Solar panels face one huge problem... you're probably moving away from the sun. Blah blah something about inverse proportion - at twice the distance from sun to earth, one receives a quarter of the solar energy.

I just want to point out that slowing down is also accelerating. If you were speed up for a month to create a force of 1g, then rotate the craft 180 degrees, before slowing down at the same rate, then we're (mostly) maintaining a constant 1g force.

Fuel is the problem, as mojo points out. I guess we need a dense source of fuel that is available from target planets.