One addition:
If you power the Arduino directly (e.g. 2x AA = ~3,00V max) you can use ( http://provideyourown.com/2012/secret-arduino-voltmeter-measure-battery-voltage/ ). This will ofc. not work if you power your setup via VoltageRegulator.
In addition to mfalkvidd's link have alook at https://www.mysensors.org/download/sensor_api_20#sleeping
It describes how to use the sleep function to put the Arduino+Radio in sleep mode.
M
