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Tidal Power on Callisto
The tidal forces working on Callisto are significant, and there should be a liquid water ocean down there somewhere which will be in motion. This energy could be tapped to power a civilisation on this moon; even more energy could be extracted from the oceans and/or rocks of the other Galilean moons, but Callisto has the advantage of being largely outside Jupiter's magnetic field and the charged particles trapped there.

Even though the amount of sunlight falling on these moons is fairly small, the tidal power could make up for this. Of course, ther is no such thing as a free lunch, so extracting tidal power will affect Callisto's orbit- but the amount of energy in the orbits of these moons is so vast that we could extract enough energy to power an Earth-like civilisation for a million years without affecting the orbit of Callisto by more than one part in ten thousand.
Idle thought: I wonder if anyone smarter than me cares to back-of-napkin an guesstimate of how present day Earth-like energy usage might compare per capita to a population totally dependent on artificial life support?
This really does depend on the efficiency of the life support system, and whether the population is human or biont, or mechanical, or virtual.
A human gives of waste heat of around 100 watts, so we can assume that this is the minimum amount of energy used by the body in a second. To obtain this energy the human needs to digest food, which itself must be grown.

An optimistic NASA report here suggests that four people could be supported by a 'bioregenerative system' in space totalling 98m3. This is about 25 square metres each; assuming that the spaceship is in Earth orbit, we can times this by the solar constant to get 34000 watts of power collected by this area. This works out to be around 0.3% efficient- I would hope that mature OA life support systems would be more efficient, but taking this as a guide for near-future energy usage, it suggests that each human will need at least 34,000 joules per second just to live.
And that's the bare minimum, without luxury frills like lights, transport, communications, manufacturing, entertainment. Ok.
34 kilowatts seems quite reasonable as a life support requirement. I'd assume that in later eras the amount of energy expended on 'frills' would increase. while the amount required to provide food, water and oxygen would decrease as the Closed Life Support becomes more efficient. So maybe the amount of energy per person might not increase very much.
Note that large sophonts, such as sophont whales and elephants, and high-energy humans such as Superiors, would need more food and energy, while a vec could probably operate on less, since they often use electricity directly. Virtual sophonts presumably require much less actual energy, maybe 100 watts or less.
Cool, I can work with those numbers. At least, it gives a starting point for developing concepts.
On a very cold world a certain amount of energy would need to be used for heating, although we can assume that insulation would be very good. And of course any construction projects, such as megastructures and terraforming, would require a lot of energy in the short, and maybe medium, term.
Tidal forces on Callisto would be insignificant compared to the other Galilean moons because it doesnt participate in the 1:2:4 Laplace resonance that Io, Europa and Ganymede are locked in. While we believe that Callisto does indeed have a liquid water layer, it is also the largest body known to not be fully differentiated because it was never tidally heated enough to fully melt its interior. This idea would probably be a good plan on the other moons but I dont believe would be particularly viable on Callisto.
The other Jovian moons are also bathed in radiation and EM fields and have dangerously active events on their surfaces. Not such a pleasant place for biologicals to live. A specially adapted vec society might do it, if we develop such in the real world future. Otherwise, it's robots all the way down.

Still, in the vicinity of Jupiter, energy will not be in short supply for colonies with a controlled population growth; IE, any viable space colony. My concern is, what if we humans don't end up physically adapting to low gravity? Either by natural or engineered means. It'll be orbital habs only until we get over that hump. 1 G is the way to be!

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