How the five-minute market works
Think of the Western Energy Imbalance Market (the real-time market, run by the California Independent System Operator, where western utilities trade power in five-minute blocks) as a shared dispatch desk for much of the West. Every five minutes it looks at what each participating utility needs and what each participating generator can deliver, then it matches the cheapest available supply to demand across state lines automatically, with no phone call and no side deal.
Arizona sits in a useful spot on that map. At midday the state's solar, both rooftop and the big plants out in Maricopa and Pinal counties, often produces more than local load can use, so the market can send that surplus west and north to utilities willing to pay for it. After sunset, when demand stays high and the panels go quiet, the same market lets APS and SRP pull in cheaper power from neighbors to cover the evening ramp (the steep climb in net demand as solar drops off and the air conditioners stay on).
The payoff, in numbers you can check
CAISO publishes a Western EIM benefits report every quarter, and it is the closest thing to a scoreboard this market has. The reports estimate gross benefits, meaning the savings from trading efficiently rather than each utility balancing alone, and they break the figure out by participating utility. Cumulatively, across all participants since the market opened in late 2014, those gross benefits have run well past $5 billion at last public report, with the total climbing every quarter.
Read those figures carefully. They are gross estimates, not the net amount that lands on any one bill, and they rest on a model of what costs would have been without the market, a comparison no one can observe directly. The honest takeaway is narrower but still real: trading in five-minute blocks is cheaper than going it alone, the operator reports roughly how much cheaper each quarter, and you can open the report and find Arizona's utilities in the table yourself.
Who trades, and who answers for it
Three Arizona utilities are in this market. Arizona Public Service joined first, around 2016, Salt River Project followed around 2020, and Tucson Electric Power came in during the early 2020s, so most of the state's customers now sit behind a utility that trades in real time across the West.
Oversight splits along familiar Arizona lines. APS and Tucson Electric Power are investor-owned and answer to the Arizona Corporation Commission, the elected five-member body that sets their retail rates, while SRP is a public power provider governed by its own elected board and is not rate-regulated by the Commission. The market itself is a wholesale matter, so the Federal Energy Regulatory Commission has the final say over its tariff, and CAISO is working through a longer transition toward more independent regional governance and a day-ahead companion market.
Worth watching this month
1. CAISO's next quarterly Western EIM benefits report is due in the coming weeks, and it will show whether Arizona's reported savings held up through the hot months; this one is routine but worth a look.
2. The Extended Day-Ahead Market (EDAM, a day-ahead companion to the five-minute market) continues its phased rollout, and any public filing on which Arizona utilities commit, and when, is the item that actually matters here.
3. Watch the West-Wide Governance Pathways effort at CAISO for steps toward a more independent regional governing body, a slow process that shapes who ultimately controls this market.
4. Arizona Corporation Commission open-meeting agendas, posted on azcc.gov, are worth scanning for any APS or Tucson Electric Power filing that references wholesale market costs or savings.
5. Keep an eye on FERC docket activity on the Western markets, where tariff changes are proposed and protested before they reach your utility; most entries are procedural, a few are not.