British thermal unit

A Btu was originally defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit at a constant pressure of one atmospheric unit.{{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}} 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%.

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.{{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}}

The unit MBtu is used in natural gas and other industries to indicate 1,000 Btu.{{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}} 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.{{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}}

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 $.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

The unit 'therm' is used to represent 100,000 Btu.{{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=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}} A decatherm is 10 therms or one million Btu. The unit quad is commonly used to represent one quadrillion (10¹⁵) Btu.

One Btu is approximately:

• {{cvt|1.0000|Btu|kJ|disp=out}} (kilojoules)
• {{cvt|1.0000|Btu|W.h|disp=out}} (watt hours)
• {{cvt|1.0000|Btu|cal|disp=out}} (calories)
• {{cvt|1.0000|Btu|kcal|disp=out}} (kilocalories)
• 25,031 to 25,160 ft⋅pdl (foot-poundal)
• {{cvt|1.0000|Btu|ft.lbf|disp=out}} (foot-pounds-force)
• 5.40395 (lbf/in²)⋅ft³

A Btu can be approximated as the heat produced by burning a single wooden kitchen match or as the amount of energy it takes to lift a {{convert|1|lb|spell=in|adj=on}} weight {{Convert|778|ft|m|0}}.{{cite book|last1=Ristinen|first1=Robert A. |last2=Kraushaar|first2=Jack J. |title=Energy and the Environment|url=https://archive.org/details/energyenvironmen00rist|url-access=registration|year=2006|publisher=John Wiley & Sons|isbn=978-0-471-73989-0|pages=[https://archive.org/details/energyenvironmen00rist/page/13 13]–14}}

{{main|Natural gas prices}}

• In natural gas pricing, the Canadian definition is that {{val|1000000|u=Btu|fmt=commas}} ≡ {{val|1.054615|u=GJ}}.{{cite web | url=http://www.energy.gov.ab.ca/about_us/1132.asp | title=Energy Measurements | access-date=7 January 2017 | publisher=Government of Alberta Province | url-status=dead | archive-url=https://web.archive.org/web/20170120191331/http://www.energy.gov.ab.ca/About_Us/1132.asp | archive-date=20 January 2017 | df=dmy-all }}
• The energy content (high or low heating value) of a volume of natural gas varies with the composition of the natural gas, which means there is no universal conversion factor for energy to volume. {{convert|1|ft3|L|abbr=off}} of average natural gas yields ≈ 1,030 Btu (between 1,010 Btu and 1,070 Btu, depending on quality, when burned)
• As a coarse approximation, {{convert|1000|ft3|m3}} of natural gas yields ≈ {{val|1000000|u=Btu|fmt=commas}} ≈ {{val|1|u=GJ}}.
• For natural gas price conversion {{val|1000|u=m3|fmt=commas}} ≈ 36.9 million Btu and {{val|1000000|u=Btu|fmt=commas}} ≈ {{val|27.1|u=m3}}

The SI unit of power for heating and cooling systems is the watt. Btu per hour (Btu/h) is sometimes used in North America and the United Kingdom - the latter for air conditioning mainly, though "Btu/h" is sometimes abbreviated to just "Btu".{{cite book|title=Masonry Heaters: Designing, Building, and Living with a Piece of the Sun|author=Ken Matesz|page=148|publisher=Chelsea Green Publishing|year=2010}} MBH—thousands of Btu per hour—is also common.{{Cite book |last=Arimes |first=Tom |oclc=32314774 |title=HVAC and chemical resistance handbook for the engineer and architect : a compilation |publisher=BCT |year=1994 |isbn=0-9640967-0-6 |location=Lexington, Ky. |page=11-12 }}

• 1 W is approximately {{convert|1|W|Btu/h|disp=out|sigfig=7}}{{cite web|url=http://app.knovel.com/web/toc.v/cid:kpASHRAE22/viewerType:toc/root_slug:ashrae-handbook-fundamentals/url_slug:ashrae-handbook-fundamentals/|title=2009 ASHRAE Handbook – Fundamentals (I-P Edition)|url-status=live|archive-url=https://web.archive.org/web/20151017172515/http://app.knovel.com/web/toc.v/cid:kpASHRAE22/viewerType:toc/root_slug:ashrae-handbook-fundamentals/url_slug:ashrae-handbook-fundamentals/|archive-date=17 October 2015|access-date=21 September 2015|publisher=American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc}}
• 1,000 Btu/h is approximately {{convert|1000|Btu/h|kW|disp=out|sigfig=4}}
• 1 hp is approximately {{convert|1|hp|Btu/h|disp=out|sigfig=4}}

