A programmable thermostat lets you preset temperatures for different times of the day, so air-conditioning is working only when you are home. The least expensive thermostat models ($30) let you set four cycles that, unless manually overridden, repeat every day. Higher-priced models ($50 and up) allow you to create settings for each weekday and for each weekend day.
These thermostats come with complete directions and are easy to install. Just remove the old thermostat, unscrewing the wire leads attached to the terminals on the back. Reattach those wires to the terminals on the new model (in a system with separate A/C and heating units there may be four leads on the back, two for each unit). AA batteries maintain the settings if the power ever goes off.
Cost: $30 to $50
Benefit: Up to 20 percent off your cooling bill