the only ones I have actualy put my hands on are the one listed below which I installed in my house.
I don't think the panels are going to be very effective when it is zero F. It should be 1/2 way between zone c and zone D in the performance tables from the SRCC
Solar Panels are rated by the SRCC. You can calculate their effectiveness as a function of outside temperature.
http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=2006008AThe good news is the colder the temp, the longer the heating season and the effectiveness is much higher in the shoulder seasons.
When I do the math for where I live, with the rated efficiency of the collector I get around 7 million BTU's per year from my double panel system. this equates to about 80 gallons of propane per year or about 1/3-1/4 of the total heat load required. I will admit the payback is still tough to justify unless you think energy prices are going to rise signifigantly, which I do.