I don't know whether the limit for economical fusion power is 1GW or 1TW or 1PW either. But if it turns out to be the higher side of that, then I'd be leaning against it. Sure, we could use the power, but right now, our power needs are low enough that we might build only one or two of them to run whole countries or even continents off of. I don't really like the idea of having One Huge Power Plant for most of a continent Seems too vulnerable to all sorts of failure modes to me. Imagine if some important part deep in the machine breaks because of some mistake somewhere, or even sabotage. The whole continent's power could be down, and it might take weeks or months or longer to make a new widget and get it installed, given that there's so little demand for them with only a couple of plants.

I'm afraid I can't; it's my personal guess, and with an admittedly large margin of error. You make a fair point that if it turned out to be more like a terawatt, that might not be out of the question.