If someone is not home during the day:
Some like the 8% rule saying that you should set your thermostat should be set back 8% higher than what you typically set it at. So if your usual home's temperature is 78 degrees, 8% of 78 is 6 degrees. The funny thing about the 8% rule is that the temperature only varies by one degree for the range of temperatures typically used in our home. Here is a quick reference table to illustrate the point depending on what your thermostat is set at.
74 degrees - really?
75 degrees - 6 degrees setback or 81 degrees
76 degrees - 6 degrees setback or 81 degrees - degrees setback or 81 degrees
77 degrees - 6 degrees setback or 81 degrees
78 degrees - 6 degrees setback or 81 degrees
79 degrees - 6 degrees setback or 81 degrees
80 degrees - 6 degrees setback or 81 degrees
81 degrees - 7 degrees setback or 83 degrees
82 degrees - 7 degrees setback or 83 degrees
83 degrees - 7 degrees setback or 83 degrees
The idea behind the 8% rule is to minimize the work the air conditioner has to do to get back to your comfortable thermostat setting.
I like the home performance way of looking at this popular question, which factors how many changes in the ceiling height, the percentage of sun struck windows, the age of your home and the type and quantity of insulation in the attic. Here is how each of these factors affects your information.
1. More changes in ceiling height = more heat gain. Changes in ceiling height indicate areas of high heat gain from the attic into the house, and the more heat gain, the harder the air conditioner has to work.
2. More sun struck windows = more heat gain. The more heat gain your home has, the warmer will it be while away and the harder the AC has to work to remove that air. It is a fact that our homes get more heat gain through the sun struck windows than an insulated attic.
3. Age of your home = indicates the age of the ductwork and insulation, and the condition they're in. If your ductwork leaks 30% of its air into the attic, it will be harder to cool the home. Also, if the insulation has settled with age and the R-value has been compromised, the cool air will more rapidly escape out of the home.
4. Type and quantity of insulation = indicates how well your conditioned air stays inside your home. You can get more aggressive with your thermostat setback if your home is properly insulated up to an R-38. Both APS and SRP home energy audit program offer a one time $250 rebate for adding more insulation to your attic depending its current condition.
If going through this list, you have little sun struck windows or have shade screens, you have added insulation, the ductwork is sealed and your airflow is balanced then you have the green light to aggressively set back your thermostat during the day and switch to a APS or SRP time of use program.
If you suspect your home is inefficient, or has some of the red flag items listed above, whether you set your thermostat back aggressively or just 8% doesn't matter. What matters is just playing the set back game during APS and SRP's peak hours. Your home is going to heat up quicker in the summer and be harder to cool regardless of your set point but the best thing you can do is just set it back. If you can, I would first see what upgrades are going to be the least expensive to fix while saving you the most money. If some upgrades can be easily done, get your home up to speed first.