I run grip warmers and a home-made heated vest, along with a phone charger (doesn't draw much). My vest draws 55-60 Watts. Not sure about the grip heaters, but probably another 20 watts or so. Never had an issue with draining the battery.
I have ridden down to 10 deg F before. It sucked, but I survived. The heated vest and grip warmers were key. I also have insulated covers that go over my hands like big mittens that keep the heat in.
I wear insulated riding pants and waterproof boots, and I can honestly say I never had a problem with cold feet or legs. I wear a balaclava over my head, but still get some air up in the helmet. I think that is the worst part for me in cold weather. Then if you seal up the helmet, it will fog up inside and the fog will start to frost over. I need one of those snowmobile helmets.