To calculate the size of the primary mover (typically an electric motor or engine), we must first establish the size of the hydraulic pump to give the required flow for the system. Once we have sized the pump, we then know the power required to drive the pump and specify the correct size motor or engine.
When calculating the power input to the pump, the total pump efficiency must be included. This efficiency is the product of volumetric efficiency. The average for axial piston pumps = 0.87. The formula shown allows for an efficiency of 80%