New to arduino - questions about sleep/interrupts/low power consumption

I'm very new to Arduino and electronics in general, but I've been programming for a long time. I've gone through the projects book with my Uno and I'm ready to try something more complex. My garage door opener is missing a second transmitter, and I thought this could be something fun to try, so I've been looking at my options. One of my constraints is that I'd like to use parts that I have on hand, one of which is an RPi with usb/ethernet ports that don't work.

Here are the general steps currently in my mind:

  1. I want to use an optocoupler to wire up the RPi to the button that operates the door opener
  2. The transmitter in my car will be my Arduino Uno, powered by some batteries
  3. I want to use a pair of nRF24L01+ transceivers for the wireless communication between the two devices, and I'm planning on writing some code using a psk or something to secure the communication (haven't fully worked through this yet)

My question:

One of the design goals I have is to be able to power the Arduino transmitter for a good long time on batteries. I don't know how to work out the power requirements for this to function the way I want. I'd like to figure it out for myself if someone can point me in the right direction. I know I'll need to use some kind of sleep mode on the Uno and use interrupts to wake it, but that's about all I know. Is what I'm thinking of doing feasible?

Have a look !