It can't have been some sort of drawing energy directly from vacuum as that would imply limitless energy, but you never hear of any of the ships either refuelling or doing anything similar. The Liberator could recharge itself quite quickly.

I'm no engineer, but I do like reading about potential spaceship designs. I would guess the Federation ships use hydrogen, which would be self-refuelling, but also limited in the sense that they could burn (especially in battle) more than they pick up. The power limit would depend on things like hydrogen density in local space and how big their hydrogen scoop is. They might have to cease using it in battle, as I imagine it would represent a vulnerable spot of the ship, and always be careful to leave a little power in reserve so they can refuel after. The Liberator, as you say, recharges, which suggests electrical energy stores. Frivilously... a pink Liberator bangs on a toy drum as the Pursuit ships slow and fall behind. It keeps going, and going, and going.