Another theory. The electrical needs of the ship and life support must be a rounding error compared to the power output of the engines. The energy required to accelerate up to and down from relativistic speeds absolutely dwarfs any onboard power use.

While the ship is under acceleration, power is no problem. You siphon off 0.1% of the astrophage drive's power, and that's more than enough for the ship's onboard power needs.

But what about when the ship isn't under acceleration? If your engines were infinitely throttleable, you could run the engine at 0.1% output just to use it as a generator. But rather than design an engine that can run at such a huge range of output levels, it may be more mass efficient just to slap some solar panels on the craft.

This also makes sense for the mission the ship was designed for. The only time it's ever in deep space is when it's accelerating. The main drive only turns off when the ship is already in orbit in the inner solar system of the target system. So the only time you ever need power but not the main drive is at a location where solar panels would work well.

I think this may explain it best. They ship could be powered entirely by astrophage. But trying to make one engine that can work at such a huge range of power outputs just isn't practical. It's easier just to have some supplemental solar panels to use while the craft isn't accelerating.