What the April 30 rule actually does
Under RCW 80.60 the utility measures net electricity produced or consumed during each billing period, and excess kilowatt hours are credited on the following period bill at the retail rate.
On April 30 of each calendar year, any remaining unused kilowatt hour credit accumulated during the previous year is granted to the electric utility without any compensation to the customer-generator.
There is no payout at avoided cost, no rollover into the following year and no discretion. Credit that has not been consumed by that date simply stops being yours.
April is a particularly awkward date for a Washington household. Credit accumulates through a bright summer, drains through a long grey winter, and the line falls before spring generation has properly recovered.
How to size against it
Start from your last twelve months of bills rather than from your available roof. The design should follow the consumption, and any installer working Washington seriously will ask for the bills before proposing a size.
Ask what percentage of your annual usage the proposed system covers. A design meaningfully above 100 percent is producing electricity that will be forfeited unless something else changes.
Ask for the monthly profile too, not just the annual total. A system that balances across a year can still accumulate a large credit through summer that is not fully consumed by the April 30 date, and only the monthly view reveals that.
The legitimate reason to size ahead is a concrete planned increase in load: an electric vehicle, a heat pump, an addition with a specific timeline. A general expectation of using more electricity later is not a reason, it is a hope.
The questions that expose a lazy design
Ask which twelve months of consumption the design was built from and ask to see them. A design produced without your bills was produced from your roof.
Ask what the savings model assumes happens to credit still unused on April 30. If the answer involves carrying it forward, the model is describing something RCW 80.60 does not permit.
Ask what the model assumes about your consumption changing. A projection that quietly grows your usage each year to absorb the production is solving the sizing problem on paper rather than in the design.
And ask what a smaller system would look like. An installer who can show you the alternative and explain why the larger one is better is a different proposition from one who only has the one answer.
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 Renton 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 under RCW 80.60 subject to the April 30 forfeit.
Then add the electricity you stop buying, from a system sized to your actual consumption. In a state with modest incentives and cheap power, correct sizing is the largest single lever you control.