1. A method of determining a fluid level Xc in a wellbore, where the wellbore extends downward from a surface and has a centrifugal pump disposed therein for transferring fluid within the wellbore using a controller operatively connected to the centrifugal pump, the method comprising: determining a zero-flow input speed Uz to the centrifugal pump at which output flow from the centrifugal pump is substantially zero; and using the zero-flow input speed Uz to calculate, with the controller, the fluid level Xc in the wellbore.