I've been running the Tourmaster Synergy brand for about 4 years now.
I have the pant liners, full coat and gloves. I used to laugh at guys with heated gear. Now I'm one of em !
My method of checking for sufficient alternator output was simple. I hooked the stuff up to my Victory V92C (28amp alternator), cranked it up and measured the voltage at the battery. On that bike with all garments maxed I still had 13v across the terminals, however it was losing the battle at idle showing 11.9. So it was still rideable in that mode and no issues at all when not maxed out, or if you just have the jacket liner and gloves running.
The Harley has (i think) a 50 amp alternator and it doesnt even know the gear is there.
So you can always do your own little test and I'll ya what....heated gear isnt cheap but once you have it its pretty darn nice. The only stone I throw at it is I hate having wires all over the place. LOL ! And...forgetting to unplug when dismounting the motorcycle.
EDIT: and good point by the fella above about bringing extra garments in case of failure. I've had no issues but a buddy of mine did recently when his heated vest gave out. It was old and heavily used so it lasted a long time.