Your comment prompted me to do the math, and it worked out a lot better than I expected.

However many computers are working on making bitcoins, the difficulty of the problem will be adjusted so that fifty are made every ten minutes, currently. (See my previous link). That rate decreases over time.

Right now there are about 10 million bitcoins in existence, iirc. Let's use that number, it's nice and easy and just a factor of 2 from the ultimate maximum of 21 million. If we've got a $1 trillion bitcoin economy, each coin is worth $100,000. We can expect people to spend up to that much in computing resources and energy to generate a coin. (If I can get a $100K coin by spending only $10K, I certainly will...and so will everybody else until things even out.)

So the next question is, what will the rate of coin production be set at, by the time it gets that big? The rate of bitcoin production decreases by about 50% every four years. So let's say we hit that $1 trillion twenty years from now, putting us at about 9 coins per hour.

That's 78K coins per year, at $100K per coin, or $7.8 billion worth of computer time and energy spent per year.

Let's say half of that is energy, call it $4 billion. (I'll neglect the energy used to make those computers, which are dedicated entirely to bitcoins.) At ten cents per kilowatt-hour, we're talking 40 billion kilowatt-hours per year. Divide by the number of hours in the year and we get 4.5 gigawatts.

In other words, a $1 trillion bitcoin economy can be run on the output of roughly five typical nuclear power plants (assuming it's 20 years from now).