Calories (th)/minute to Gigawatt

Running at 10 km/h burns about 8,000–12,000 cal(th)/min (8–12 kcal/min) depending on body weight — that's roughly 560–840 W of total metabolic power. Sprinting can hit 25,000 cal/min briefly. But here's the catch: only 20–25% becomes mechanical work; the rest is heat, which is why you get hot. A 70 kg runner at marathon pace (~12 km/h) burns roughly 12,000 cal/min and must dissipate about 700 W of waste heat through sweating.

Before SI standardisation, the calorie was the dominant energy unit in biology because it was defined by water's heat capacity — and most biological calorimetry involved water baths. Measuring oxygen consumption in liters per minute and converting to cal/min via the caloric equivalent of oxygen (4.825 kcal/L O₂) was standard practice. The per-minute rate matched the natural timescale of spirometry measurements. Modern papers have mostly switched to watts, but the older literature is vast.

Metabolic rate scales with body mass to the 0.75 power (Kleiber's law). A 3 g mouse produces about 36 cal/min; a 70 kg human about 1,200 cal/min; a 5,000 kg elephant about 30,000 cal/min. Per kilogram, the mouse is 12× more metabolically active than the elephant. This is why small animals eat constantly and have rapid heartbeats — they burn through their energy reserves much faster relative to their size.

In the late 1800s, Wilbur Atwater burned thousands of food samples in a bomb calorimeter — a sealed steel vessel submerged in water — and measured the temperature rise in cal/min to calculate total energy. He then subtracted energy lost in digestion (measured via feces and urine calorimetry) to derive the "physiological fuel values": 4 cal/g for protein, 4 cal/g for carbohydrate, 9 cal/g for fat. These Atwater factors, over 120 years old, are still the basis for every nutrition label worldwide — remarkably accurate despite their crude origin.

Most wrist-based trackers are 15–30% off for cal/min estimates — some studies found errors up to 93%. They estimate from heart rate, which correlates loosely with metabolic rate but is confounded by temperature, caffeine, stress, and fitness level. Chest-strap heart monitors are better (10–15% error). Gold standard is indirect calorimetry with a face mask measuring O₂ and CO₂, accurate to about 3%. For most people, tracker estimates are directionally useful but not precise.

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.