The appendices of 'Consider Phlebas' has information on Culture energy production when describing the motivation, or lack thereof, for the Culture-Idiran war:
And energy was, if anything, even more generally available (than matter) through fusion, annihilation, the grid itself, or from stars, taken either indirectly as radiation absorbed in space or directly tapped at the stellar core.
So The methods would be:
1. Fusion - not the most efficient I think, but easy if you don't have a lot of antimatter stored and can grab some hydrogen and helium.
2. Annihilation - I think this means matter/antimatter annihilation. I don't think antimatter is very plentiful in nature, so it has to be manufactured using some other form of energy. Stored antimatter is however much more powerful than any of the other methods save perhaps the energy grid.
3. The grid itself - I assume somehow using finely tuned fields to extract energy from the grid (explained below).
4. Stellar radiation - Culture 'field technology' is ill-defined, but ships can and do project fields with about any properties you can think of. I think they could probably produce fields that would absorb 100% of the stellar radiation shining on them or focus the radiation into something that could absorb a lot. I doubt this would be used for space travel because it wouldn't be as powerful as other methods and because there is not that much stellar radiation between the stars.
5. Stellar cores - I imagine this would be using wormholes to displace incredibly hot and dense matter from the cores of stars to the ship and using the energy contained therein. Like #4, not very useful between the stars because there is a limited range. This could be used when near stars to produce large quantities of antimatter for later use however.
I don't recall a description in any of the books of a Culture ship running out of fuel, so my guess is that they are able to get all the energy they need from the grid.
Ships do take time to produce munitions such as CAM (compressed antimatter) warheads. 99
My guess is that they are able to use energy from the grid to create antimatter.
In surface detail there is an armada of ships from a level or two lower in the technological hierarch from the culture where it is mentioned that they will work on fusion but be much more powerful if they have time to create their own antimatter, so a higher civilization provides them with some antimatter, but importantly not from their flagship. So fusion is apparently enough for space travel.
The Energy Grid
The cosmological model in the culture universe is explained in 'Excession', the first book from the series that I read. The universe we see is like the surface of an expanded balloon. This balloon resides inside another balloon, which resides inside another balloon, etc. Inside our balloon is another balloon, and inside that is another balloon, and on until supposedly there is some universe creation engine creating these expanding universes like Russian nesting dolls. The three dimensional space and matter in our universe exists on the surface of one balloon. Between it and the next outer balloon is a region of hyperspace called ultraspace. Between it and the next inner balloon is a region of hyperspace called infraspace.
Seperating those areas from their counterparts of the other balloons is something called the 'energy grid'. It is a chaotic energy field. Ships propel themselves through hyperspace by using the fields created by their engines to 'push' against the grid in hyperspace. At one point a ships is described as diving from infraspace through normal space to ultraspace and back again. I imagine the engines are pushing so hard off the lower grid in infraspace that it pushes them to ultraspace where it has to push off the higher grid to increase or maintain velocity in what would look like a sine wave with real space in the middle, hopping back and forth between infraspace and ultraspace.
The energy grid can be used as a weapon, something that happened in the Idiran war was using those engine fields to distort the grid and cause ripples large enough to penetrate real space where they would annihilate anything they came in contact with.
I assume getting energy from the grid entails those finely tuned engine fields somehow. 99