W’ Balance is a fixed amount of work, expressed in kJ, that you can do above Critical Power.

Another way to look at it is to consider it as the amount of anaerobic work your body is able to put in at any given time during the ride.

W’ Bal is also known as the Anaerobic Work Capacity. ie how much anaerobic work your body is able to put in at any given time during the ride.

See also W’ and Aerobic Work Capacity.