Washington State has been behind in the race to adopt solar energy, and perhaps for good reason. Seattle is not known for having a lot of sunshine. Years ago I remember seeing a t-shirt that read: "Seattle Rain Festival, January 1 - December 31."
But cloudy and rainy days do not preclude getting energy from solar power. Solar cells are actually more efficient, and degrade more slowly, the cooler the temperature. So by that metric, WA could be considered a better place for solar than AZ. Not that there aren't challenges in the Evergreen State. Those trees we love so dearly do block the sun from reaching solar panels, regardless of whether the sky is blue or grey.
When I bought my house here in 2014 it needed a new roof. The roofer I chose offers solar roofing tiles but didn't suggest them for my house. When I specifically asked about solar tiles, figuring the time to install them was when I was re-roofing the house anyway, the company explained that because of the shading on the south-facing portions of my roof, I wouldn't get enough energy benefit for it to be worthwhile. And this was from someone who stood to profit from upselling me to solar rather than standard composite roof tiles.
New technology may be able to improve the performance of solar panels in rainy climates. The concept takes advantage of graphene, an impressive material composed of single layers of graphite that has been proposed as a solution to myriad technical challenges but so far remains a material with great potential, in search of an economically viable commercial application. Solar cells that generate energy in the rain and in the dark are just the latest proposal for this "wonder" material.
Researchers at the Ocean University of China in Qingdao added a layer of graphene to a dye-sensitized solar cell. The concept is that positively charged ions in rainwater bind to the graphene, creating a sort of capacitor that can generate an electric current. The process works equally well during the day or night. This research is still in the very early stages, however. Initial testing demonstrated the ability to generate a current, but researchers used saltwater as a stand-in for rain. Real rain contains various salts, in concentrations that may or may not mimic those used in the lab. The trick is to make use of real rain with sufficient conversion efficiency to generate measurable energy.
If graphene-coated solar cells can be made into a scalable product, they have the potential to overcome one of the disadvantages of solar power: the inability to generate power at night. The Pacific Northwest seems to be an ideal climate for such a technology, but commercial development, if even feasible, is many years away. Meanwhile, there are a great many commercial buildings in Seattle with no trees nearby that look like good candidates for solar panels, despite the clouds and rain.