I bought a givi bag for my trip this summer, fantastic bit of kit. I wired in a cig adpater direct from my battery under the panels and cable tied onto the central bracket near the light mountings.

Watch out you don't interfer with your sterring lock and also be careful when routing the wires, there are some hot parts you need to avoid.

Finally, it makes sense to wire in a switch to save the battery in the event of leaving it plugged in when your not running the bike!