Therm (US) to Kilocalorie (th)

Residential US natural gas prices typically range from $0.80 to $2.50 per therm depending on region, season, and utility. The wholesale Henry Hub benchmark translates to about $0.25 per therm at $2.50/MMBtu. Delivery charges, taxes, and utility markups roughly triple or quadruple the commodity cost by the time it reaches a home meter.

The average US home using gas for heating consumes about 500–900 therms per year, depending on climate, insulation, and home size. Homes in mild climates like Southern California may use under 300 therms; homes in Minnesota or Wisconsin can exceed 1,200 therms. Gas water heaters alone account for roughly 150–250 therms per year.

One US therm equals exactly 100,000 BTU, while one MMBtu (million BTU) equals 1,000,000 BTU — so 1 MMBtu equals 10 therms. Wholesale gas markets and pipeline contracts use MMBtu; residential utility bills use therms. The two are straightforward to convert, but confusing them by a factor of ten is a common mistake in energy cost comparisons.

Retail billing in therms gives homeowners manageable numbers — a winter month might be 80–120 therms at $1–2 each. Wholesale pipeline contracts deal in millions of BTU (MMBtu) because the volumes are enormous and the industry standardized on BTU-based pricing in the early 20th century. One MMBtu equals 10 therms, so converting is simple. The Henry Hub benchmark price of $2.50/MMBtu translates to about $0.25/therm before delivery charges, taxes, and utility markup roughly quadruple it at the meter.

A typical US residential furnace rated at 80,000–100,000 BTU/h uses about 0.8–1.0 therms per hour at full output. High-efficiency condensing furnaces (95%+ AFUE) extract more heat per therm, so they cycle less often. On a cold winter day, a furnace might run 8–12 hours total, consuming 6–10 therms. That translates to roughly $5–$25 per day depending on local gas prices.

Most foundational biochemical data — ATP hydrolysis (~7.3 kcal/mol), glucose oxidation (~686 kcal/mol), amino acid combustion values — were measured and published in kcal th before SI adoption. Rewriting decades of literature, lecture notes, and exam banks to kJ would introduce conversion errors and confusion. The field maintains kcal th by convention while acknowledging SI equivalents.

The standard free energy change (ΔG°) for ATP → ADP + Pi is approximately −7.3 kcal th/mol (−30.5 kJ/mol). Under actual cellular conditions, the value is closer to −12 to −14 kcal/mol because reactant and product concentrations differ from standard state. This energy drives muscle contraction, nerve impulses, protein synthesis, and virtually every energy-requiring process in living cells.

The classic Atwater factors (4 kcal/g carb, 4 kcal/g protein, 9 kcal/g fat) are averages from 19th-century bomb calorimetry, adjusted for digestibility. They can be off by 5–25% for specific foods. Almonds deliver ~20% fewer usable calories than labels claim because cell walls trap some fat from digestion. High-fiber foods also overcount. The FDA allows ±20% tolerance on label accuracy, so a "200 kcal" bar could legally contain 160–240 kcal.

Complete aerobic oxidation of one mole of glucose (C₆H₁₂O₆) releases approximately 686 kcal th (2,870 kJ). The human body captures about 38–40% of this in ATP; the rest dissipates as body heat. This is why exercise makes you warm — over half the food energy your muscles consume is released as thermal energy rather than mechanical work.

Fat molecules are highly reduced — their carbon atoms are bonded mostly to hydrogen, with very little oxygen. Oxidising them releases maximum energy because every C-H bond is converted to C=O and O-H bonds. Carbohydrates are already partially oxidised (they contain oxygen in their structure), so less additional oxidation is possible. Gram for gram, fat stores 2.25× more energy, which is why evolution favored fat as the body's long-term energy reserve — it packs the most kcal per gram of tissue weight.