Arizona grid peak demand explained: why 4pm to 7pm costs the most

Arizona grid peak demand does not happen at noon. On August 7, 2025, the hottest working day of that summer, demand on the APS system bottomed out at 8am and peaked at 6pm. That single fact explains most of how Arizona electricity is priced.

Peak demand is the highest instantaneous moment of use in a period. The entire grid, every power plant and every transmission line, has to be built large enough to survive that moment, even though that moment lasts an hour and happens a handful of times a year. Capacity that sits idle 8,700 hours a year still has to be paid for.

The short version

The real hourly load shape

Most articles about peak demand describe it. This is what it actually looks like.

The chart below is real hourly demand for the APS balancing authority (EIA respondent code AZPS) on August 7, 2025, the day APS set what was then its all-time record. The data comes from EIA’s Hourly Electric Grid Monitor (opens in a new tab), reported in local time. Arizona does not observe daylight saving, so local hours are straightforward.

View the numbers as a table

APS system demand by hour, August 7, 2025 — full dataset

Period	APS demand ( MW)
12am	6668 MW
2am	5886 MW
4am	5603 MW
6am	5625 MW
8am	5401 MW
10am	6007 MW
12pm	6778 MW
2pm	7698 MW
4pm	8229 MW
6pm	8499 MW
8pm	8232 MW
10pm	7636 MW

Source: EIA Hourly Electric Grid Monitor, respondent AZPS, demand, local hour ending, August 7, 2025.

Three features of that curve drive everything downstream.

The trough is at 8am, not overnight. Demand falls to 5,401 MW mid-morning, after the overnight cooling load ends and before the day’s heat builds.

The peak is at 6pm. Demand climbs all afternoon and tops out at 8,499 MW in the hour ending 6pm, then stays above 8,200 MW through 8pm.

Demand is still 7,581 MW at 11pm. The grid does not get a break at night in an Arizona August. Air conditioning runs long after sunset because the building envelope is still shedding heat it absorbed all day.

SRP’s curve on the same day peaked an hour later still, at 8,784 MW in the hour ending 7pm. Combined, the two systems were pulling roughly 17.3 GW into the early evening.

Why does electricity cost more from 4pm to 7pm in Arizona?

Because that is when demand peaks and the most expensive generation has to run. APS prices on-peak energy at 4pm to 7pm on weekdays year round for every plan it currently offers new customers, and charges roughly 34 cents per kWh in that window against about 12 cents off-peak.

The pricing follows the physics. During those hours the cheap baseload plants are already running flat out, so the marginal kilowatt-hour comes from whatever is left: peaking gas turbines, market purchases at the worst possible time, or storage. Time-of-use pricing is an attempt to pass that real cost difference through rather than averaging it across the day.

On APS Rate Schedule TOU-E, the published bundled energy charges are $0.34396 per kWh on-peak in summer, $0.12345 off-peak, and $0.03495 in the winter super-off-peak window of 10am to 3pm on weekdays from November through April. That last figure is worth noticing. On-peak summer power costs roughly ten times what winter super-off-peak power costs on the same plan.

One caveat on the hours. Saver Choice Plus (R-2), which has been closed to new customers since December 1, 2021 but still serves existing ones, runs 3pm to 8pm rather than 4pm to 7pm. Check your own plan sheet rather than assuming.

The solar timing problem

Here is the awkward interaction. Solar output follows the sun, so it peaks near solar noon and falls off through the afternoon. Grid demand peaks at 6pm. Those two curves are offset by roughly six hours.

At 6pm in August, a fixed rooftop array facing south is producing a small fraction of its midday output while the grid is at its annual maximum. This is why the value of exported solar has fallen so far, and why utilities have moved on-peak windows later into the evening over the last decade.

It also explains why self-consumption beats exporting under current Arizona tariffs. APS credits exported energy at $0.06171 per kWh under Rate Rider RCP, while on-peak retail energy runs about $0.34396. Every kilowatt-hour you can use inside the house during the peak window instead of buying it is worth several times what the same kilowatt-hour earns exported at midday. The full mechanics are in APS solar buyback rates explained.

More requests than the grid can take

The demand curve is getting taller, and the data-center pipeline is the main reason.

In direct testimony filed in the pending APS rate case (opens in a new tab), APS witness Jessica Hobbick stated that APS is already contractually committed to approximately 3,296 MW of data-center load and is engaged with prospects representing a further 16,908 MW of potential load. She notes directly that this figure “surpasses Arizona’s current system peak demand of roughly 8,200 MW.”

APS publishes the same shape as an uncommitted queue of 19.0 GW against a 2025 peak of 8.7 GW and a 2035 forecast of 12.0 GW. Data centers accounted for about 5 percent of the 2025 peak.

APS’s corporate position is that it serves “all customers that locate to our service territory, regardless of business type or class of customer,” qualified by doing so “in a measured and thoughtful way.” A more direct framing came from Patrick Bogle, the company’s data-center strategy director, who told AZ Family in July 2025: “We can serve you, but just not now,” and “We do not have the energy and transmission infrastructure to support the amount of energy that’s being requested of us.”

Attribute that one to Bogle rather than to APS corporate messaging. The two are not saying quite the same thing.

What the utilities are building

Meeting a taller curve means buying capacity, and the numbers are large.

APS has secured roughly 7,300 MW of renewable power, battery storage and natural gas, split as about 2,480 MW of solar power purchase agreements, over 500 MW of wind, roughly 3,460 MW of storage and about 520 MW of gas. A separate 2024 all-source RFP seeks another 2,000 MW or so, in service from as early as 2028.

