Why we [might] need fusion power:
a) Shipping. Big container ships *can't* be sail-powered (the largest sailing ships ever are about 1% of the size we'd need). But existing marine nuclear power is a proliferation and/or meltdown risk. If we can make a small fusion reactor, we could make our existing global supply chains carbon-neutral.
b) Continuous industrial processes that can't survive a power glitch, much less nighttime on a windless night (no renewables). There are a lot of those.
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c) Cold winter nights in the far north. Most folks think solar power is great—it peaks during daytime in summer, when you want air conditioning! But that's less useful if you need heat to avoid freezing on a winter night.
d) Space travel beyond roughly Jupiter orbit (not much sunlight that far out).
e) Speculatively: fusion reactors as a controlled source of neutrons for destroying existing radioactive waste stockpiles through transmutation.
Of these, (a) and (b) are critical.
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@cstross Fusion power actually worries me. Because it comes with the implicit assumption that when our power is "clean" we can use as much of it as we like. And we can't: Earth can only shed waste heat via well-understood radiation into space. Which isn't enough if our power usage continues to increase at its current rate, even "clean" we cook ourselves within a couple of centuries.
