Radiant heat not only warms the floor - it also supplements the heating of the room itself. Here's a link to a radiant floor supplier, Warmly Yours (corny name, huh!). Very nice site. Warmly Yours Radiant Floor Heat
I put down electric under floor heating in my home bathroom. Very easy - no fancy wiring and you can buy a thermostat to control the temp of the floor. Be aware that most manufacturers only make products to go under tile or stone. Only a few - like Warmly Yours - are for use under wood floors.
I actually used the Easy Heat brand because the product was a single wire that you lay down in a serpentine pattern and I was installing a stone tile floor in an irregularly shaped room. The room is only 5'x8'. The under floor heat is the primary heating for the entire room - no baseboard or forced air heating in the room. (I am just finishing the installation so I can't verify that the room will be toasty to 40 below but, hey, this is Cincinnati, not Nome!!!)
The other style is a thin rectagular or square mat, like what Remco described, that is put down under the floor in one piece. YOU CANNOT CUT THE MAT because you would end up cutting the embedded heating element. That's not such a big deal if you can find a size that will cover most of the floor. I think that Lowes and Home Depot sell the mat style.
I'm not certain if the electric radiant heat would also work on DC current. No reason why it would not work but you'd have to check the current load - might require a 27 bank battery pack to get you through the night!!!