Using fiddle factors that have served me well over Elefant rallies and sidecar trips to Finland (knock 10% off what the bike manufacturers claim for not being at optimum RPM, knock another 5% off at 5C then another at 0 then another at minus 10 and so on, add 5% to what any accessory claims for bad connections and extra wiring) and the Wee has enough electrical power for UK lattitudes solo with both headlights. You only need the switch if your pillion will run their hair straighteners/every form of heated kit going and/or you will use all this gear for trips under 15 miles.
If you don't want the wiring hassle, either switch off your jackets for 15 minutes in each half hour and when 15 miles from your destination or get into the habit (if practical) of plugging into a trickle charger after each two up ride. I doubt you can take enough power to kill the engine, it'll be next day when the cold battery won't want to crank.
I do have one fitted but have never used it in anger. If it's warm enough for Karen to ride I don't need the heated kit.
If you want to go all extreme-adventure-adventurer there is also the option to keep the battery in the sleeping bag with you, or get your Special Friend to do it!
Andy