The most expensive hours in Arizona: how summer on-peak pricing reshapes the afternoon

How the peak window gets built into your bill

Two rate ideas do most of the work here. The first is time-of-use (a plan that sets different prices for power by the hour), where weekday afternoons and early evenings in summer carry an on-peak price several times the off-peak one. At last published schedule, APS runs its main on-peak window from about 4 to 7 p.m. on weekdays, while SRP's time-of-use plans use a longer afternoon-into-evening window, and its EZ-3 plan compresses the costliest hours into a three-hour block. Check your own sheet, because the hours differ by plan.

The second idea is a demand charge (a fee based on your single highest hour of use during the peak window, measured in kilowatts, not total kilowatt-hours). On APS and SRP demand plans, one hour of running the dryer, the oven, and the air conditioner at once can set a charge that lands on the whole month's bill. Summer here runs roughly May through October, with the steepest prices in the June-through-August stretch, so this is a five-month problem, not a one-week one.

The cheapest grid resource is the load you move

Shifting flexible use out of the peak window is the lowest-cost thing on the Arizona grid, because it costs nothing to build. Pool pumps are the clearest case: a pump that runs several hours a day can usually run overnight or mid-morning with no loss, and moving it off the 4-to-7 window can trim a meaningful share of a summer bill. Laundry, the dishwasher, and electric-vehicle charging move the same way, onto a timer or a delayed start.

Pre-cooling is the other lever, and it leans on the physics of a well-insulated house. Running the air conditioner hard before the peak begins, then letting the thermostat drift up a few degrees during the costly hours, shifts the cooling work into cheaper time. It does not work for every home, and a leaky or poorly shaded house in a Phoenix July may give most of it back, so treat it as something to test against your own next bill, not a guarantee.

Where this meets rooftop solar and demand response

Rate design does not sit alone. If you have rooftop solar, the credit you earn for power sent back to the grid is set separately (an export rate that the Arizona Corporation Commission has stepped down for APS customers over several years, with SRP running its own solar price plans), and that credit is usually worth less than the on-peak retail price you avoid by using your own power during the afternoon. That math pushes solar households toward pairing panels with a battery, or toward the same load-shifting everyone else does.

Demand response is the grid's paid version of the same move. SRP's Cool Rewards and the APS smart-thermostat programs let the utility nudge your thermostat up for short stretches on high-demand afternoons in exchange for a bill credit, and you can usually override a given event. These programs matter most on the worst days, the ones late in a long heat stretch when a monsoon storm has not yet broken the temperature and the whole region's air conditioners pull at once.

Worth watching this month

1. Your own summer rate sheet, since APS and SRP publish current peak windows and prices on their price-plan pages and the hours can change between plans and seasons.

2. The Arizona Corporation Commission's open dockets on APS rates and solar export credits, which set terms for investor-owned customers and are routine to track but occasionally consequential.

3. SRP board meetings and any posted price-process notices, since SRP sets its own rates outside the Commission and its calendar is public.

4. The close of the summer season, roughly in October, when peak pricing eases and it is worth checking whether your plan still fits your usage.

5. Monsoon and extreme-heat alerts from the National Weather Service Phoenix office, since the costliest demand days tend to cluster in long, storm-free heat.