Steve,
It all depends on what you are trying to achieve. If it's just to keep water above the point where it starts to thicken (around 4 degrees) then a couple of large fish tank heaters will do the job, which will be a few hundred watts.
If you want 'heated' water then electrically you're going to need something with a lot of ummmph. That might be the e-bay one mentioned by john at 2kw, although I wouldn't leave this one all night.....it will burn out by being on constantly. So what you'd need to do would be to put a timer into the line so that it came on around the 4am time, given you 3 or 4 hours before setting out.
But you would need to do your own tests to find out the temperature of the tank water at different ambiant temperatures because that will change over the months.
Final warning. I'm not a great advocate of 'hot water' on cold glass during winter months. You'll be putting glass under extreme stress which some types can take whilst others will crack.