How much will occasionally running my air conditioner increase my electric bill?

Question

I live in Ohio where the temperature has been in the mid to upper 80's recently. When I get home from work, I turn on the air conditioner to 74 degrees for about 1 hour, then I turn it off. I work with kids all day and really need to cool down when I first get home.

My roommate says it costs a lot of money to turn the A/C on and off, but I feel that is if I'm leaving it on to cool to a low temperature, turning it off till it's hot again, then turning it back on. Either in dollars or a percentage, how is turning on the A/C for an hour or so per day likely to change my electric bill?

The house has two stories and a basement. Five small bedrooms altogether. It's an older house, probably from the early 20th century.

It is important to note a few things:
#1- Our central air system doesn't include a timer option or anything. It's a rental so we can't make those changes.

#2- I am only home about 2 hours per day other than when I'm asleep.. so the sprint v. pace example isn't pertinent. Why pace myself for the mile when I only need a short sprint anyway?

Answer 1

You are charged by the kilowatt for power, whether you use it for light, cooking a meal, or running your A/C.

To determine how much energy your A/C uses, find the wattage. If it is a window unit, it should tell you on the nameplate behind the grille. If it is a central unit, it gets a bit more involved.

Watts = Volts x Amps.
Kilowatt = 1000 Watts

Your indoor unit uses 120V and the blower motor pulls 3-4 amps. or 360 to 480 Watts/hour (.36 ~ .48 kW)
The outdoor unit uses 240V and 21-24 Amps = 5040 ~ 5760
Watts/hour (5.04 ~ 5.76 kW)

If you are charged .04 per kW, your A/C costs you .21 to .25 per hour to run the A/C.

This is subject to change, based on your cost per kilowatt, and the true Wattage your system uses.

Answer 2

I'm not sure what the dollars or percentages would be, but you'd actually be better off if you set your a/c on auto. The reason is because when you do this, the a/c will only come on when the house needs cooling, run for just a few minutes, then shut off when the temperature matches the desired setting. When you come home the house will be comfy, and your a/c won't be working so hard for that hour that you have it running. Think about it. It takes an hour for your a/c to bring the temp down from upper 80's to 74. You're consuming a lot of energy, while you wait for the house to get comfortable; when you could be just using it in short spurts, and come home to a comfy house. Look at it like you're running a mile. If you start out sprinting, you might make it, but you're really going to be tired because of all the energy you've spent, but if you run at an easy pace, you'll still get there, and be a lot less tired because you've used less energy.
It'll cost you less to stretch your energy, and use it in spurts over the course of the day, than to use a big burst of it in one hour. That's what the thermostat is for.

Answer 3

If you need only your den to be cooled then you can go for a timer based section in the house and balance your electricity cost.