Why is solar-powered heating harder to do that solar-powered air conditioning?

So if solar-powered air conditioning is all the rage, why not solar-powered heating? After all, a solar hybrid heat pump can supply either heating or air conditioning.  And, the solar panels can be used to run the direct-current motors in these heat pumps in either mode.  So, if air conditioning is a no-brainer application for leveraging these, why not heating?  It really comes down to location, timing, and delta T.

In the winter...

1.  The highest heating load is normally when the outside air temperatures are coldest and when there's no chance for radiative heating (sunlight coming through your windows), and that's at night.  It may not come as a surprise that there's not sunlight to create solar power at nighttime. 

2.  The sun hits northern latitudes (in the northern hemisphere) at a lower angle of trajectory in the winter, and that sunlight is likely to be reflected or absorbed by the earth's atmosphere (clouds and other water vapor, dust, pollution) before ever reaching solar panels.  In short, the sun usually doesn't have much intensity in the winter.

3.  On a related note, the days are shorter, so the sun's simply not up long enough to generate a ton of BTUs.  Even a southern sunny location like Palm Springs sees around 37% less Peak Sun hours (where the sun's irradiance is at 1,000 watts/square meter) in December than in June.

4.  And, yes, some areas are seasonally cloudy.  Grand Rapids, for instance, has relatively sunny summers.  But, the winters, not so much.  With only a bit more than an hour of solid sunlight in an average winter day, don't become too dependent on your solar panels for heating.

5.  Finally, the temperature difference (delta T) between the outside air temperature and the target indoor temperature is more in the winter than in the summer.  How so?  Well, if it's 95°F outside in the summer, and you want to get your space down to 73°F, that's 'only' 23°F.  But, if you want to get from 11°F to 73°F, that's a bit longer of a hike.  That's why heat pumps always have a lower HSPF (heating seasonal performance factor--used for heating) than they do SEER (Seasonal Energy Efficiency Ration--used for cooling), even though both use the same formula:  BTUs produced / watts-hours consumed.  Much like driving a car at 110 mph (only recommended on a closed course) is less efficient on the gas tank than driving at 55 mph (less fun), it's more work to heat than cool, and work = energy.  A good SEER value is 22 and a good HSPF value is 12.  So, more energy is needed in the winter, but less solar is available. 

So, is all lost for leveraging your solar-powered heat pump for actual winter heating?  Not at all!  In many places (e.g., the southwestern U.S.), winter days tend to be a bit longer, a bit sunnier, and a bit less cold, making them a potential perfect candidate for handling the majority of the heating.  And, if you're in one of the aforementioned short, cloudy, cold day locations, think of any solar-powered heating winter hours you can reap from your unit as a bonus, the frosting on the cake to the wonder of summer solar-powered air conditioning.