In the Winter, for a location at 40 degrees latitude, the sun is lower in the sky and the average flux received is about 300 Watts per sq. meter
A typical household Winter energy use is around 2000 KWHs per month or roughly 70 KWH per day. This is equivalent to about 72 hours of incident sunshine on your rooftop or roughly 9 days per month of steady sunshine (which most locations won't receive).
But, at the moment solar collectors are only about 10% efficient so the true yield of 1000 square feet of collecting area is only about 22 KWH under 8 hours of sunshine in the Winter. This represents 1/3 of the typical daily Winter energy usage and it assumes the sun shines on the rooftop for 8 hours that day.
Another example calculation for Solar Energy which shows that relative inefficiency can be compensated for with collecting area.
How Solar Energy is Used:
Thermal Conductivity of some Materials:
Heat flow through a wall:
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