To Run the AC, or Not to Run the AC?
Excerpt from Valley 101: A Slightly Skewed Guide to Living in Arizona, a collection of Clay Thompson’s columns for The Arizona Republic. (Originally published September 24, 2000.)
Q: My friend turns his air-conditioning off when he goes to work, goes on vacation or plans on being out of the house for more than a few hours. It gets up to 100 degrees in his house. Is he really saving money on his electric bill?
A: Coincidentally, as this is written, the air-conditioning is off here at the Valley 101 offices on the No. 9 parapet of the Dark Tower.
Our masters, who are sipping cool drinks and being fanned and fed grapes by interns, tell us this is a temporary malfunction, but in some of the more primitive areas of the newsroom there are mutterings of human experiments being carried out.
Nonetheless, brow dripping, we soldier on.
We have reason to believe that your friend—and we don’t mean to get too technical here — is a dope. We got partial confirmation of this by consulting with the estimable Scott Harelson, a Salt River Project
Yes, said Harelson, if you are going to be gone for a long time, like a couple weeks or so, you might savemoney and energy by turning the air-conditioner off. However, the unit is going to have to work really,
really hard to cool the place off when you get home, because the whole house—walls, floors, furniture, pots and pans and the goldfish—are going to be pretty well baked by then. So your savings might be
But if you’re only going to work or going to be out for a little while, SRP recommends turning the thermostat up just 4 to 6 degrees above your comfort level. Turning the air-conditioning off for a short-term outing will not save money or energy because of the effort involved in cooling the house down.
Our masters insist the air-conditioning will be on again soon.
Meanwhile, they have called for more grapes.
It’s not just about the outside temperature. It’s also about the ambient temperature. Your a/c doesn’t just cool the air. It also has to cool the internal structural components (e.g. walls, floor, ceiling), and the contents of the structure (e.g. furniture, carpet, appliances), all of which retain heat to one degree or another, depending on what they’re made of, and many of which generate their own heat, such as computers, televisions, refrigerators, clothes dryers, light bulbs, etc. Humans generate between 300 and 2500 BTUs (British Thermal Units) per hour, depending on what activity they’re engaged in. So, your a/c has to not only cool the air, but it also has to cool everything in the house down to whatever temperature you’ve chosen, meanwhile compensating for some things/people that are constantly putting out their own heat. Remember, also, that an air conditioner is working to pull moisture out of the air, and that makes the unit work even harder. The trick is not to turn off the a/c but to find a thermostat setting, for “away” times, that can save electricity but doesn’t require the a/c unit to work overtime on reducing the ambient temperature and humidity after you come home.
You’re better off enrolling in “dynamic peak pricing” (“superpeak” as called by APS) and “super-cooling” your home (and contents) during off-peak times (7pm to noon and all weekend) and turning off during the peak hours. Especially if you’ll be away during the day and don’t mind coming home to an hour of 80 degree house. Cool your contents and walls and they will absorb the heat. The savings can be substantial and it helps distribute load on the grid which delays construction of additional power plants (and their associated environmental effects, though our power here is relatively clean with hydro and nuclear, other than the dirty coal plant up north). You can save a double-whammy if you have an electric car that you charge overnight. Wi-fi connected thermostats can really help you tweak your usage too if you have a variable schedule.