The hour the utility borrows your air conditioner

How an hour of your thermostat becomes a power plant

It is a little after 5 on a July afternoon in Phoenix, the kind of day when the bank sign reads 113 and the asphalt gives off heat through your shoes. The air conditioner has run most of the day. Then the thermostat on the hallway wall clicks up two degrees on its own, and a few minutes earlier a text arrived from the utility saying a peak event is on. You signed up for this last spring, probably for the bill credit, maybe because a neighbor mentioned it.

Both big Phoenix-area utilities run a version of this. APS calls its program Cool Rewards; SRP has its own smart-thermostat program for customers with an eligible device (a Nest, ecobee, Honeywell or similar). On a high-demand afternoon the utility sends a signal that eases each enrolled thermostat up a few degrees for the length of the event, usually an hour or two. One house barely registers. Tens of thousands moving together, at last public report, become a demand response resource, which is a block of electricity demand the grid can lean on instead of starting another power plant.

The honest math on the credit

What a household gets is modest and worth reading carefully. The utilities typically offer a one-time enrollment credit plus a smaller recurring credit each cooling season, on the order of a few tens of dollars, though the exact figures move year to year and are posted on each program page. The trade is comfort for a short window on a handful of the worst afternoons, with the house drifting a couple of degrees warmer before it settles back.

Honesty requires saying the savings are not dramatic. Most of the value is the credit itself, not a big cut in usage, and how much the program delivers for the grid depends on how many events get called and how often people opt out. You can override any single event from the app or the wall unit, which the utilities allow by design; do it every time and you still keep the credit, but the shared resource shrinks.

Who runs it, and who answers for it

The two programs look alike and sit under different roofs. APS is an investor-owned utility regulated by the Arizona Corporation Commission, so its demand-side programs, credits, and event rules are set and revised through ACC dockets that any resident can read online. If an APS term seems off, the Commission is the public venue.

SRP is a different animal. It is a public power utility, a political subdivision governed by its own elected board and council rather than by the ACC, and it sets prices and programs through its own public process. Checking the rules, in other words, means two different front doors depending on which utility serves your meter, and a complaint that fits one may have nowhere to land at the other.

Demand side, not supply side

This is the mirror image of the desert build-out this desk usually covers. Utility-scale solar and batteries add supply to meet the summer peak; a thermostat program subtracts demand from the same hour. It is also distinct from a home-battery virtual power plant, which pushes stored electricity back onto the grid. One adds megawatts, the other quietly removes them, and both are aimed at the same stubborn afternoon.

Timing is the whole point. As rooftop solar fades toward evening, Arizona's net peak slides later, roughly into the 6-to-8 pm window on the hottest stretches, which is exactly when easing air-conditioning load is most useful. Monsoon humidity can push cooling demand even when the thermometer dips, so the events do not always track the single hottest hour of the day.

Worth watching this month

1. Watch the APS Cool Rewards program page for any change to enrollment terms or credit amounts before the 2027 cooling season, which is routine housekeeping but worth a look.

2. Watch the Arizona Corporation Commission docket system for APS demand-side management filings, where proposed changes to event rules would surface first and public comment is allowed.

3. Watch SRP's board and council meeting notices for any price-process items touching its smart-thermostat program, since SRP changes run through that body rather than the ACC.

4. Watch the U.S. Energy Information Administration's monthly Arizona electricity figures for how high summer peak demand ran, which gives context for why these programs exist.

5. Watch for either utility to report how many events it called last summer and total enrollment, numbers that matter more than the marketing and are not always published promptly.