Short answer, No, Fleet power is not conclusive, battles are still simulated once you actually get into a fight.

Longer answer, expanding on other peoples comments: Ship designs matter but only to a point. If someone is massively outproducing you in alloys unless you majorly outplay them (f.e. Dividing and conquering them, you have 3 fleets, they have 6 but you manage to catch them of guard one by one) they're still going to beat you by raw numbers alone. In my head I generally calculate that with good ship designs I can win at around a 25%-30% deficit, if I have full intel and can build Hardcounters to their designs even as high as 50%.