That's true - for some reason I was thinking of tides as following the moon's orbit, when of course they're caused by the earth spinning underneath the moon (ti

That's true - for some reason I was thinking of tides as following the moon's orbit, when of course they're caused by the earth spinning underneath the moon (tides occcur daily, not monthly. Always check your working against reality, kids). Weirdly, the resulting pool of energy is pretty much the same size - wikipedia quotes the rotational kinetic energy of the Earth as being 2.138E29J. So we're still talking about an insane amount of energy. If you use 17.7TW as the total power consumption of all of humanity, if we were to somehow derive ALL of our energy needs from tidal generation, it would take 358 million years to deplete the rotational energy of the earth.