Psybeam has a power of 65 always.
You must be talking about PsyWave. The power of the move is calculated using the formula below:
(X + 5) × (User's level / 10)
X is a number between 1 and 10 that is randomly chosen by game.
So if this came from a Level 100 (no STAB), it would be between 60 and 150. With STAB, 90 to 225.