Kardashev Level 1, 2 and 3 correspond to using the power of an entire planet, star, and galaxy respectively. In terms of steam engines, that's 10^16, 10^26, and 10^36 W. This rather neatly allows us to generalize the Kardashev levels to non integer values, using the formula (Log10[P]-6)/10. Currently our P is about 10 TW, or Kardashev level 0.7. Pre-industrial consumption, mostly based on agriculture, was level 0.5, comparable to the power consumption of all ants. The Roman Empire at its peak was about 0.36.