Power is a statistical parameter representing combined strength of your Troopers. For that reason, the only ways to increase it are buying or upgrading troopers. You can't get it from any other source. On top of that, Power's not used for anything, aside from odds evaluation during picking fights, and Rankings.
Power is basically computed with the following formula: =4*T+L (where T – number of troopers you have, L – sum of their levels). In other words, it equals 4 times the number of troopers you have plus their Levels put together. For example, you have 3 troopers and they are all level 6, which would be 4*3+18=30 .