Ak - I only use a 25 watt because of our temps. If I use a higher watt bulb, the water gets warm and slimey. You can use any size (40, 60, etc) to meet your needs. Inside a shelter, the birds body heat helps too, but not enough to keep the water from freezing. You might want to test one out this year and see if it will do the job. There are great metal ones too, but don't use them on one gallon plastic waterers - it might melt them. The metal heater bases work great on metal waterers in any size. You may want to try that too or at least see what temps they are meant to work in. I realize what works here may not work there, as you got it much colder than we do.
Jody