I quickly discovered that regular Arduino board (e.g., Arduino Uno) is not feasible for this project due to its high power consumption. This page was extremely useful in obtaining solid overview in the domain of low-power arduino world.
My requirements were following:
we have 1200mAh battery and we want it to run for 6 month.
6 months is cca 4500 hours.
those 1200mAh must last for 4500 hours.
thus the device cannot draw more than 0.26mA (1200/4500).
Arduino Uno without any energy saving draws cca 50mA, so that the battery will run out in 24 hours.
As you will see later in this article, it is easy to construct a bare arduino board which draws only 0.005mA, periodically measures information from sensor and trasmit it wirelessly via 433mhz transmitter.