The read out on the unit says i would get 1.7 hours with fresh batteries and the heater set at the lower setting of 750 watts. There's not a lot of space to heat in a sedan so if the thing ran 50% of the time, i'd get over 3 hours of heat. I do like your electric blanket Idea though...that'd be cozy... maybe too cozy 😴🛌I think a 900-1400 watt heater would drain the power tool batteries quick.
I just a 12v electric blanket to cover my legs, as the hearing stering, seats and toque keep me pretty warm. Hehe