I'm working on the Engineering Minigame now, and I'm running into a slight problem. The ship's power requirements are going to vary quite widely between when the ship launches (with 785 people on board) and when it's toward the end of the total mission (with 3000 people on board).
As fantastical as the propulsion system's power requirements are, accelerating the vessel at 1g for decades, the power requirements to simulate sunlight dwarf that number. The ship's engines will draw 6.12E+09 kWh of power per day. The lighting system for the agriculture system, by contrast, draws 2.67E+11 kWh. Agriculture draws 43 times as much power as the engines.
But, of course, that's not the whole picture. I was performing a worst case analysis of agriculture output for the complete ship's compliment of 3000. The agriculture output for 785 people would be a fraction of that amount. It doesn't exactly scale proportionally, because some long lead item like lumber would be growing for future consumption, but for the purpose of argument, let's say our power requirements for 785 people is roughly 1/3 of our peak agriculture, so 8.90e+10. (Still 14.5 times the amount of power as the engines).
There comes the issue of how power systems scale. With thermodynamic systems, the bigger the plant, the more efficient it operates. Now each system has slightly different rules, but a fusion plant will operates much like a fission plant:
A nuclear reaction generates a lot of energy, which is carried off (in the form of heat) by either pressurized water or liquid metal to a heat exchanger. That heat exchanger converts unpressurized liquid water into pressurized steam. In the process of condensing back into water we use that energy to spin a turbine which spins an alternator, and out of the alternator comes the electricity that we all know and love.
As the diagram indicates, there are a lot of moving parts in this system. If you have a lot of smaller plants you increase the number of parts that have to be monitored and maintained. The parts for a smaller plant also operate less efficiently, but that's a topic for breaking out my college thermodynamics textbook to explain properly.
On the other hand, having one giant plant that is normally operating at a fraction of its rated power is not efficient. It's partly down to quirks in the Rankine cycle, and partly because energy losses from spinning bearings and energy leakage occur at roughly the same rate no matter what speed the plant is running at. A large generator running at half speed consumes more fuel to produce the equivilent amount of electricity as a than a half-sized generator running at full speed.
After balancing a million wrong answers in my head, I've come up with is the concept that three generators would need to be running at all times to keep the colony alive. And thus, in the beginning, only one generator would need to be operating at peak efficiency to run the colony.
Because the colony is on its own, we need to maintain triple redundancy. For every generator we have running, we need to have one generator ready to take its place, and one generator out of action for maintenance or refurbishment. In short, the colony will have 12 nuclear power stations, spread around the gravity ring.
The particle accellerator is necessity for getting our deuterium molecules up to the energy where we can force them to fuse. We will assume that there are multiple redundant loops to allow us to take one or more down for service. The power of the loops comes from its circumference, and while a 500 meter in diameter loop is nowhere near the power of something akin to the Large Hadron Collider, it's still far more powerful than the 6.4m torroid of the ITER. Because we are operating by tapping a continuously accelerated stream of plasma, the reactor can run continuously. Or, at least as long as the parts, shielding, and ablative components hold out.
Scattered around the ship are megajoules of flywheel based energy storage. This allows the ship to whether spikes in power demand as well as momentary losses of power brought on by switching from on unit to another. The reactor design also acts as the heating system for the habitation spheres.
Built into the design is a pumping system to move water from one sphere's reservoir to another in order. This system serves two purposes. First, it allows mass to be re-distributed quickly and easily to correct for a mass imbalance in the ring. Second, it allows water from a sphere that has an operating reactor to be moved to a sphere with no operating reactor and vice verse.