my roommate and I are in disagreement, if we leave the a/c off or turn up the thermostat so that it doesn't come on as much during the day and then turn it on when we come home in the evening, does it save money? does the a/c have to work harder and use up more electricity cool the apartment down in the evening?
The USEPA says both turning the temperature up and turning the AC completely off while you are not at home saves $$$.
The question is how much will either option save and will it be worth it.. for your comfort.
If I remember correctly simply turning the temperature up saves 2% per degree (F) that you turn it up. (so if you turn up the AC temperature from 74F to 78F you will save up to 10%.of your cooling cost.).
If you turn it up while you are away and at night and turn it down again while you are home you will save about 1/3 to 1/2 as much — depending upon how long you turn it up, etc.
In extremely high humidity your comfort may matter most because at higher temperatures you may not be comfortable and it may take much longer to cool down.
Programable thermostats are inexpensive (I have seen them as low as $25) and can be used to do this automatically — as someone else said — it can be programmed to cool your apt down an hour or so before you get home so you will not feel too much of a difference and still save $$.
As to your last question — does the AC work harder and use up more electricity when you cool down the Apt in the evening.. yes a bit. But the savings from turning it off when the temperature coasts upward AND from the lower temperature difference when you are holding your Apt at a higher temperature..should outweigh the extra electricity used when you cool it down again. (The temperature differential between inside and outside the Apt and the time this temperature differential is trying to be achieved is directly related to the cost of cooling.)