Here is the brake specific fuel consumption map for the Prius 1.5L (Gen 2 & Prius C) and 1.8L (Gen 3, V, & Gen 4):
That dark grey "island" of peak thermodynamic efficiency occurs between about 20 & 35 KW of power (~27-47 horsepower). Here is a ballpark of the power required to overcome aerodynamic drag and rolling resistance at steady speed on flat ground for the gen 2 prius:
MPH HP
5 0.4
10 0.7
15 1.2
20 1.7
25 2.4
30 3.2
35 4.2
40 5.5
45 7.0
50 8.7
55 10.8
60 13.3
65 16.1
70 19.4
75 23.1
80 27.2
85 31.9
90 37.2
95 43.0
100 49.4
Add another ~1 hp to run all of the computers, another 2-8hp to run accessories like the water pump and air conditioning. But basically the load you put on your engine holding a steady ~80mph is the load you want to drive at when accelerating. And if you're not accelerating you should try to be coasting.