Editing British thermal unit (section)

== Definitions ==
A Btu was originally defined as the amount of heat required to raise the temperature of one [[Pound_(mass)|pound]] of liquid water by one degree [[Fahrenheit]] at a constant pressure of one [[atmosphere (unit)|atmospheric unit]].<ref name=Smith>{{cite book |title=Introduction to Chemical Engineering Thermodynamics |edition=6 |last1=Smith |first1=J. M. |last2=Van Ness |first2=H. C. |last3=Abbott |first3=M. M. |others=B. I. Bhatt (adaptation) |isbn=0-07-049486-X |publisher=Tata McGraw-Hill Education |date=2003 |page=15 |url=https://books.google.com/books?id=oF3OmdnxCHUC&pg=PA15}}</ref> There are several different definitions of the Btu that differ slightly. This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature. As seen in the table below, definitions of the Btu based on different water temperatures vary by up to 0.5%.

{| class=wikitable
|-
! Variant
! Energy (J)
! Notes
|-
| Thermochemical
| ≈1,054.35{{efn|4.184 × 453.59237 × {{sfrac|5|9}}}}
| Originally, the thermochemical Btu was defined as the heat required to raise the temperature of one pound of water from its freezing point to its boiling point, divided by 180 (the temperature change being 180&nbsp;°F). The basis for its modern definition in terms of SI units is the conceptually similar ''thermochemical [[calorie]]'', originally defined as the heat required to raise the temperature of one gram of water from freezing to boiling divided by 100 (the temperature change being 100&nbsp;°C). The thermochemical calorie is exactly 4.184&nbsp;J by definition of the International Organization for Standardization (ISO).<ref name= Smith/> The thermochemical Btu is calculated by converting from grams to pounds and from Celsius to Fahrenheit.<ref name=convert>The pound is 453.59237 grams by definition; see {{Cite web |url=http://ts.nist.gov/WeightsAndMeasures/Publications/upload/h4402_appenc.pdf |publisher=United States National Bureau of Standards |title=Appendix C of NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement |page=C-12 |url-status=dead |archive-url=https://web.archive.org/web/20061126120208/http://ts.nist.gov/WeightsAndMeasures/Publications/upload/h4402_appenc.pdf |archive-date=26 November 2006
}} One degree [[Fahrenheit]] is exactly {{sfrac|5|9}} of a degree Celsius by definition.</ref>
|-
| {{convert|59|°F|1}}
| ≈1,054.80<ref name=Thompson>{{cite web |url=http://physics.nist.gov/cuu/pdf/sp811.pdf |title=Guide for the Use of the International System of Units (SI) 2008 Edition |first1=Ambler |last1=Thompson |first2=Barry N. |last2=Taylor |publisher=[[National Institute of Standards and Technology]] (NIST) |page=58 |url-status=live |archive-url=https://web.archive.org/web/20160603203340/http://physics.nist.gov/cuu/pdf/sp811.pdf |archive-date=3 June 2016}} NIST Special Publication 811.</ref>
| Used for American natural gas pricing.<ref name=HenryHub>The Btu used in American natural gas pricing is "the amount of heat required to raise the temperature of 1 avoirdupois pound of pure water from {{convert|58.5|to|59.5|F}} at a constant pressure of 14.73 pounds per square inch." See {{cite web|work=NYMex Rulebook|title=Chapter 220: Henry Hub Natural Gas Futures|publisher=[[New York Mercantile Exchange]] (NYMex)|url=https://www.cmegroup.com/rulebook/NYMEX/2/220.pdf|access-date=6 January 2017|url-status=live|archive-url=https://web.archive.org/web/20161110004509/http://www.cmegroup.com/rulebook/NYMEX/2/220.pdf|archive-date=10 November 2016|df=dmy-all
}}</ref>
|-
| {{convert|60|°F|1}}
| ≈1,054.68<ref name=Sørensen>
{{cite book
 |title=Renewable Energy Focus e-Mega Handbook
 |first=Bent |last=Sørensen
 |publisher=Academic Press
 |date=2008
 |isbn=9780123747068
 |page=5
 |url=https://books.google.com/books?id=e493O0is564C&pg=PA3
}}</ref>
| Mainly [[Canada|Canadian]].{{Citation needed|date=January 2017}}
|-
| {{convert|39|°F|1}}
| ≈1,059.67<ref name=Sørensen />
| Uses the [[calorie]] value of water at its maximum density ({{convert|4|°C|1|disp=comma}}).{{Citation needed|date=January 2017}}
|-
| IT ||≈1,055.06{{efn|4.1868 × 453.59237 × {{frac|5|9}}}}
| An early effort to define heat units directly in terms of energy units, and hence to remove the direct association with the properties of water, was made by the International Steam Table Conferences. These conferences originally adopted the simplified definition that 860 "IT" calories corresponded to exactly 1 [[Watt#Origin_and_adoption_as_an_SI_unit|international watt-hour]] (not the same as a modern watt-hour). This definition ultimately became the statement that 1 IT calorie is exactly 4.1868&nbsp;J.<ref name=Smith/><ref name=Koch>
{{cite book
 |title=VDI Steam Tables
 |publisher=Springer
 |edition=4
 |date=2013
 |last=Koch
 |first=Werner
 |page=8
 |isbn=9783642529412
 |url=https://books.google.com/books?id=bJ_wBgAAQBAJ&pg=PA8
}} Published under the auspices of the {{lang|de|Verein Deutscher Ingenieure}} (VDI).
</ref> The Btu is then calculated from the calorie as is done for the thermochemical definitions of the Btu and the calorie, as in [[International standard]] [[ISO 31-4]] ''Quantities and units—Part 4: Heat'' and [[British Standard]] BS 350:Part 1:1974 ''Conversion factors and tables''.<ref name=convert/><ref>{{cite book |title=Scientific Unit Conversion: A Practical Guide to Metrication |first=Francois |last=Cardarelli |others=M.J. Shields (translation) |edition=2 |publisher=Springer |date=2012 |page=19 |isbn=9781447108054 |url=https://books.google.com/books?id=-ZveBwAAQBAJ&pg=PA19-IA35}}</ref><ref>{{cite book |title=BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors |date=1974 |publisher=British Standards Institution |page=59}}</ref>
|}

