What temperature should I set my thermostat in Arizona summer? There isn’t a single right answer to this, and anyone who gives you one without knowing anything about your house, your schedule, or your tolerance for heat isn’t giving you useful information. There is a framework for understanding what different thermostat settings actually cost, what they actually feel like to live with, and where the tradeoffs land in an Arizona summer specifically, because the thermostat math in Phoenix or Lake Havasu in July is genuinely different from the same calculation somewhere with a mild summer.
The setting that makes sense is the one that accounts for all of that, rather than a number someone read somewhere and repeated.
What the Efficiency Ranges Actually Mean for Your Thermostat
Every degree the thermostat setpoint drops in summer costs money in Arizona in a way that’s more significant than the general guidance suggests. The Department of Energy’s recommendation of 78 degrees when home gets cited constantly, and it’s a reasonable baseline for a moderate climate running a properly sized system. In Arizona that number needs more context.
The efficiency hit from dropping below 78 isn’t linear. The gap between the indoor setpoint and outdoor temperature is what the system is working against, and in Arizona that gap is already extreme. Setting the thermostat to 72 when it’s 115 outside asks the system to maintain a 43-degree differential. Setting it to 78 asks for a 37-degree differential. That six-degree difference in setpoint produces a larger difference in system runtime and energy consumption than the same six degrees would in a climate where outdoor temperatures peak at 85. The system is working at the edge of its capacity regardless of the setpoint, and lower setpoints push it further past that edge.
A reasonable starting range for an occupied Arizona home in summer is 76 to 80 degrees. Below 76, the cost increases become significant, and the system runs nearly continuously in peak heat, trying to maintain a setpoint that the outdoor conditions are fighting hard against. Above 80, the comfort tradeoff becomes real for most people, and the heat stress risk for vulnerable household members becomes worth taking seriously. Within that range, the right number depends on the specific house, the insulation quality, the window exposure, and what the household can actually tolerate.
Cost Impact
The cost impact of thermostat settings in Arizona is real enough to be worth understanding specifically rather than approximately. APS and SRP both publish data showing that each degree of setpoint reduction in summer increases cooling costs by roughly two to three percent. In a home spending $300 a month on electricity in July — which is not an unusual number for a larger Arizona home running a system that’s working hard — dropping the setpoint four degrees costs an additional $25 to $35 per month. Over a five-month cooling season, that’s $125 to $175 in additional cost from a thermostat setting.
Time of use rate structures change this calculation significantly for customers enrolled in them. APS and SRP both offer rate plans where electricity costs substantially more during peak demand hours, typically 3pm to 8pm in summer. A household on a time-of-use plan that pre-cools the home to 74 degrees before the peak window starts and lets the temperature rise to 80 during peak hours is managing both comfort and cost in a way that a fixed setpoint can’t accomplish. The pre-cooling strategy takes advantage of off-peak rates to do the heavy lifting before the expensive window, and the thermal mass of the home maintains reasonable temperatures through the peak period without the system running constantly at peak rates.
What People Actually Live With
The numbers that work on paper don’t always work in a house with a two-year-old, or a home office where sitting still for eight hours at 79 degrees isn’t comfortable, or a house with a west-facing bedroom that gets direct afternoon sun through single-pane windows regardless of what the thermostat says. The efficiency ranges are starting points for a real conversation rather than final answers.
78 degrees is comfortable for most people moving around a house doing normal activity. It’s less comfortable for people sitting still for extended periods, for anyone doing physical activity indoors, and for households with young children or elderly family members who regulate temperature less effectively. The household that can live comfortably at 78 saves meaningfully compared to the one that needs 74 to function well, and neither number is wrong — they just produce different cost outcomes.
What tends to work better than a fixed setpoint in Arizona is a schedule that matches the thermostat to the actual use of the house. Cooler during occupied hours when comfort matters, allowing the temperature to rise during unoccupied hours when nobody is there to feel it, pre-cooling before peak rate windows for households on time-of-use plans. Smart thermostats do this automatically once programmed correctly. A programmable thermostat does it manually. A fixed setpoint that never changes does none of it and costs more than it needs to across a five-month Arizona cooling season that adds up faster than most households track carefully.