Expected percentage of collisions for uni-directional wireless transmissions
I can accept some collisions, but I would like to estimate how many
  they are so that I can lower the bit rate of the transmission (and
  therefore the transmitting power)


Lowering the bit rate will extend the transmission time which ironically may increase the overall power consumption.

If you have an average value of all transmissions per hour I believe the probability that two transmissions occur in a certain time frame can be calculated assuming a Poisson Distribution. If the probability of one transmission in a certain time frame is (say) 0.1 then the probability of more than one transmission is 0.00468. See this calculator. I assumed X was 1 and that the probability, \$\mu\$ was 0.1 and plugged in the numbers.

Remember that if a transmission is (say) 20ms long then you need to consider a period that is up to 60ms wide because a colliding transmission could occur right at the end of either transmission.