===Prefixes===
Units of kBtu are used in building energy use tracking and heating system sizing. Energy Use Index (EUI) represents kBtu per square foot of conditioned floor area. "k" stands for 1,000.<ref>{{cite web|title=VOLUNTARY BUILDING ENERGY PERFORMANCE SCORE SYSTEMS|website=Oregon Secretary of State Administrative Rules|url=https://secure.sos.state.or.us/oard/displayDivisionRules.action?selectedDivision=1091}}</ref>

The unit '''MBtu''' is used in natural gas and other industries to indicate 1,000 Btu.<ref name="EIA"/><ref name="Price">{{cite book |last=Price |first=Gary D. |title=Power Systems and Renewable Energy: Design, Operation, and Systems Analysis |publisher=Momentum Press |date=2014 |page=98 |isbn=9781606505717 |url=https://books.google.com/books?id=skICBAAAQBAJ&pg=PP9}}</ref> However, there is an ambiguity in that the metric system (SI) uses the prefix "M" to indicate '[[Mega-]]', one million (1,000,000). Even so, "MMBtu" is often used to indicate one million Btu particularly in the oil and gas industry.<ref name="APS">{{cite web |title=Energy Units |url=https://www.aps.org/policy/reports/popa-reports/energy/units.cfm |publisher=[[American Physical Society]] |access-date=26 December 2016 |url-status=live |archive-url=https://web.archive.org/web/20161231123621/http://www.aps.org/policy/reports/popa-reports/energy/units.cfm |archive-date=31 December 2016}}</ref>

Energy analysts accustomed to the metric "k" ('[[kilo-]]') for 1,000 are more likely to use MBtu to represent one million, especially in documents where M represents one million in other energy or cost units, such as MW, MWh and $.<ref>Cook, Warren C (2018): https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Home%20Energy%20Score%20Report%20Example.pdf 
 https://www.oregon.gov/energy/Data-and-Reports/Documents/BER-Chapter-1-Energy-Numbers.pdf</ref>

The unit '[[therm]]' is used to represent 100,000 Btu.<ref name="EIA">{{cite web|url=http://www.eia.gov/tools/faqs/faq.cfm?id=45&t=8|title=What are Mcf, Btu, and therms? How do I convert prices in Mcf to Btus and therms?|publisher=[[Energy Information Administration|U.S. Energy Information Administration]]|date=6 April 2016|access-date=30 December 2016|url-status=live|archive-url=https://web.archive.org/web/20161225220005/http://www.eia.gov/tools/faqs/faq.cfm?id=45&t=8|archive-date=25 December 2016}}</ref> A decatherm is 10 therms or one million Btu. The unit ''[[quad (unit)|quad]]'' is commonly used to represent one quadrillion (10<sup>15</sup>) Btu.<ref name="APS"/>