Federal Income Tax Credit
You may take a credit on your Federal taxes of up to 30% for solar equipment. The phase outs start in 2020 as follows (2020-26%, 2021-22%, after 2021-no tax credit).
You may include any labor costs properly allocable to the onsite preparation, assembly, or original installation of the residential energy efficient property and for piping or wiring to interconnect property to the home.
The credit is limited to your tax liability but you can carry forward the remainder to subsequent years.
You must reduce the basis of your home by the amount of any credit allowed. This means that when the day comes that you sell your home, the credit would effectively be added to your gain.
Washington Sales Tax Exemption
The Sales Tax exemption in WA State Expires June 30, 2018 – The sales tax exemption is taken at the point of sale, and you must provide a completed Buyer's Retail Sales Tax Exemption Certificate to the seller and/or installer.
Renewable Energy Advantage Program (REAP)
Washington State also has a Production Incentive program called Renewable Energy Advantage Program (REAP) that extends through June 2020. It is an annual payment made to reward property owners for installing made-in-Washington solar systems to incentivize renewable energy manufacturing in our state. – this credit incentive makes payments to the customer based on energy (per kw hour) produced and is available through your electricity provider.
For additional information, this is a good summary: