It makes no difference. The pad must be fed by the auxilliary battery if it's using 12V. But whatever the source, ultimately the energy comes from the ICE. Whether you're draining the aux or the hybrid batteries, the ICE has to recharge it.

 

It's like using the rear window electric defrost. Are you leaving it on your entire commute? I bet it feels NICE ! A 10 amp draw is going to cost you. Resistance type heaters are one of the least efficient loads you can have. Is it worth it? That's for you to decide...

Most power ports are fused at 10 to 15 amps max. So your probably on the edge of smoking it. The fuse protects the wires so no damage can occur. But if the fuse opens you'll lose power on that circuit until you replace it. And don't go and put a larger capacity fuse in there unless you like the smell of smoking wires and enjoy spending time on your back under the dash replacing burned wire, or paying someone to do it.

 

I don't think a 120 watt- draw can drop your mpg by 10%-something else is going on.I think this implies that you are only using 1200 watt-hr per hr on your trips.This is less than 2 hp;2hp will barely get you moving-You are probably using more like 12-20 hp while you are moving.Something else is causing the drop in mpg.This 120 watts is roughly the difference in day(no lights) and night-(lights on) driving.Of course-if the vehicle has DRL-it is drawing some power all the time.Charlie

 

The defrost on the HCH shuts off automatically after 15 minutes.

 

... assuming the inverter you're using is 100% efficient.