View the numbers as a table

APS secured new capacity by resource type, MW — full dataset

Resource type	Value ( MW)
Storage	3460 MW
Solar	2480 MW
Wind	500 MW
Gas	520 MW

Source: APS newsroom, 'APS Secures its Largest-Ever Energy Supply', November 20, 2024.

Note that storage is the single largest block. That is a direct response to the shape problem above: batteries are how a utility moves midday solar into the 6pm peak at grid scale.

On the money side, Pinnacle West investor materials indicate roughly $10.35 billion of APS capital investment planned across 2025 to 2028, rising from about $2.40 billion in 2025 to $2.70 billion in 2028, with rate base growing from $12.23 billion at year-end 2024 to $15.73 billion in 2028.

SRP’s 2023 Integrated System Plan (opens in a new tab), approved by its board on October 2, 2023, plans for “greater than 40% growth in energy demand by 2035.” Its build list through 2035 includes 7,000 MW of renewables, 1,500 MW of battery, 1,000 MW of pumped hydro and 2,000 MW of firm gas, against 1,300 MW of coal retirements.

Utilities recover capital spending through rates. That is the mechanism connecting a taller demand curve to a larger bill, and it is covered in more detail in why your Arizona electric bill keeps going up and tracked on the APS rate increase page.

The record peaks, and how fast they are falling

The grid is under real strain but it is not failing. All three major Arizona utilities set all-time peak records in August 2025, and both APS and SRP broke those records again in summer 2026. The constraint is showing up as delayed interconnection for very large new loads, not as shortfalls for existing customers.

The record sequence, from utility announcements:

One correction worth noting, since several outlets got it wrong: TEP’s record was set on August 6, not August 7. TEP’s own release puts August 7 at 2,501 MW, one megawatt lower.

All three utilities also exceeded their own 2025 forecasts, which the Arizona Corporation Commission flagged in August 2025 (opens in a new tab). Forecasts were APS 8,491 MW, SRP 8,436 MW and TEP 2,434 MW.

Reliability is the other half of the story

A stretched grid is most fragile exactly when it is most needed: the hottest summer afternoons and evenings, when demand peaks and there is the least headroom to spare. In Arizona, losing power in that window means losing air conditioning in genuinely dangerous heat.

That changes how a home battery is evaluated. Under a tariff-only analysis, storage competes against the difference between the export rate and the retail rate. Under a reliability analysis, it is doing something a cheaper system cannot do at all.

Solar paired with storage does two separate jobs here: it keeps essential circuits running through an outage, and it lets a home draw on stored energy during the 4pm to 7pm window instead of buying at on-peak rates. A battery is not automatic, though. It adds cost, and whether it earns that cost back depends heavily on which utility and which plan. The honest version of that analysis is in solar plus battery in Arizona.

Common questions

What time is peak demand in Arizona?

Late afternoon into early evening. On the APS system’s 2025 record day, demand peaked in the hour ending 6pm at 8,499 MW and stayed above 8,200 MW through 8pm. SRP peaked an hour later, at 7pm.

The mid-morning trough, around 8am, is the daily minimum. Demand then climbs continuously for ten hours. This is why utility on-peak windows sit in the late afternoon rather than at midday, despite midday being the hottest part of the day by air temperature.

What are APS peak hours?

4pm to 7pm, Monday through Friday, year round, for every rate plan APS currently offers new customers. Weekends and holidays are off-peak. Summer on-peak energy runs about $0.34396 per kWh on Rate Schedule TOU-E against roughly $0.12345 off-peak.

One exception matters: Saver Choice Plus (R-2), closed to new enrollment since December 2021 but still serving existing customers, uses a 3pm to 8pm window instead.

Is the Arizona power grid running out of capacity?

It is under real strain. APS, SRP and TEP all set record peak demand in August 2025, and APS and SRP broke those records again in 2026. APS has disclosed a queue of roughly 19 GW of potential data-center load against a system peak near 8.7 GW.

The grid is not collapsing. It is being pushed hard, and the visible symptom is that very large new loads are being told to wait rather than existing customers being curtailed.

Does a stretched grid mean more outages?

Peak strain raises risk during the hottest stretches of summer, when demand is highest and the system has the least headroom. That is the window in which an outage in Arizona is most consequential.

Solar paired with a battery addresses this in two ways: it keeps essential circuits energised during an outage, and it shifts stored energy into the expensive peak hours. Neither benefit comes free, and the cost side is covered separately.

How much is APS building to keep up?

APS has secured roughly 7,300 MW of new renewable, storage and gas capacity, with storage the largest single block at about 3,460 MW. A further all-source RFP seeks around 2,000 MW more, in service from as early as 2028.

Pinnacle West investor materials indicate roughly $10.35 billion of APS capital investment across 2025 to 2028. A figure of “10 GW through 2028” circulates widely but does not appear in any APS or Pinnacle West document.

Related reading

• Why your Arizona electric bill keeps going up traces grid investment through to the line items on a residential bill.
• The APS rate increase page tracks the pending rate case.
• APS rate plans explained compares the plans and their on-peak windows.
• APS solar buyback rates explained covers why exported energy is worth so much less than avoided on-peak energy.

Demand, capacity and tariff figures were verified against EIA, APS, SRP and ACC publications on August 10, 2026. Peak records are broken regularly and capital plans are revised, so confirm current figures against the linked sources.

• Rate Cases
• APS
• SRP
• Arizona