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# Units of Measurement for Primary Energy Sources

The mass and volume of fossil fuels and biomass can be expressed in different ways. The growth of renewable energies has popularized certain units of measurement such as watt-peak and meters per second.

## Measuring Non-Renewable Energies

• is measured in barrels ("bbl" or "b"). This is the most common unit. The output of a refinery is quantified in terms of barrels per day ("b/d" or "BPD"). When the crude is carried by an oil , the cargo is expressed in tons because the cost of shipping depends more on its weight than its volume. Oil can be very dense and heavy or less dense and light, meaning that the same volume of oil can have a very different mass and cost more or less to ship. The in service stations is sold by the liter or the gallon.
• Natural gas is measured in cubic meters (m3). In the United States and the United Kingdom, it is also measured in cubic feet, with 1 cubic foot (CF) equal to 0.028 cubic meters. The volume of gas is measured under specific physical conditions. The density of gas, i.e. its mass per unit volume, expressed in kilograms per cubic meter, changes according to temperature and pressure conditions. This property is key from a commercial standpoint: the higher a gas’ density, the more molecules it contains and the more energy it yields when it is burned. This is why gas volume is always measured at a temperature of 15°C and a pressure of 750 hectopascals (hPa). The volume of is measured in cubic meters, cubic feet or gallons. For transportation by LNG carrier, its mass is measured in tons.
• is measured in tons and millions of tons.
• The fuel used in nuclear plants is manufactured from oxide in the form of cylindrical pellets weighing 7 grams each. One pellet provides as much energy as a ton of coal.

## Measuring Renewable Energy Sources

• When wood is used as a energy source, its volume is measured in steres (“st”). Biomass in the form of wafers or granules is expressed in tons.
• Wind turbines convert the energy of the wind into electrical energy. A wind turbine starts to turn at a minimum wind speed of 4 meters per second (4 m/s). The turbine’s productivity — the amount of it produces in a specified period — doubles when wind speed reaches 7 meters per second. However, if wind speed exceeds 25 meters per second, the turbine shuts down and produces no more energy. This is why the prevailing winds of any proposed site are studied carefully before a decision is made to install a wind turbine.
• Hydroelectric plants use turbine to convert the energy released by flowing or cascading water into electricity. The size of the turbine is proportional to river flow — measured in cubic meters per second — and the waterfall height or altitude difference between the upper and lower reservoir, expressed in meters (m). In addition to these two factors, weight due to gravity is taken into account when calculating the electrical power produced. Thus, for a flow of 1 meter per second, a hydraulic system produces 9.8 kilowatts (kW) of electricity per meter of waterfall. These data are crucial when determining where to locate a hydroelectric power plant.
• systems recover from the Earth for home heating or for conversion into electricity. Before this type of system is installed, the internal heat of the potential site is taken into account. It rises by 3°C every 100 meters on average, but can vary significantly according to the geological features of the location. This value is called the geothermal gradient.
• Solar panels capture solar thermal energy and convert it into heat to produce hot water or into electricity. Their angle or tilt, expressed in degrees (°) is key: they recover the most energy when perpendicular to the Sun’s rays. Another pivotal factor is orientation, because south-facing panels capture more energy. Geographical and climate factors are also taken into account when picking a site. The watt-peak (Wp) is the unit used to measure the maximum electrical power produced by a when correctly oriented and angled at a temperature of 25°C and of 1,000 watts per square meter. A multiple of watt-peak — the megawatt-peak (MWp), equivalent to 1 million Wp — is used to express the maximum power produced by a solar power plant under the same conditions.