How do APS and SRP time-of-use plans change how I should run my AC?

By Michael Fitzgerald, Founder & Editor, Phoenix Cooling Guide
Technically reviewed by Tom Mattingly, Owner & President, AZ Trusted Air LLC (ROC 317368)
Published 2026-09-18

Every Phoenix electric bill from May through October answers two questions at once: how much cooling the house needed, and what hour of the day it needed it. The second question is the one most Valley homeowners have never priced out. Both APS and SRP charge more for a kilowatt-hour used in the late afternoon than for the same kilowatt-hour used at nine in the morning, and a four-ton condenser fighting a 112-degree afternoon has no idea which side of that line it is running on unless a thermostat schedule tells it.

What follows is the on-peak window each utility currently publishes, what a demand charge does that an ordinary energy charge does not, how to find the plan name already printed on a bill, and whether a switch made in July is worth making. Rate figures move with tariff filings and billing cycles, so this page names the documents to check rather than repeating numbers from calculator sites that contradict each other.

What are the on-peak hours for APS and SRP, and how expensive is peak cooling?

APS publishes the window in its filed tariff rather than on a marketing page, and the tariff is the version worth trusting. Rate Schedule TOU-E defines the residential on-peak period as 4 p.m. to 7 p.m. Monday through Friday, year-round, with a Super Off-Peak period of 10 a.m. to 3 p.m. on weekdays that applies only in the November through April billing cycles. All other hours are off-peak. The same schedule defines summer as the May through October billing cycles — which is the half of the year a Phoenix cooling load decides the bill, and the half with no super off-peak window to hide in.

The three-hour shape is recent. The Arizona Corporation Commission voted to cut residential on-peak from five hours to three, and APS states plainly that removing two hours required spreading cost across the remaining three on-peak hours and all off-peak hours. Customers left on frozen residential plans keep the older clock: Saver Choice Plus on-peak remains 3 p.m. to 8 p.m. weekdays, and business plans remain 3 p.m. to 8 p.m. as well. Two neighbours on the same Phoenix street, with identical equipment, can be running against different windows.

SRP runs on a third and fourth clock. Manage Demand 5-10 p.m. and Save (E-16) sets on-peak at 5 p.m. to 10 p.m. weekdays year-round and offers its lowest super off-peak pricing from 8 a.m. to 3 p.m. every day. Conserve 6-9 p.m. and Save (E-28) narrows on-peak to 6 p.m. to 9 p.m. weekdays. Time-of-day customers receive off-peak pricing on six observed holidays. For the May through October 2026 billing cycles SRP is temporarily lowering prices by $0.0038 per kWh because fuel and purchased power cost less than forecast, with prices returning to posted levels in the November 2026 billing cycle.

Measured as exposure, the APS Time-of-Use 4pm-7pm Weekdays plan and SRP's Conserve 6-9 p.m. and Save each put 3 hours a weekday at the highest price, while the frozen APS Saver Choice Plus plan and SRP's Manage Demand 5-10 p.m. and Save each put 5 hours there. Per-kilowatt-hour peak prices are a separate matter: they live in the APS tariff sheets and the SRP ratebook, they move with filings and with the temporary reduction, and third-party calculator pages disagree with one another — and sometimes with the utility — on facts as basic as whether the APS residential window is three hours or five. One peak figure is published without ambiguity: on the APS Time-of-Use 4pm-7pm Weekdays with Demand Charge plan, the summer demand charge is $19.585 per kW applied to the single highest on-peak hour of the billing month, and APS's own worked example puts a 5 kW peak hour at $97.93.

Which rate plan is best for heavy summer AC use?

Residential plans at both utilities come in two shapes. Energy-only plans charge for kilowatt-hours at a price that changes by clock hour. Demand plans charge for kilowatt-hours as well, usually at a lower price, and then add a separate charge for peak power draw. A household that cannot control the second number will often pay more on a demand plan than it saves on the first, and a household that can control it will do the reverse. Heavy summer use does not settle the question. Predictable summer use does.

The practical test is whether the house holds a pre-cool. A well-sealed 2010s home in Gilbert can be chilled to 74 by 3:30 p.m. and coast to 79 by 7 p.m. with the compressor barely cycling. A 1978 block home in central Phoenix with flex duct running through an attic that reaches 150 to 170 degrees in July will shed that pre-cool in under an hour, and the air conditioner restarts inside the expensive window anyway. The duct leakage nobody can see is what decides whether a rate plan strategy works, which is why plan shopping and duct condition are the same conversation in the Valley.

