonized world that is renewable-powered, the land area required to maintain today's British energy consumption would have to be similar to the area of Britain.

Footnotes and references

  1. DUKES - the Digest Of UK Energy Statistics - is at the gov.uk website
    The original link, to the website of Village Eye is no longer active
  2. Sustainable Energy Without The Hot Air p329
  3. "Electricity Generated in Ontario", webroots.ca website