Keeping your home at a comfortable temperature when the mercury is up into the triple digits can be difficult and expensive. Depending on where you live, you may have a couple of air conditioner options to choose from. Take into account the size of your home, how long the temperatures are going to stay high, the humidity, and who is home during the hottest part of the day. Now, consider the following options and go from there.
If you live in an area of the country that does not have long summers, they are only very hot for a week or two, you may want to go with a couple of window units. This is especially true if no one is home during the hottest part of the day. Window units allow you to cool off only the rooms that are in use so you do not pay to cool the whole house off. However, if things get very hot for more than a few weeks, you will need a unit in every room. If this will be more than three units you should consider having a whole-house system installed because four or more units will not save you any money.
Another consideration for window units is if you are renting your home. You can buy a couple of AC units and then take them with you when you move out. Of course, if things are normally very hot all summer, the landlord should be responsible for installing a whole house unit.
If you live in a larger house and need to cool more than a couple of rooms at a time, you should go with a whole-house unit. You can have a zone system installed so that you can set the system to cool certain rooms at certain times. Perhaps you want the living room, kitchen, and den kept cool during the day but only need the bedrooms cooled overnight. A zone unit will allow you to set the zones and then the different temperatures for each zone throughout the day. This will save you from paying to cool the whole house all day and night.
A whole-house unit is going to cost more initially than putting a window unit in every room. However, if you have many rooms, a whole-house unit will be less expensive to run. You must decide if it will be worth it to put in a big unit if it will only be needed for a couple of weeks.