Solar radiation is absorbed during the day and released gradually during the cold nights. This is the equivalent energy available from 4.25 trillion barrels of oil. Well a typical household in the North Eastern part of the United States uses between 1000 and 2000 gallons of fuel oil per year. A square foot per gallon is based on the calculations below. In other words one square foot of will harvest about two gallon's worth of number 2 fuel oil.
Lets be conservative though and simplify things.
Let's say one square foot is needed to harvest the equivalent energy in one gallon of oil. This means that if you use 2,000 gallons of fuel per year you will need about 2,000 square feet of collector surface to meet all your heat energy needs. You will also need an adequate heat storage and transfer system to make use of the heat you collect.