Even though Washington is cloudy many months there is a good solar potential here. Seattle City Light suggests a rule of thumb of 1000 kilowatt hours per year for each 1000 watts of generating capacity. There are several incentive programs and tax breaks that can greatly improve the payback period for solar investments.
Solar FAQ with Incentives and payback periods – http://www.seattle.gov/light/solarenergy/solarfaq.asp
A typical home system consists of 20-30 solar panels mounted on a roof, plus an inverter that converts DC to AC and provides an interconnect to the electric meter. From April to October the system generates more than is consumed on a monthly basis and power is fed back into the grid with net metering. At night and during winter the grid provides power.
Save on energy infographic – https://www.saveonenergy.com/todays-solar-panels/