Reply #58: Humans currently use a bit over 400 exajoules each year.

[phantom power]

Which could, in principle, be supplied with about 4000 1-gigawatt nuclear reactors.

For various reasons discussed in detail by Paul Chefurka, I suspect that relatively few of those 4000 reactors will ever be built, although I note gloomily that those same reasons will prevent relatively little of those 400 exajoules from being replaced by any means. Nuclear, wind, solar or otherwise.

But anyway, the number you should memorize is "400 exajoules per year." That is what humans currently use. And it provides a good baseline for thinking about energy usage.