Why the payback has been long
Seattle City Light is a municipal utility with a largely hydroelectric supply, and it has charged around 11 cents per kWh for residential power. That is well under the Washington state average of roughly 15 cents and far under the national average of about 18.4 cents.
Solar savings are the price of the electricity you no longer buy, so a low rate produces low savings per kilowatt hour regardless of how good the system is. This is arithmetic rather than a criticism of solar.
Seattle also sits in one of the cloudier populated parts of the country, so the same panel produces less here than it would in eastern Washington. Both terms in the calculation work against a fast payback.
Anyone telling you Seattle solar pays back in six or seven years is either using a rate that is not yours or a production figure that is not your roof. The record here shows payback in the low teens for a reason.
What changes the calculation
City Light raised rates about 6 percent this year and has outlined increases approaching 10 percent a year for both 2027 and 2028. Compounded, that is a materially different rate at the end of the decade than the one you pay now.
That matters because a solar system is a twenty-five year asset priced today against electricity purchased over that whole period. Rising rates raise the value of every kilowatt hour the system displaces, for the rest of its life.
So the escalation assumption in a Seattle projection is not a technicality, it is close to the whole argument. Ask what annual rate increase the model applied and what it was based on.
Ask to see the projection at a lower escalation rate too, and at zero. A case that only works at an aggressive escalator is a bet rather than a calculation, and you should at least know which one you are making.
The April 30 forfeit
Washington net metering under RCW 80.60 credits excess kilowatt hours at the retail rate on the following period bill. On April 30 of each calendar year, any remaining unused credit accumulated during the previous year is granted to the utility without any compensation to the customer-generator.
April is an unkind month for that line to fall. A Seattle household builds credit through a bright summer, spends it through a long grey winter, and the reset lands just as spring production is picking up again.
The consequence is that a system generating an annual surplus donates the surplus every spring. There is no bank, no cheque and no carry forward.
So build to your consumption, not to your roof. Ask what percentage of your annual usage the design covers, ask to see the twelve months of bills behind it, and require a specific reason, such as a planned electric vehicle or heat pump, for anything above it.
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 Seattle 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, so there is nothing at state level to replace what ended. Guides describing a federal plus state stack are describing other states.
What does exist is the sales and use tax exemption under RCW 82.08.962, which removes state and local sales tax from equipment and installation labour for systems up to 100 kW AC through December 31, 2029, and retail-rate net metering with the April 30 forfeit.
And the electricity you stop buying, which in Seattle is cheap today and is scheduled to become less cheap. Ask for the projection with the escalation assumption stated explicitly, and ask to see it at zero as well.