Horsepower (British) to Kilocalories (th)/minute

A horse can sustain about 0.7 hp over a working day, and briefly peak at 10–15 hp during a gallop or heavy pull. Watt's definition was deliberately generous — he wanted his steam engines to look good compared to the horses they replaced. A fit human can sustain about 0.1 hp and peak at ~1–2 hp briefly. So a horse is roughly 7× a human in sustained output, which aligns well with historical accounts of animal labor replacing human workers.

Because American engineering inherited the British unit directly — the US was a British colony when Watt defined horsepower in the 1780s. Both equal 550 ft·lbf/s = 33,000 ft·lbf/min = 745.7 W. The "international" horsepower adopted in 1956 formalised this same value. The only reason it's sometimes called "British" is to distinguish it from the metric horsepower (PS) used in continental Europe, which is 1.4% smaller.

Watt's own improved steam engines: 10–20 hp. Brunel's SS Great Eastern ship engines: 8,000 hp. The Rolls-Royce Merlin (WW2 Spitfire): 1,030–1,760 hp depending on variant. Concorde's Olympus 593 engines: 38,000 hp each (with reheat). The Rolls-Royce Trent XWB (A350 engine): about 97,000 hp. In 240 years, British engines went from 20 hp to 97,000 hp — a 5,000-fold increase.

Almost. "bhp" stands for "brake horsepower" — power measured at the engine output shaft using a dynamometer (historically a brake). "hp" can technically mean the gross figure including power consumed by accessories. Since 2005, European regulations require "net" power (engine with all standard accessories), so bhp and hp are effectively identical for modern cars. The "b" in bhp is mostly a British tradition to emphasize that the number is a real dynamometer measurement, not a theoretical calculation.

From 1910 to 1947, Britain taxed cars by "RAC horsepower" — a formula based on cylinder bore and number of cylinders, not actual power. This incentivised narrow-bore, long-stroke engines with terrible performance. A car rated at "10 RAC hp" might actually produce 30–40 real hp. The tax warped British car design for decades, producing underpowered engines that only made sense as tax dodges. The system was scrapped in 1947, but its legacy shaped British car culture for years after.

A MET (Metabolic Equivalent of Task) is the ratio of activity metabolic rate to resting metabolic rate. Sitting = 1 MET; walking = 3.5 METs; running = 8–12 METs. Researchers prefer METs because they normalize for body weight — a 50 kg woman and a 100 kg man both register 8 METs while running at the same pace, even though their raw kcal/min differ by 2×. This makes METs portable across populations. To get kcal/min from METs: multiply METs × body weight in kg × 0.0175. The Compendium of Physical Activities lists METs for over 800 activities, from accordion playing (1.8) to wrestling (6.0).

Cross-country skiing uphill can hit 15–20 kcal/min (1,050–1,400 W metabolic), making it one of the highest sustained metabolic rates in sport. Rowing and swimming at race pace reach 12–18 kcal/min. Cycling at elite level sustains 15–25 kcal/min. But the absolute champion is sprint running: Usain Bolt's 100m final produced roughly 80–100 kcal/min of metabolic power for 9.58 seconds. Of course, no one sustains that for long.

VO₂ max (maximum oxygen consumption) converts to kcal/min via the caloric equivalent of oxygen: 1 liter of O₂ consumed ≈ 5 kcal. An elite endurance athlete with VO₂ max of 80 mL/kg/min (70 kg person = 5.6 L/min) can sustain roughly 28 kcal/min at maximum effort. An untrained person at VO₂ max of 35 mL/kg/min maxes out around 12 kcal/min. This is why fit people can sustain higher power outputs — they literally process more oxygen.

Because their energy accounting is in kilocalories: food energy in kcal, basal metabolism in kcal/day, exercise expenditure in kcal/min. If a client eats 2,000 kcal and you want them to "burn 500 kcal," it's immediately useful to say "run at 10 kcal/min for 50 minutes." Saying "exercise at 700 W" is technically correct but meaningless to most clients. The kcal/min rate connects directly to the dietary energy balance equation.

Yes — EPOC (excess post-exercise oxygen consumption) is measured as elevated kcal/min above resting rate after exercise. After intense interval training, your metabolic rate might stay 0.2–0.5 kcal/min above baseline for 12–24 hours. That sounds tiny, but over 24 hours it adds up to 200–700 extra kcal — a meaningful amount. However, the fitness industry wildly oversells this: moderate exercise barely budges EPOC. You need truly brutal intensity to get a significant afterburn.