The two demand charges are not built the same way, and the difference matters more than the price. The APS summer demand charge of $19.585 per kW is applied to the highest single on-peak hour in the billing month, so one Tuesday in July with the oven, the pool pump, the dryer and a four-ton condenser all running between 5 and 6 p.m. can set the charge for all thirty days. APS does provide a demand limiter that reduces the charge after a rare spike — once in the summer months of May through October and up to three times in a calendar year on the Time-of-Use 4pm-7pm Weekdays with Demand Charge plan — plus a demand charge credit once in a rolling twelve-month period.

SRP's Manage Demand 5-10 p.m. and Save calculates its monthly demand charge per kW from 60-minute intervals during on-peak hours, based on the average of each day's highest demand period rather than the worst hour of the month. One bad evening costs less on that structure than it does on the APS demand plan. Neither utility will tell a homeowner which plan is cheapest in the abstract, and neither should — both offer comparison tools that run the arithmetic against the household's own recorded usage, which is the only version of the answer that means anything.

For Phoenix-area homeowners who want this looked at on their own system, Phoenix Cooling Guide recommends AZ Trusted Air LLC (ROC 317368), whose owner Tom Mattingly reviews this guide. (623) 252-6403 · second opinion on a quote.

How do I find out which plan I am on and whether it is costing me money?

Start with the bill rather than with memory. Plan names are specific and easy to confuse: Time-of-Use 4pm-7pm Weekdays, Time-of-Use 4pm-7pm Weekdays with Demand Charge and the frozen Saver Choice Plus are different products at APS, and Manage Demand 5-10 p.m. and Save, Conserve 6-9 p.m. and Save and the Basic Price Plan are different products at SRP. SRP directs customers to check the bill or SRP My Account to see which plan applies and to switch there.

APS makes eligible customers a Monthly Plan Comparison showing whether another plan would have cost less based on how and when the household actually used energy. Plans can be changed as often as once per billing cycle, and changed back, either by logging in at aps.com or by calling customer care — 602-371-7171 in metro Phoenix. Two things a homeowner might reasonably fear losing are not at risk: APS states that Budget Billing and the Energy Support income discount are not tied to rate plan, so switching does not forfeit either.

One line-item detail explains a good share of confused bill comparisons. APS bills carry adjustors as separate items — the Power Supply Adjustment and the Renewable Energy Adjustment Charge among them — which can change annually with Arizona Corporation Commission approval, independent of base rates. A bill that rose while usage held steady has not necessarily proved anything about the rate plan.

On the SRP side, the price comparison display requires at least 12 calendar months at the residence, which quietly excludes recent buyers and the snowbird household that just took occupancy. SRP also retired several residential plans: its Board approved the retirement on February 27, 2025, and the Time-of-Use, EZ-3, Residential Demand Pilot and Electric Vehicle plans are closed to new enrollment and will be discontinued as of or before the November 2029 billing cycle, with at least 120 days' notice to affected customers. A household still on one of those plans is not in trouble today, but it is on a clock, and the comparison it should be running is against the plans that will still exist.

Does switching plans mid-summer make sense?

The usual objection to a mid-summer switch is that it is a bad time to experiment. In the Valley the opposite is closer to true, because the months that decide the annual bill are the months being tested. The safety net is real on both sides: APS permits a change as often as once per billing cycle with the option to switch back, and SRP's 90-day trial lets a customer keep the new plan or return to the previous one, with changes made through SRP My Account or by calling (602) 236-8888.

One SRP mechanic catches people who switch or compare around the season boundary. Seasonal rate changes are applied by meter read date: if the meter is read on May 14, all usage and demand charges on that bill are billed at the summer rate even though part of the usage happened in April. A bill that looks like proof a new plan failed may just be the first bill that crossed into summer pricing. Compare full billing cycles, not calendar months.

A plan change is worth pairing with a thermostat change, because the plan only pays if the schedule moves. SRP's published guidance is to set the thermostat three degrees below the preferred temperature starting three hours before on-peak begins, then three degrees above the preferred temperature during on-peak, then back to normal afterwards. SRP also notes that some homes need more lead time depending on size, age and building materials, and that a smart thermostat's learning features may need to be disabled before it will pre-cool at all. A 3,000-square-foot two-storey in Peoria with west glass frequently needs four hours of lead time where a small block ranch needs two — see the pre-cooling strategy page for how to find the right lead time for one house.

Two situations argue against switching now. The first is a household about to change its own behaviour — a new work-from-home schedule, a returning college student, a pool pump replacement — because the usage history the comparison tools rely on is about to stop describing the house. The second is failing equipment. A system short-cycling or losing capacity in August will post a demand peak no rate plan can soften, and the honest first step there is diagnosing why the system cannot keep up, not moving to a different price for the same wasted kilowatt-hours.