Our home is a 2 story house(about 2200sq ft + basement) after reading some posts here I have set 75F on first floor and 77F on second floor which are both comfortable settings for us. But, i'm thinking to save some energy bill during the hours me and my wife are working. So the settings would be around 78F on first and 80F on second floor between 8AM and 4PM. My concern is, would the AC have to work hard at 4PM to bring he temperatures from 78 to 75 and utilize more energy than keeping the temperatures same through out the day? which is preferable? Temperatures around in this area during summer are between 80-100F.
tl;dr Trial-and-error is your best bet. There's no standard answer.
There's really no such thing as "working hard". Efficiency is mostly a matter of minimizing total system runtime needed to keep you comfortable during a given 24-hour period.
Since every home and every HVAC system is different, you have a few considerations:
- How quickly your HVAC system can recover X number of degrees
- Whether the humidity that accumulates during the day is a concern
A couple tips:
Discontinue cooling up to an hour before you leave, but keep the fan running at least periodically. Your home won't probably start warming up enough to be uncomfortable.
Gradually increase the "away" temperatures until you find that recovery isn't happening in a timely manner or humidity becomes a problem.
Your climate and mine are similar, and I find that I don't need to maintain cooling at all during the day for all but a few days out of the year. Again, keep the fan on a periodic cycle to prevent stagnation and hotspots due to solar gain.
Here is the deal. It's all about temperature differential multiplied by time.
Imagine you have a tire with a small hole in it (the heat loss of your house) and you are trying to keep it pumped up (your HVAC). Would it be harder to keep it pumped to 100 psi or 50 psi for 8 hours?
Bringing the temperature down after it has been allowed to rise is actually cheaper than keeping low all day while you are not there. Many people believe it is cheaper to keep it low all day but if you do the math it is cheaper to set-back and then adjust the temperature when you are home.
Set back and save!