For natural warmth without using electricity try the Cozy Cushion. It works like and electric blanket but safer and without the electricity. Your pet will snuggle warmly on the faux fur double thick thermal cushion. A core layer of thermo-reflective material uses your cats' own body heat to warm the entire cushion - no cords to worry about and not electricity costs. The fabric is heavy duty and machine washable.
You can find this great product at your favorite pet store or order online from Doctors Foster& Smith. I use it for my older pets year round and for additional warmth in the winter.