Impossible to say without knowing what climate you're in, how well insulated the building is, how warm you are going to keep the place, and what you pay for electricity. If you said where the condo is, what city, that might help. Someone in a similar place in the same area might know. You may be able to call the utility and find out what last year's bill was, but of course that will also include the cost of electricity used for things other than heating.
I'm wondering if anyone knows approximately what it would cost to heat a 600 sq. foot condo in a brick building per month? It has forced air heating AND electric baseboards.
