I had a mattress heating pad, and the actual battery consumption would range wildly depending on the dial's setting and the ambient temperature.
Minimum was about 24AH consumption in 8 hours, maximum could be as high as 70AH.
The 12v pads do not heat up as much or as fast as household 120vac electric blankets.
I would turn mine on to high about 2 hours before getting into bed, and it would be a very nice 120F, then I would turn it all the way down, or Off.
If I did not turn it all the way down, I would wake up completely overheated and it would take a good half hour before the mattress cooled down enough.
Later on I had acquired a wattmeter to plumb inline and saw the voltage drop at the ciggy plug was extreme. Maximum amps was 6.2 amps.
When I cut off the Ciggy plug and about 3 feet of 18awg wire, and put some anderson powerpoles in place, maximum amperage increased to 7.4 amps, and it heated up much faster and got hotter and actually consumed slightly less battery power overnight. I was at the end of the second winter at this point and not long after, half of the mattress pad failed to heat up. I think it was the wires themselves just got worn out, as opposed to the extra voltage and amperage able to flow through the heating pad from cutting off the ciggy plug, but it certainly could have been a contributing factor.
I went last winter without replacing the pad, and only rarely missed it, but I am in a mild climate that rarely goes below 40f in winter
How much battery capacity and solar for it will vary pretty widely depending on your preferences/ needs.
More blankets is much cheaper, but climbing into an already warm bed when it is cold is heavenly.
