Which is more energy efficient? Or is it about the same?
 

I recently bought a house which did not have central air conditioning. Well today im having it installed yeah.gif My question is which will be more energy efficient (cheaper on the bill lol) out of these two...

I can just leave it on all day while im at work and it will shut off when it reaches the temp then come on for short bursts throughout the day to maintain that temp.

or

Im getting a programmable thermostat so i can have it turn off when i go to work then turn on about an hour before i come home so the house is cool when i get there.

Is there really a difference? Or would the short bursts through the day add up to about the same amount of time its going to take for it to cool the entire house after being hot all day?

IIRC it's most energy efficient to have it on all day at a higher temperature than you would want by about 10 degrees, then bring it down to temp a half hour before getting home.

I think in reality it has a lot to do with how well your house is insulated.

Second option. Turn the A/C on right before you get home.

Your A/C is basically a heat pump that will "pump" heat from the inside of your house to the outside. The amount of heat coming into your house due to the 2nd law of thermodynamics will be dependant on time and the temperature differential from the inside of your house to the outside. The smaller the delta temperature the less heat is transfered into your house. The less amount of heat that is being transfered into your house, the less amount of heat the A/C needs to "pump" out.

Or in other words, your A/C will have to do a lot less work if you turn it off while you're gone, and then just turn it back on right before you get home.

Think about the sun beating down on your house. If you leave the A/C on all day, it's constantly battling with the outside temperatures and the sun heating up your house. When the sun is setting, it will immediately start to cool your house.

By running the A/C during the day it will be fighting to keep temps low. Do what other mentioned - kick it on as late in the day as possible to return it back to normal temps.

I leave my house at 68 degrees when I'm home. When I leave for work in the morning, I turn it up to 71 and manually lower it when I get home in the evening. Our electric bill for my 3,000 sq. ft. house averages $120-180 in the summers and as low as $80 in the off-months (spring-summer and fall-winter transition). Actually, I don't think it's ever been over $180. It's a newer home (4 yrs old) and well insulated - that has a lot to do with it. YRMV.