You may need 10w per sq. ft. to achieve room temperature for an insulated house. 5w per may prevent freezing in an insulated well house.
Using this,
http://www.degreedays.net/
we can get an idea of your winter temps. You also may want to know your Outside Design Temperature which for my area @ 39 N. Lat. is +14F.
And,
http://www.amazon.com/Fahrenheat-Marley-WHT500-Utility-Heater/dp/B001OEPXXU
If you don't want to gamble $100, wire up a few 100w bulbs to a t'stat and see how many it takes to give enough heat.
In a medium size house, the center can be at 45F and the pipes in the walls can be freezing.
It comes down to a lump sum for insulation plus a small electric bill or you paying a larger annuity to the PoCo. If the payback period is less than 10 years, insulate.