Computing the average hours of sunshine at given location&orientation
Between September and March (ie winter, assuming a Northern Hemisphere location) The sun rises in the South East, is Due south at midday and sets in the South west. Your South facing window would get sun for as long as the sun is up. The North West facing window would only get a glimpse of the sun around sunset (and it would then be low and weak)

Between March and September (Northern hemisphere summer) The sun rises in the North East, it is in the south between roughly 6am and 6pm, then sets in the North West. Your Southern window would get 12 hours of sun each day. The North West window would get some sun in the evening, and this sun would be low (weak but shining into the room)

Overall the South facing window would get a lot more sun than the North West window. The reason is that the sun is always above a point on the Earth that is between 23.5 degrees North or South of the equator. For anyone in the Northern Hemisphere, the sun is usually South of them. 

A quantatative estimate of this could be done, but the actual values you got would depend on the Latitude and precise aspect of the two windows. However the result from SunCalc looks to be robust. The North West facing window will only get a little sun on summer evenings, and little or nothing during the winter.