*HUGE noncanon alert*
While I can't speak for the official canon, my own thought was that subspace drives relied on an enormous bank of capacitors. It makes more sense than batteries, since you're going to need a high volume of energy discharged in a very short period of time, and there's probably some risk of an accidental discharge causing damage to the ship's entire electrical system if some numb-skull CO decides to leave the drives charged at all times. The jump drive's function is to tear a small tunnel through subspace between two points in normal space. This is sufficient for most intrasystem jumps (hence, fighters can return to base without being leashed to a jump node), but longer jumps require access to a pre-existing subspace tunnel.
To determine relative energy requirements of a ship's jump, I apply some geometric reasoning.... Consider a standard where the subspace drive onboard any ship must open a jump conduit of some specified length, and this length is equal for all ships. What you need to concern yourself with now is the radius of the tunnel that the ship needs to create. That's pretty easy to tell, just looking at the subspace animation for each type of ship. (You could argue, here, that subspace missions always occur in a tunnel that is really big, relative to the ships inside. Those also occur in the intersystem subspace conduits, which I'll address below, not the smaller conduits formed directly by a ship's subspace drive, so seeing them with a constant radius of Really Big™ doesn't conflict with the reasoning presented here.) Now, when you double the radius of a circle, you square the area, therefore, you square the energy requirements. This is why you have fighters, with small banks of capacitors that charge relatively quickly, that can jump through subspace on just a few seconds notice, while corvettes, destroyers, and other large ships will need several minutes forewarning ahead of a jump. Normally, this is a non-issue. When the Aquitaine is approaching a jump node, because they're so frakking slow, they know exactly how long they have before they reach the node and can start charging their jump drives at the optimal moment. In situations where you're jumping into an area, then immediately trying to jump out (such as "The King's Gambit," mentioned above), or in an ambush, where the vessel's crew didn't know they'd need to jump, recharge times become an issue.
Jump nodes, for all intents and purposes, are the terminal points of a natural, self-sustaining subspace tunnel that can be accessed by a ship's subspace drives. The node, as rendered on one's HUD, represents a safe radius, where the normal charge on a ship's subspace drive will allow it to produce a subspace tunnel long enough to intersect the pre-existing tunnel between systems. In situations where caution must be foregone, it is therefore possible to apply extra power to the subspace drives to access a subspace conduit relatively far from the node, such as the end of the third mission in Second Front*, at the expense of doing damage to subsystems across the entire ship.
What I've not addressed at any point are issues of general accuracy of a jump, when exiting subspace and navigational errors. I figure that by the FS2-era, even GTVA subspace technology has reached a point where they can perform some very Shivan-esque maneuvers, such as jumping in right on top of a hostile convoy, as long as they have intelligence on that convoy's location/route. (Shivans, on the other hand, likely have some means of sensing ships on the other side of the subspace barrier, so they don't need recon data or prior intelligence about a convoy's route; they can just spontaneously decide to ambush it.) When you're talking about capital ships as large as the ones in FS2, a few meters up and to the left of where you wanted to arrive doesn't really mean much. As for major navigational errors, fire the helmsman or repair your nav subsystem before exiting subspace.
Again, none of that is canon, but it is really nifty, and it's the logic I follow whenever I have to babble about subspace in my missions.
* -- Shameless plug? I think so!
