[kurthr]

The rule of 72 is useful. It would give roughly doubling every 35 years for a 1000years means 28 doublings or a bit over 256million x increase in power usage. Only off by ~50%.

I'm guessing that would put the population over a quadrillion, as well, with efficiency increases.

[HPsquared]

Can also think in terms of half-life.

For example at an inflation rate of 2%, that rule of thumb means the value of money has a half-life of 72 years.