I've used Katz brand circulating tank heaters. I really like them. I really like that you can get them in higher wattages. My last one I got was 1000W, and I installed it on a Geo Metro. It warmed up that little engine in about 20 minutes which was great. Definitely not one you'd want to keep plugged in overnight though. I put it on a Christmas light remote so I could plug it in when I got home, and turn it on from inside the house ~20 minutes before I left. It worked great.