What should I set my thermostat to in an Arizona summer?

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

Nearly every thermostat recommendation a Phoenix homeowner finds online was written for a climate where the air conditioner runs four months a year and the house cools itself overnight for free. Neither is true in the Valley. The cooling season runs from April into October, the attic above the ductwork reaches 150 to 170 degrees on a July afternoon, and overnight lows in midsummer frequently stay in the 80s, so the house never gets the free recovery that a single national number quietly assumes.

The second thing national advice misses is that Phoenix households do not all buy electricity the same way. APS and SRP price the same kilowatt-hour differently depending on the hour, and the two utilities do not even use the same peak window. A degree is not a fixed amount of money here; it is a different amount of money at 2 p.m., at 5 p.m. and at 11 p.m. What follows is the range Valley households actually use, what is verifiable about per-degree savings and what is not, and how the published APS and SRP schedules change the answer.

How much does each degree above 78 actually save?

A figure of roughly three percent per degree circulates constantly in Arizona HVAC marketing, and it deserves to be treated as a rule of thumb rather than a measurement. Public-sector energy pages do not agree with one another on it: some express the saving per degree of setpoint change, others express it as a share of annual cooling cost from a multi-degree setback held for part of the day, and the two framings produce very different answers for the same house. No primary study supporting a single Phoenix number turned up in the sources checked for this page. Where a number cannot be verified, the honest answer is that it depends on the house, the system and the rate plan.

What can be verified is what a degree costs at a given hour. APS Rate Schedule TOU-E sets the on-peak period at 4 p.m. to 7 p.m., Monday through Friday, year-round, with a super off-peak period from 10 a.m. to 3 p.m. on weekdays during the November–April billing cycles and all other hours off-peak. That three-hour peak is recent. The Arizona Corporation Commission voted to shorten on-peak hours in APS residential time-of-use plans from five hours to three, and APS states that removing two hours required repricing to spread costs across the remaining three on-peak hours and all off-peak hours. Frozen plans did not move with it: Saver Choice Plus customers keep a 3 p.m. to 8 p.m. weekday on-peak window. A full comparison sits on the APS versus SRP time-of-use page.

SRP's windows are later and plan-specific. Conserve 6-9 p.m. and Save prices on-peak weekdays from 6 to 9 p.m. year-round, with the lowest-cost super off-peak energy from 8 a.m. to 3 p.m. every day. Manage Demand 5-10 p.m. and Save runs on-peak from 5 to 10 p.m., with the same 8 a.m. to 3 p.m. super off-peak window. SRP has also applied a temporary price reduction of $0.0038 per kWh across the May–October 2026 billing cycles, with prices returning to published levels with the November 2026 billing cycle. A household that does not know which window applies to its own meter is guessing at the price of a degree.

On demand-charge plans the arithmetic changes shape entirely. APS defines demand as the highest single hour of energy use during on-peak hours, 4 p.m. to 7 p.m. weekdays, within the one-month billing period, measured in kilowatts, and on the Time-of-Use 4pm-7pm with Demand Charge plan the summer demand charge is $19.585 per kilowatt. APS describes that plan as offering its lowest per-kWh prices while adding the demand charge. One degree that forces a compressor to run flat out through a single 5 p.m. hour can therefore cost more than the same degree held quietly all month, which is the subject of the demand charge explainer.

Should I turn the AC off when I leave, or turn it up?

The attic above the ductwork runs 150 to 170 degrees on a July afternoon, which is why the duct leakage that costs a Valley homeowner most is the leakage nobody can see: cooled supply air lost into that attic, and 160-degree attic air drawn into leaky returns, while the house is unoccupied and nobody is standing there noticing the system run.

A house allowed to drift to 88 or 90 degrees needs a long, full-output run to come back down, and in most households that run begins at exactly the wrong time of day, as APS on-peak pricing starts at 4 p.m. and SRP's on-peak window opens at 5 or 6 p.m. depending on the plan. APS's own guidance for the Time-of-Use 4pm-7pm with Demand Charge plan tells customers to pre-cool the home during lower-cost off-peak hours before 4 p.m. on weekdays. SRP is more specific still: three hours before on-peak hours begin, set the thermostat three degrees below the preferred temperature. The mechanics are covered in the pre-cooling strategy guide.

SRP's Bring Your Own Thermostat program pays a $50 bill credit per thermostat on enrollment, to a maximum of two, plus a $25 bill credit per eligible thermostat for each year enrolled — $150 in a first year with two thermostats. Events run May 1 through October 31, adjust the thermostat up to 4 degrees above the current temperature, number at least one and at most 15 per summer, skip holidays, and usually last no longer than three hours, sometimes with pre-cooling beforehand. APS Cool Rewards pays a $50 enrollment credit per thermostat and a $35 participation credit per thermostat at the end of each event season — $85 in a first year with one thermostat — for events between June 1 and September 30, never more than 20 a summer, each up to three hours, with the $35 credit applying for up to five thermostats per home.

