[Edit]
Here's some rough math.
From wiki, assume a typical ion engine can produce 150mN of thrust from 4,000 W of power input.
Using a space station solar panel as an example of solar collection in space, each space station solar panel is 420 square meters in size and produces 31,000 W of power.
One space station solar panel would then provide (31,000 W / 4,000 W) * 150 mN = 1,162 mN, or .001162 N of force.
The force required to accelerate 100 tons at 1g requires 996,402 Newtons of force.
To generate that much force, you would then need 996,402 N / .001162 N = 857,488,812 space station solar panels worth of power.
As one space station solar panel is 420 square meters, then that requires 857,488,812 * 420 square meters = 360,145,301,040 square meters of solar panels.
Assuming square construction, each side would need to be 600,121 meters, or 373 miles long.
I assure you, just using high thrust engines makes infinitely more sense than building a pv-based ship scaled up so far that the ship's dimensions are nearly 400 miles long on each edge. At least for any time soon ..
High Isp solar electric systems would not exploit the Oberth effect (likely they would start in high Earth orbit) so they don't have a high acceleration need from that.
If you want to accelerate to 15 km/s in 1 week, that's 2.5 milligees.
The power/weight ratio of nuclear rockets actually sucks, compared to chemical rockets. Conveying heat through a solid/fluid interface is awkward and slow compared to just making it in situ by combustion.
