The Arizona Corporation Commission is the part of state government which regulates most utility companies in Arizona. The five members are elected and serve four year terms. They have established the Renewable Standard and Tariff, rules that require each utility they regulate to generate or purchase electricity from renewable sources like solar in an amount that increases each year until the current goal, 15% of electricity sales in 2025 is met. You can look at any of the cases decided by the ACC and even listen to their hearings live on the internet by going to their website: http://www.azcc.gov/
To reach this goal, each utility like Tucson Electric Power (TEP) sets up a proposed plan for each year and submits it to the ACC for approval. Each utility user is charged a small monthly fee set by the ACC and marked on your bill as REST charge. That money goes into a pool that is used to pay rebates to those who install solar panels, solar water heaters, etc.