I want to wire up an alarm sensor on my window so my arduino will know when it's opened. I can't run wire to the sensor so it needs to be wireless. I know you can get DIY wireless window kits that run on batteries and last years before you have to change the battery.
How would I do that with an arduino? All I need to know is "is the alarm sensor tripped" so I think an XBee is way overkill. How can I do something like this in the most battery efficient way possible?