Why this number carries the whole quote
Most items in a solar quote are checkable facts. The sales tax exemption either applies or it does not, the equipment has a datasheet, the price is the price.
The annual production figure is different. It is a model output with assumptions inside it, and small changes to those assumptions move it substantially.
In a high-rate state an overstatement is painful but survivable, because there is a lot of value per kilowatt hour to absorb it. Washington averaged around 15 cents per kWh in mid-2026, so there is less room.
With the federal residential credit gone and no state credit existing, there is also nothing else in the stack to make up the difference. The production estimate is doing more work in a Washington quote than in almost any other state.
The questions that test it
Ask for the annual figure in kilowatt hours per year, not only as a dollar saving. A dollar figure conceals both the production assumption and the rate assumption in a single number.
Ask what data source produced it and whether it uses location-specific irradiance for your address rather than a regional or statewide average. Puget Sound is cloudier than the state as a whole, so a statewide input flatters it.
Ask what shading analysis sits behind it, and specifically what it assumed about tree growth over twenty-five years. A model built on today canopy will overstate a mature suburban lot in ten years.
Ask what soiling and degradation assumptions it used. Panels lose a small amount of output each year, and a model that holds production flat for the whole term is overstating the back half of the projection.
And the rate assumption beside it
The other half of the savings figure is what a kilowatt hour is worth, and that varies enormously across western Washington. Puget Sound Energy has charged around 17.7 cents per kWh while Seattle City Light has charged around 11.
So the first question is which utility serves your address and whether the projection used its actual rate rather than a state average. Both an overstatement and an understatement are common and both come from the same generic input.
The second is escalation. Utilities across the state have proposed steep increases, so some escalation is defensible, but a compounding assumption over twenty-five years can carry most of the headline savings by itself.
Ask for the projection at zero escalation alongside the main one. If it still works, the case is robust. If it only works at an aggressive escalator, you now know exactly what you are betting on.
Rebuilding the Washington arithmetic
The 30 percent federal Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, so a cash or loan purchase in Federal Way receives no federal tax credit. Section 48E survives at 30 percent for third-party owners under leases and power purchase agreements.
Washington has no state income tax and therefore no state solar credit.
What exists is the sales and use tax exemption under RCW 82.08.962 on systems up to 100 kW AC through December 31, 2029, covering labour as well as equipment, and retail-rate net metering with the April 30 forfeit.
Then add the electricity you stop buying, calculated from a production estimate you have interrogated and a rate that is actually yours. In Washington those two numbers are the quote.