The APS Cool Rewards FAQ advises that on time-of-use plans a smart thermostat reduces on-peak use mainly by scheduling pre-cooling before on-peak hours start on weekdays, and suggests considering turning off geofencing or occupancy-sensing features that auto-adjust the thermostat; a geofence that lets the house drift and then restarts cooling when a phone crosses the boundary at 5:30 p.m. works directly against the pre-cool. Second, a full shutdown belongs to absences measured in days, and the settings that suit an empty Valley house are a separate question covered under vacant home settings.

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.

What is the best sleeping temperature without spiking the bill?

Overnight is the cheapest comfort a Phoenix household can buy on a time-of-use plan, and many run it backwards. APS Rate Schedule TOU-E puts on-peak at 4 p.m. to 7 p.m. weekdays, with all other hours off-peak. SRP's Conserve 6-9 p.m. and Save closes its on-peak window at 9 p.m., and Manage Demand 5-10 p.m. and Save closes at 10 p.m. Under all three schedules, the hours between bedtime and sunrise are off-peak. A setpoint of 74 to 78 degrees held from 10 p.m. to 6 a.m. therefore runs on the cheapest energy of the day, while the same 74 degrees at 5 p.m. runs on the most expensive.

The sleeping figure most often quoted, 82 degrees, comes out of an older setpoint table built for a climate where night air does most of the work. A Phoenix house in July gets no such help. Overnight lows frequently stay in the 80s, the slab and framing are still releasing the afternoon's heat at midnight, and the attic is still warm at 2 a.m. Holding 82 indoors is a different proposition when the outdoor air is 88 than when it is 65, and a setpoint that has to be overridden at 2 a.m. saves nothing at all.

In two-story Valley homes the sleeping setpoint is often a duct problem wearing a thermostat's clothes. With the thermostat downstairs and the bedrooms upstairs, reaching 78 degrees in a bedroom can mean setting 73 or 74 downstairs and air conditioning an empty living room all night to get there. That gap is paid for every night of a seven-month cooling season. Balancing dampers, sealing attic ducts, adding return air upstairs or zoning the system treats the cause; the thermostat only pays for the symptom, as set out under hot upstairs in a two-story home.

From July into September, comfort stops being purely about temperature. Dew points climb during monsoon season, and a system set very low can satisfy the thermostat quickly, short-cycle, and leave the house cool and clammy because the coil never runs long enough to drain moisture. Two adjustments help more than another degree: leave the fan on AUTO rather than ON, since a fan that keeps running after the compressor stops re-evaporates moisture off the wet coil and pushes it back into the house, and run ceiling fans in occupied rooms only. Widely cited federal guidance holds that moving air lets a household hold a slightly higher setpoint at equal comfort, because fans cool people rather than rooms.

Are the standard 78/85 recommendations realistic at 115 degrees outside?

Central air conditioners are rated at a 95-degree outdoor condition. Every capacity and efficiency figure printed on the label describes the machine on a 95-degree day, and capacity falls as outdoor temperature rises above that point. Phoenix spends much of June, July and August well past it. That is why correct sizing here is a Manual J load calculation against a local design temperature near 110 to 115 degrees rather than a rule of thumb about square feet per ton, a point covered under what size AC a house needs. Equipment sized by rule of thumb can hold 78 degrees in May and steadily lose ground at 4 p.m. in July, and no thermostat setting repairs that.

So the 78-degree question really splits in two. In a house that holds 78 degrees at 115 outside with the compressor cycling off occasionally, the number is realistic and the only remaining question is what it costs. In a house where the system runs without stopping and the indoor temperature climbs anyway, 78 is not a setting, it is a wish. The usual causes are measurable rather than mysterious: duct leakage into a 150 to 170 degree attic, thin or displaced attic insulation, west-facing glass with no shading, a condenser coil loaded with monsoon dust, or equipment that was undersized the day it was installed.

The 78 home, 85 away, 82 asleep trio is worth understanding before anyone treats it as law. It traces back to an older ENERGY STAR programmable-thermostat setpoint table rather than to a current federal recommendation, and ENERGY STAR's present public guidance does not hand out one recommended summer temperature; a homeowner can confirm that directly at energystar.gov. The durable part of federal advice is directional rather than numeric — the smaller the gap between indoor and outdoor temperature, the lower the cooling cost — which is precisely the guidance that gets less comfortable as the outdoor number rises toward 115.

There is also a floor under all of this that has nothing to do with money. During a Phoenix heat warning, an 85-degree indoor setting in a home with infants, older adults, anyone taking medication that impairs heat regulation, or animals left indoors is a safety decision rather than a budget decision, as the guide on pets and heat safety sets out. The practical method for finding a household's own number is unglamorous: move the setpoint one degree, hold it a week, and compare hourly usage data for similar weather days inside an APS or SRP account. That produces a real answer for one specific house, which is more than any national figure can offer.