yarzy,
You have gotten a lot of very good responses from everyone. However, the Prius power management schema is very complicated. Those of us who have delved into the technical items regarding this vehicle have come to an understanding about how engine RPM is translated into a combination of ground movement and electrical generation while the vehicle is moving.
There are ICE rpm sweet spots where the ICE is running most efficiently. The computer system is able to determine this based upon programming and onboard sensors. The beauty of HSD in general is that the computer's goal is to make the vehicle as efficient as possible regardless of what you are doing with the vehicle: accelerating, braking, cruising, going uphill, going downhill, etc.
I have found that when you are cruising on the highway at a posted speed of 70 mph on flat road, the MFD indicates that you being powered by the ICE and providing electrical power to the battery. Without going into what the PSD is doing specifically (it helps to see one of the online demonstrations) the vehicle is running the ICE at an RPM which is most efficient for that ground speed, and the surplus RPM is then directed into MG1 for power generation.
If you have 5 or 6 bars in this situation, and there is only minimal draw on the battery from the 12v system or A/C you will cruise at 6 bars on the battery most of the time. Even with A/C on at these speeds you should continue to see 5 or 6 bars. I have never experienced the car at cruise acting as if the ICE were stressed because it was having to generate power for a/c and help move the vehicle forward.
By removing all the belt driven accessories off from the ICE (except for the water pump), there is no power loss from the engine through accessories. This actually frees up a lot of the engine's power generation ability and directs it into one place, the PSD and through that MG1 and the drive train. There is lots of talk about electrical conversion loss, however I'm convinced through logic only, and not numbers, that there is conversion loss, if not overall power loss if the a/c system was belt driven.