Watt & Ohm

[Rocknwolf]

Last night my friends and me discussed about power ratings of amplifiers.
The fact is that we have a 100Head with two mono cabinet out with the classic impedience switch 4,8,16 ohm.
We usually connect it to our 4X12 cab, setting 16Ohm both.
Sometimes I try to connect the head with only 2 speakers of my stereo cab (it's a marshall 1960) setting both impediences to 8 Ohm.

The question is:
How is the impedience related to power?

I mean: 100W from head go straight to 100W into one cab (both setted to 16Ohm)
if I set head + cab to 8 ohm, how much power goes into my half cab? 100W or 50W?

[Track Rat]

Ohms refers to the load. Amps run at their rated power output only into the load they were designed to drive. Anything higher or lower, impedence wise, and the amp will do something else. Say an amp is rated at 100 watts into 8 ohms. If you try to drive a 16 ohm load, you won't get all 100 watts. Drive the same amp into 2 ohms, you get more than 100 watts but only till the amp blows up. Some amps are more flexible than others when it comes to the impedence range of loads they'll safely drive.