power supply - How can I turn on and off a switch every 20 minutes?

I want to make the above switch which will stay on for 20 minutes and then off for 20 minutes. This should be continuous. Actually, I want to place this switch for my air conditioner which I do not want to run the whole night but off itself every 20 minute interval and restart after 20 minutes. This will save my electricity by half.. since the thermostat function of the air conditioner is not of much use and also I do not want my appliance to run for the whole night.

The device I'm going to switch takes between 5 and 15 Amps circa.