Horsepower (British) to Gigawatt

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.

1.21 GW is very real — it's about the output of a large nuclear reactor. Doc Brown needed it for the flux capacitor, but a single lightning bolt actually delivers far more instantaneous power (up to 1,000 GW) for a few microseconds. The movie got the pronunciation slightly off: Christopher Lloyd famously said "jigawatts," which is technically an acceptable older pronunciation but not the standard one.

It varies enormously. The UK peaks at about 50 GW; Germany around 80 GW; the US about 750 GW; China over 2,000 GW of installed capacity. But installed capacity and actual consumption differ: the US averages about 450 GW of actual demand. Developing nations can operate on strikingly little — some small African nations manage on under 0.5 GW for millions of people.

The Three Gorges Dam in China holds the record at 22.5 GW of installed hydroelectric capacity — enough to power a country the size of Switzerland. It has 32 main turbines each rated at 700 MW. Its annual output of ~100 TWh makes it the world's most productive power plant, though the Itaipu Dam on the Brazil-Paraguay border occasionally produces more in a given year due to higher capacity factor.

The world added roughly 420 GW of new solar capacity in 2023 alone — more than doubling the pace from just two years earlier. Total global solar capacity surpassed 1,600 GW by end of 2024. China installed over 200 GW in a single year, which is more than the entire US solar fleet accumulated over decades. At current trajectory, solar will exceed 5,000 GW globally by 2030.

A category 5 hurricane dissipates about 600,000 GW of heat energy through cloud formation alone — dwarfing human power infrastructure. A major volcanic eruption releases energy equivalent to thousands of GW sustained over hours. The Gulf Stream carries about 1.4 million GW of thermal power northward. Even a modest thunderstorm generates 10–100 GW. Nature operates on power scales that make our entire grid look like a nightlight.