Kilocalories (th)/hour to Terawatt

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.

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.