Horsepower (British) to Kilocalories (th)/hour

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.

In microgravity, muscles never work against their own weight — even walking requires zero effort. ISS astronauts exercise ~2.5 hours/day burning 400–600 kcal/h on resistive machines and treadmills with bungee harnesses, yet still lose 1–2% muscle mass per month. The problem is not total energy expenditure but the absence of constant low-level gravitational loading that Earth provides 24/7. Ground-based standing and walking burn only 80–120 kcal/h but provide continuous mechanical stimulus that exercise bursts cannot fully replace.

Most machines use crude formulas based only on speed/resistance and assume a 70–80 kg user. They often report gross calories (including resting metabolic rate you'd burn anyway) rather than net additional calories from exercise. Studies show treadmills overestimate by 15–20%, ellipticals by 25–40%, and stationary bikes by 10–15%. The machines have an incentive to flatter you — higher numbers keep you coming back. Always discount the displayed number by at least 20%.

Surprisingly little extra. The brain uses about 20% of resting metabolic energy (~15–20 kcal/h) regardless of what you are thinking. Intense mental work — chess tournaments, exams, complex coding — increases brain glucose consumption by only 5–10%, adding roughly 1–2 kcal/h. Chess grandmasters who lose weight during tournaments are not burning it with their brains — they lose it through stress hormones elevating heart rate, skipping meals, and disrupted sleep. The brain is always "on" at nearly full power; thinking harder barely moves the needle.

Almost linearly for weight-bearing exercise: a 100 kg person burns roughly 60–70% more kcal/h than a 60 kg person walking or running at the same speed. For cycling and swimming (where body weight is supported), the difference is smaller — maybe 20–30%. This is why heavier people find it "easier" to create a caloric deficit through exercise, and why lightweight people need to exercise longer for the same caloric burn. It's simple physics: moving more mass requires more energy.

Basal Metabolic Rate for adults is typically 55–85 kcal/h (1,300–2,000 kcal/day), depending on age, sex, weight, and muscle mass. It accounts for 60–75% of total daily energy expenditure — far more than exercise for most people. This is why crash diets backfire: severe calorie restriction can drop BMR by 10–20% (metabolic adaptation), reducing your burn by 200–400 kcal/day. Your body literally becomes more efficient, fighting your weight loss efforts.