In the State of Washington, there are two incentives that are often conflated when solar power is being discussed: Net Metering and the Washington State Production Incentive.
Net Metering
Net Metering is an arrangement wherein you trade kilowatt-hours (kWh) with your utility. If your home or building is producing more electricity than it is using the excess flows out to the grid, running your meter backward.  
As a result, a solar array in Seattle will feed a lot of excess kilowatt-hours into the grid during the summer. Net Metering allows you to "bank" that extra energy with your utility to be used in the winter months when solar production drops and electricity consumption rises.
The fiscal year for Net Metering is from May to April. If you still have kWh “in the bank” at the end of April you forfeit their value. For this reason, systems are designed to not exceed 100% of the consumer's annual production. Net metering does not have an expiration date, but there is a threshold in State law below which your utility must offer net metering.  Some utitilties have enough solar on their system to have exceeded that threshold, but, with a couple of exceptions, none have considered ending their net metering program so far.
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