Power bred sheep are modern factory sheep who are explicitly bred for either wool, meat, milk, etc.
I'm not a native speaker so I roughly translated what they are called in the german language.
The opposite of them are old race sheep: Those are normally less trouble to hold but won't grow as fast, won't produce as much wool and milk and have (from a production aspect) unwanted properties, too. Some people can live from 100 old race sheep when you know someone who knows people who like special stuff. Most people need at least 300-400 power bred sheep to be productive.
As far as I know you can hold 6-8 sheep on 2 acres all year round - without aditional foodering. Is anyone disagreeing? When you think of a medium productive pasture, I mean.