Do programmable thermostates really save energy?

I installed a programmable thermostat last night. I basically have the a/c set for 77 all day and was wondering: which is more energy efficient and less “stressful” on the a/c unit: to keep the temp steady all day or having the temp set at 82 while I am gone then making the unit stay on til the temp reaches 77 when I get home?

Thermostats. No “e.”

This is a common question which has been debated and answered at-length on here, complete with proofs and calculations. The short answer is - allowing the house air conditioner to keep the house at a higher temperature during the day will save you energy, and thus money. For the details, I recommend searching on here; the question is typically posed with respect to “if I turn my heat down during the day in the Winter will I save money”, but it’s the same principle.

One caveat about this may be the humidity aspect - if you need to keep the house at a lower humidity during the day, then you may want to go ahead and keep the A/C at a lower temperature. That’s something only you can really decide.

I haven’t done any detailed simulations of the different scenarios possible with a programmable thermostat for my home (although I have the necessary software and engineering knowledge, I lack the time- it would take several hours that I bill at $75-$100 per hour). I am a licensed mechanical engineer that designs and specifies A/C systems and controls as part of my job (but I’m probably not licensed in your jurisdiction- this is general advice and not engineering services; consult a local engineer if you have specific questions).

My anecdotal experience is that if you are away from the home during the peak A/C load times (noon-5 or 6 PM) and set your thermostat to a temperature that can be cooled down to comfort levels from in a reasonably short time (say, an hour of running continuously), you will save a lot of money. My personal experience is that I used to have a traditional thermostat that I would forget to bump up to the low 80s when I would leave for work, and my summer electric bills would be around $150/month. With a programmable thermostat that would step up while I was away and start cooling down about 1/2 hour before I would get home, my summer electric bills are around $100/month- without affecting my personal comfort one bit (well, it would be a little warm when I would first arrive home, but I would be coming in from 100 degree outdoor temps to around 80 deg inside and falling, so it wasn’t uncomfortable). Since the T-stat I bought was around $50, it paid for itself in a month.

There are a lot of factors that contribute to the potential savings, including geometry of the house (exterior surface area vs. volume), construction of exterior walls and roof, amount of window area, exterior shading from trees and adjacent buildings, etc.

As far as wear and tear on the A/C unit, the programable t-stat reduces the number of start & stop cycles on the unit by a small amount. This reduces the wear and tear on the unit by a small amount, and will extend the life of the unit by a small amount.