Even after my installation, I often wondered if I can expect SOME solar production on a dark & miserable Seattle day. I think I found the "edge of the envelope" answer a few days ago...
Last Friday, Dec 20, Seattle had the darkest day on record, since the University of Washington started recording total solar radiation 20 years ago. They recorded only 0.37 megaJoules/square meter that day, beating the Dec 14, 2014 record of 0.39. On Dec 20 my system recorded 0.93 KWh of production.
For comparison, the July daily peaks are around 27 megaJoules/square meter. I didn't get my system until late August, but my peak production so far was 52.8 KWh on Aug 28. Average for the first 4 months was 20.4 KWh/day, which is very close to my average consumption.