Thank you for all the help. Now I feel this project is a bit more doable. I have ordered some cheap NRF24L01+ modules from eBay, as well as some small solar cells and supercaps. Now I will learn to use the transceivers, and testing their range and differend master/slave setups.
Looks like i have som work to do with the atmega328P also. I would like it to run in the same voltage range as the NRF24L01+ (1.9 - 3.6V) which means I have to use a slower clock. I would also like to remove the arduino bootloader so I avoid the bootloader delay waste a lot of power during powerup.
@ex-Gooserider
I have been thinkin about using bathroom scales as well. But I am under the impression that the load cells in these scales are of a very bad quality. And for some reason it feels a bit wrong to buy 30 bathroom scales, remove one component and then throw avay the rest. 
But if i can't find anything else, it's worth a try. 
LEGO-lars