There are a variety of incentives available in Washington State that significantly reduce the cost of a solar system – and help maximize your Return on Investment.
Federal Tax Credits
The Federal Government allows you to claim a Tax Credit of up to 30% of your total system cost on your tax return. What is more, you can roll it over, meaning that if you can’t use the full value in the first year, you can claim the remainder in the next year. There are two programs in the IRS code: The Residential Energy Efficiency Credit in section 25D of the tax code is for individuals and is claimed on the Form 1040, and Form 5695.  This credit is applicable to your Alternative Minimum Tax (AMT) as well.
For solar that is purchased and installed on a business, the Investment Tax Credit program in section 48 of the tax code provides a 30% credit + MACRS depreciation up to 85% of the project cost, or 'basis'.
This incentive is is good through 2021 and is reduced after 2019.

A tax credit is a dollar-for-dollar reduction in the income taxes that a person or company claiming the credit would otherwise pay the federal government. The ITC is based on the amount of investment in solar property. Both the residential and commercial ITC are equal to 30 percent of the basis that is invested in eligible property which have commence construction through 2019. The ITC then steps down to 26 percent in 2020 and 22 percent in 2021. After 2021, the residential credit will drop to zero while the commercial and utility credit will drop to a permanent 10 percent

Commercial and utility projects which have commenced construction before December 31, 2021 may still qualify for the 30, 26 or 22 percent ITC if they are placed in service before December 31, 2023. The Treasury and IRS are currently drafting guidance which will inform solar developers of which percentage of ITC they will qualify for depending on when they started their project

Washington State Production Incentive
The Washington State Production Incentive is funded by the state government and managed by your local utility. This program pays you for every kWh you produce, so a separate meter is used to record that.
The program has been renewed and revised but the legislature added a total commitment limit of $110M through June 30, 2029.  This commitment may have already been met, meaning that projects that are in the installation pipeline now will be enough for the program to reach that limit.
WSU provides a public dashboard of program statistics here.
Project Types
Residential projects are 12 kW and less and earn $0.14/kWh + a bonus of $0.04/kWh for panels that are made in Washington.  The annual maximum earnings are $5,000.
Commercial Projects are greater than 12kW and earn $0.04/kWh + the made in WA bonus, with an annual maximum of $25,000.
Community Solar Projects can be up to 1 MW and earn the same rates as Residential projects. 
Residential Example
For a 5-kW Residential system in Seattle, on a South-facing 4:12 roof with 95% solar access, the annual production would be around 5263 kWh per year,
Production Incentives are paid on top of the Net Metering benefit you get from your utility.
Net Metering
Net Metering, an incentive available since 1996, is a contract that you sign with your utility allowing you to trade power with them at retail rates. When you produce more energy than you consume the excess kilowatt hours are fed into the grid and credited to your account. When you produce less than the consume, you purchase electricity from the utility. See: Net Metering Explained.
Sales Tax Exemption
Washington State ended the Sales Tax Exemption for solar systems except for projects that are 500kW and larger.
Rebates, or "cash back incentives," for solar systems in Washington vary from utility to utility. Check out your utility's website to determine if rebates are available in your service area.
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