Physics puts a floor under the cost of thinking. A brain runs on 20 watts, one AI prompt on 0.24 Wh: here is the real energy math of intelligence.
Sit with a hard problem for an hour, a proof, a contract clause, a strategy whose end you cannot see, and your brain will draw about 20 watts. That is roughly what it draws while you stare at the ceiling. The brain is around 2% of body mass and takes something like 20% of resting metabolic energy, and the extra cost of genuinely hard thinking is small next to that baseline (Quanta Magazine). The organ that produced the thought costs less to run than the lamp above the desk. Ask a machine the same question and the accounting changes shape. In 2025 Google published the first serious first-party measurement of inference: the median text prompt to its Gemini apps consumes 0.24 watt-hours of energy, emits 0.03 grams of CO2 equivalent and uses 0.26 millilitres of water, counting the accelerator, the host CPU and memory, idle backup capacity and data center…