Calories (th)/hour to Terawatt

Ecologists track energy flow through ecosystems: sunlight → plants → herbivores → predators. Each link is quantified in cal/h or kcal/h per square meter. A temperate forest floor receives roughly 500,000 cal/h/m² of sunlight; plants capture 1–2% as biomass. A field mouse consumes about 3,000–5,000 cal/h in food energy. Expressing everything in cal/h makes the efficiency losses at each trophic level immediately visible.

A factor of 1,000. Since 1 kcal = 1,000 cal, 5,000 cal/h = 5 kcal/h. Nutrition and exercise science almost always use kcal/h (the "food Calorie" per hour), while laboratory calorimetry might use cal/h for precision measurements. The confusion between small and large calories has caused countless errors in student lab reports. When reading older literature, always check whether "calorie" means the thermochemical calorie (4.184 J) or the kilocalorie (4,184 J).

A hibernating black bear's metabolic rate drops to about 15,000–25,000 cal/h (roughly 17–29 W) — only about 25% of its active resting rate. Its body temperature drops just 5–6°C (unlike true hibernators that cool near freezing), and heart rate falls from 40–50 to 8–10 beats per minute. The bear burns about 4,000 kcal/day entirely from fat reserves, losing 15–30% of body weight over 5–7 months of hibernation.

In pre-SI European engineering, heating systems were often rated in kcal/h. A standard European radiator might be rated at 1,000 kcal/h (1,163 W). German and Italian heating catalogs from the mid-20th century used kcal/h exclusively. The conversion to watts was mandated by EU directives in the 1970s-80s, but older buildings across Europe still have heating system documentation in kcal/h. Italian plumbers still sometimes think in "frigorie" (negative kcal/h) for cooling.

Radioactive decay heat in spent nuclear fuel rods: a few hundred cal/h per rod years after removal. Slow corrosion reactions in sealed containers. Heat generation in composting piles (2,000–10,000 cal/h per kg of compost). Bacterial metabolism in soil samples. The continuous heat loss through a single-pane window: about 200,000 cal/h per square meter in winter. These are processes too slow for per-second measurement but too fast to ignore over hours.

The Sun delivers about 173,000 TW to Earth's surface. Human civilisation uses roughly 18 TW total. So we'd only need to capture 0.01% of incoming solar energy to power everything — an area of solar panels roughly 400 km × 400 km, about the size of Montana. The challenge isn't total energy availability; it's cost, storage, transmission, and the fact that sunlight is spread thin and intermittent.

Imagine 18 trillion light bulbs burning continuously, or 9 billion people each running a 2 kW heater non-stop. That 18 TW figure includes everything — electricity, transport fuel, industrial heat, cooking, heating. About 40% comes from oil, 27% from coal, 24% from gas, and the rest from nuclear and renewables. The US alone accounts for about 3 TW despite having only 4% of world population.

Replacing all 18 TW of human energy with clean sources would require roughly 60–75 TW of installed solar capacity (accounting for ~25% average capacity factor). That's about 40 times current installed solar. At 2023 installation rates of ~0.4 TW/year, it would take 150 years — but installation rates are doubling every 2–3 years. If that exponential trend holds, we could theoretically reach 60 TW of solar within 15–20 years.

Earth radiates about 47 TW of geothermal heat from its interior, driven by radioactive decay and residual primordial heat. That's 2.5× human energy consumption, but it's spread across the entire surface at extremely low density (~0.09 W/m²). Iceland, sitting atop a mantle plume, exploits geothermal for 90% of its heating. Globally, geothermal electricity capacity is only about 16 GW — a tiny fraction of what's theoretically available.

No — the terawatt scale is a very recent phenomenon. In 1800, global human power consumption was about 0.5 TW (mostly biomass burning). By 1900 it reached 1 TW with coal industrialisation. We crossed 10 TW around 1985. The jump from 1 to 18 TW in just 120 years tracks almost perfectly with global population growth times rising per-capita energy use. Pre-industrial humans used about 0.1 kW each; Americans now average 10 kW per person.