Horsepower (British) to Watt

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 standard USB charger draws 5–10 W, while fast chargers pull 18–65 W and some proprietary ones hit 120–240 W. The charger itself consumes about 0.1–0.3 W even when nothing is plugged in — so-called "vampire power." Over a year, a plugged-in-but-idle charger wastes roughly 2 kWh, costing pennies but multiplied across billions of chargers worldwide it adds up to gigawatt-hours of waste.

Both are identical — 1 W = 1 J/s — but the watt was named in 1889 to honor James Watt, who quantified engine power decades before the joule was formalised. Giving power its own name made practical engineering simpler: saying "a 60-watt bulb" is far catchier than "a 60-joules-per-second bulb." The naming also followed a 19th-century tradition of honoring scientists with SI units — volt, ampere, ohm, and watt all came from this era.

A resting adult generates about 80–100 W of thermal power, roughly equivalent to an old incandescent light bulb. During intense exercise this spikes to 300–500 W total metabolic output, though only 20–25% becomes mechanical work — the rest is waste heat. This is why a packed lecture hall gets stuffy fast: 200 students produce about 20 kW of heat, equivalent to running 20 space heaters.

A single lightning stroke delivers about 1–5 billion watts (1–5 GW) of instantaneous power, but only for 1–2 milliseconds. The total energy per bolt is surprisingly modest — roughly 1–5 billion joules compressed into microseconds, equivalent to about 250 kWh or one month of a US household. You could theoretically power a town for a second, but capturing it is impractical because the pulse is too brief and unpredictable.

Watts measure the rate of energy flow (like the speed of water through a pipe), while watt-hours measure total energy consumed over time (like the total volume of water). A 100 W bulb running for 10 hours uses 1,000 Wh (1 kWh). Your electricity bill charges per kWh, not per watt — so a 2,000 W heater running one hour costs the same as a 100 W lamp running 20 hours.