Energy measurement

The units of measurement used by human beings to express the quantity of energy sources are numerous. There are measures for physical quantities and measures for the energy or heat content.
Among the most well-known units of measurement of physical quantities, we find tons used for crude oil and for coal; barrels used for crude oil; cubic metres used for gas, and litres used for petrol and diesel oil. Comparing sources of energy, with different units of measurement of physical quantities, is an extremely complex, if not impossible, task. In fact, from a kilo of petroleum we do not obtain the same amount of energy produced with a kilo of coal or a cubic metre of gas. The official measurement unit for energy is the Joule (J). Among the most common units measuring energy mention should be made of the kilowatt/hour (kWh), used especially for electric energy (in fact it is used to calculate electricity bills). In order to measure the production of large electricity plants or the national consumption, the Terawatt/hour (TWh) is used, which corresponds to a billion kW/h.
The most common units measuring heat include the BTU (British Thermal Unit) and the kilogram calorie (kg-cal) and especially the Tonne of Oil Equivalent.
The Tonne of oil equivalent is the most common at international level because it is connected to one of the most important and widely used fuels: oil. By measuring the different energy sources in terms of Tonne of oil equivalent, a comparison becomes possible and they can be aggregated, a vital operation to calculate how much energy a country consumes in a year or how much energy is still available under the surface (oil and natural gas fields and coal mines).
But what is Tonne of oil equivalent? Basically, one Tonne of oil equivalent represents the quantity of heat which can be obtained from a tonne of oil. In practice, if we measure coal in terms of Tonne of oil equivalent, it means we are considering the quantity of coal capable of producing as much heat as a tonne of oil.
(Let’s remember that: 1 Kcal = 4.186 J = 1,16 x 10-3 kWh = 1×10-7 TOE).
How can we calculate the physical quantities corresponding to one Tonne of oil equivalent of coal or natural gas? In other words, how many kilograms of coal are needed to reach one Tonne of oil equivalent of coal and how many cubic metres of natural gas to produce one Tonne of oil equivalent of gas?
To calculate that equivalence and use the units measuring physical quantities (kilograms, litres, cubic metres) of the different energy sources, we resort to calories.
We know that one tonne of oil contains 10 million kilogram calories (kg-cal), whereas one tonne of pit coal contains 7 million kg-cal. Therefore one Tonne of oil equivalent of coal, since it measures the quantity of coal containing as many kilogram calories as one tonne of oil, is equal to approximately 1.43 tonnes (measure of the physical quantity) of coal. Calculations are easier if we take, for example, vegetable fuels, containing 2.5 million kilogram calories per tonne of material. In this case, to obtain 10 million kg-cal (the calorific content of a tonne of petrol) we need 4 tonnes of vegetable fuels; therefore one Tonne of oil equivalent of vegetable fuels correspond to 4 tonnes of vegetable fuels.
If we know the contents, in terms of calories, of the physical units measuring the different energy sources we can calculate all the Tonne of oil equivalent equivalents. The following table reports the “Net Heat Value”, i.e. estimates based on international average values leading to the conversion of the content of calories into the various units measuring the physical quantities of some of the most common fossil energy sources.

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