You will need either a generator or high quality deep cycle battery. It really depends on how you want to run your practice sessions. I like to fly 6 flights in a session, that's 6 flights of 1 P-09 or (starting this week) 1 F-09 each flight. I take 4 flight packs with me and I use my Optima deep cycle battery to charge two of the packs at the field. It works out just right, I have little time spent waiting around for a charge. I found that if I have two or three packs then I spend time waiting. If you have 4 packs and a deep cycle battery (like the Optima, others don't last long) then you are good for 6 flights a session. If you have fewer packs or if you want to fly more than 6 flights in a session then you must have a generator. I find I get only 2 charges out of my Optima battery, so any more flight time means you must have a generator.

That's how I do it.