When I was picking up my ST generator head from Tom Osborn (Central Georgia Generator) I saw a ST 7.5 for something like $30 more than the ST 5. Since they both have the same shaft with the only real difference being more copper windings, I got the ST 7.5 for my 6/1 running at 800 rpm. I can get 4,400 watts sustained at 800 rpm. I have a very heavy two step pulley on the ST head allowing me to run the 6/1 at 600 rpm or 800 rpm depending upon needs. If I ever want to move the ST 7.5 to another, perhaps bigger, prime mover, I own a bigger generator head already. This setup can start very large loads using the larger ST head, and lots of spinning cast iron (I.e. 90 lbs on the ST spinning at 1800 rpm + the 6/1 cast iron flywheels at either 800 or 600 rpm). My emergency power needs in south florida are associated with August and September hurricanes. 100+ degree F daytime temperatures are common with high humidity after a hurricane. I figured the bigger generator head would run cooler and better at around 50% load, plus allow bigger loads to start.