Following up on a point that has been made elsewhere by some people and myself, let's reword Sandro's comment here...
At the moment, any way I slice it, I can't come to any conclusion other than Commanders in
Solo have a tougher time than those in Open. Despite the
possibility of PvP in Open, which can slow things down, Powerplay is predominantly a PvE game mechanic. The main way to get merits is via PvE actions and the only way to really support your power is through PvE activities. People in Open (and group) have the
possibility to wing up to increase their effectiveness, especially when doing military strikes, because everyone gets merit points for taking out targets. Those doing deliveries in Open can wing up for protection against NPC interdictions, and the odd player interdictions. Players in solo must face those NPCs alone!
My suggestion is, to balance this, players in solo receive a PP bonus, perhaps 4x, to reflect the fact that hey cannot wing up.
However, later, FD can implement NPC wings, and this will redress this balance issue. At which point, Open would carry the higher risk, and a bonus for Open could be considered.