• 1 ton of cooling, a common unit in North American refrigeration and air conditioning applications, is {{convert|12,000|Btu/h|kW|2|abbr=on}}. It is the rate of heat transfer needed to freeze {{convert|1|ST|kg|0}} of water into ice in 24 hours.
• In the United States and Canada, the R-value that describes the performance of thermal insulation is typically quoted in square foot degree Fahrenheit hours per British thermal unit (ft²⋅°F⋅h/Btu). For one square foot of the insulation, one Btu per hour of heat flows across the insulator for each degree of temperature difference across it.
• 1 therm is defined in the United States as 100,000 Btu using the {{not a typo|Btu_59 °F}} definition. In the EU it was listed in 1979 with the BTU_IT definition and planned to be discarded as a legal unit of trade by 1994.{{cite web|url= https://www.legislation.gov.uk/eudr/1980/181|title=Council Directive of 20 December 1979 on the approximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EEC (80/181/EEC)|access-date=25 October 2024}} United Kingdom regulations were amended to replace therms with joules with effect from 1 January 2000.{{cite web|url=https://www.legislation.gov.uk/uksi/1995/1804/made|title=The Units of Measurement Regulations 1995|access-date=7 November 2019}} {{As of|2013}} the therm was still used in natural gas pricing in the United Kingdom.{{cite web|url=https://www.ofgem.gov.uk/gas/wholesale-market/gb-gas-wholesale-market|publisher=Office of Gas and Electricity Markets|title=The GB gas wholesale market|quote=The wholesale gas market in Britain has one price for gas irrespective of where the gas comes from. This is called the National Balancing Point (NBP) price of gas and is usually quoted in price per therm of gas.|access-date=13 January 2013|url-status=live|archive-url=https://web.archive.org/web/20130930030713/https://www.ofgem.gov.uk/gas/wholesale-market/gb-gas-wholesale-market|archive-date=30 September 2013}}
• 1 quad (short for quadrillion Btu) is 10¹⁵ Btu, which is about 1 exajoule ({{val|1.055|e=18|u=J}}). Quads are used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used 99.75 quads in 2005.{{cite book |last1=Husher |first1=John Durbin |title=Crises of the 21st Century: Start Drilling-The Year 2020 Is Coming Fast |publisher=iUniverse |date=2009 |page=376 |oclc=610004375 |isbn=9781440140549}}. One quad/year is about 33.43 gigawatts.

The Btu should not be confused with the Board of Trade Unit (BTU), an obsolete UK synonym for kilowatt hour ({{convert|1|kW.h|Btu|0|abbr=on|disp=or}}).

The Btu is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in Btu required to generate 1 kW⋅h of electrical energy. A typical coal-fired power plant works at {{convert|10500|Btu/kWh|kWh/kWh|abbr=on}}, an efficiency of 32–33%.{{cite web |url=http://www.npc.org/Study_Topic_Papers/4-DTG-ElectricEfficiency.pdf |title=Electric Generation Efficiency |publisher=National Petroleum Council (NPC) |editor1-first=David K. |editor1-last=Bellman |date=18 July 2007 |url-status=dead |archive-url=https://web.archive.org/web/20081120094040/http://www.npc.org/Study_Topic_Papers/4-DTG-ElectricEfficiency.pdf |archive-date=20 November 2008 |access-date=30 March 2012 }} Working Document of the NPC Global Oil & Gas Study.

The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of {{convert|1|lb|kg|spell=in}} of water by one Celsius degree. It is equal to 1.8 Btu or 1,899 joules.{{Cite web |url=http://www.diracdelta.co.uk/science/source/c/e/centigrade%20heat%20unit/source.html |title=Centigrade Heat Unit |access-date=10 February 2017 |archive-url=https://web.archive.org/web/20161225090911/http://www.diracdelta.co.uk/science/source/c/e/centigrade%20heat%20unit/source.html |archive-date=25 December 2016 |url-status=dead }} In 1974, this unit was "still sometimes used" in the United Kingdom as an alternative to Btu.{{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}}

Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.{{cite journal|title=Does the history of food energy units suggest a solution to 'Calorie confusion'?|last=Hargrove|first=James L.|date=2007|journal=Nutrition Journal|doi=10.1186/1475-2891-6-44|volume=6|page=44|pmc=2238749|pmid=18086303 |doi-access=free }}

• Conversion of units
• Latent heat
• Metrication
• Ton of refrigeration

{{notelist}}

{{reflist}}

• {{cite web

|url=http://www.legislation.gov.uk/uksi/1995/1804

|title=The Units of Measurement Regulations 1995

|date=13 July 1995

|website=www.legislation.gov.uk

|publisher=HMSO

|access-date=23 February 2018

}}

• {{cite web

|url=http://www.nrcan.gc.ca/energy/natural-gas/5641?destination=node/1233

|title=Natural Gas: A Primer

|date=27 November 2015

|website=www.nrcan.gc.ca

|publisher=Natural Resources Canada

|access-date=23 February 2018

|archive-url=https://web.archive.org/web/20180224052827/http://www.nrcan.gc.ca/energy/natural-gas/5641?destination=node%2F1233

|archive-date=24 February 2018

|url-status=dead

}}

{{Imperial units}}

{{United States Customary Units}}

Category:Units of energy

Category:Imperial units

Category:Customary units of measurement in the United States