Southern California at sea level does not really get too cold in comparison with most everywhere else, but it will drop into the high thirties on occassion mid winter. I actually like using a lot of heavy blankets, but sometimes climbing into a 45 degree bed is more than a little unsettling.
So I bought this on Amazon on Monday:http://www.amazon.com/Heated-Mattress-El...7850&sr=1-1
I installed it under my sheets, under my covers, and even though it was not cold out(57f), I cranked it up to high and let it go 3 hours before climbing into bed.
At first It was nice and toasty, and I turned it down to one( out of 7). A few minutes later I was no longer using any covers. I unplugged the ciggy plug and cranked up my intake and exhaust fans.
It did not help, I had to get out of bed and turn on the rest of the fans.
Eventually I was able to climb back into bed but the mattress was still radiating a surprising amount of heat.
So lesson learned, wait for actual cold nights before using this product.
Now for the product details. It Claims a 4 amp draw. It was pulling over 6 amps when I had it cranked up to 7, so perhaps their "4" number is the average amount of amps consumed per hour.
The temp control is also the thermostat, so if it is left outside the covers the heating pad will get hotter than if is is inside or above the covers.
The ciggy plug seems to be one of the better designs. It does not pull out easily, and does not get hot. The Ciggy plugs which came with my DC to DC laptop converters(which can pull just as much amperage) are much more problematic.
The mattress heating pad must be safety pinned to the mattress to keep it tight and spread out. It comes with nice stainless pins and tabs on the cornersso you do not have to worry about hitting any wires.
Now, 6 amps is a significant draw on the battery. Most starting batteries in Vans are group 27 size, and contain 100 to 115 amp hours.
So if the battery was healthy and fully charged, and the heating elements were powered 100% of the time, the battery should last 16.67 hours until it registered 10.5 volts. This is considered 100% discharged, which no lead acid battery should ever be subjected to if you want it to last.
50% is a better target, so 8.3 hours of this mattress heating pad should leave you with 50% of the battery left, plenty to start the Van in the morning. The alternator will fairly quickly (within 2.5 hours depending on van and cabling length and thickness) bring the battery back up to 80% but many hours are needed to bring it from 80% to 100% so many consecutive nights of doing this will eventually cause your battery to leave you stranded. It will need many many hours of driving, or much better, a 10 amp battery charger overnight to get it back to 100% of it's remaining capacity.
The instructions say it is a good idea to crank the pad upto seven while you are driving before bed so the alternator is powering it.
Now, I doubt the thing will actually run at a 100% duty cycle So one should really add another 4 hours to that.
I will update this thread when it gets cold and let you know exactly how many amp hours it consumed overnight.
Overall I imagine climbing into a toasty bed in the winter will be a beautiful thing, even 'down by the river' as I choose to live.
It does come in a smaller and a larger size. I imagine the amp draw is either lower or higher than mine depending.