While alkaline cells generally more energy per-cell than rechargeable types (NiMH or NiCd) this energy may not be accessible to the appliance using that cell - particularly if it is a device that draws a lot of current. /// Internal resistance is the culprit.
NiCd and NiMh cells, on the other hand, typically have a much lower internal resistance over their charge life and this resistance (which varies depending on state of charge, temperature, age, condition of the cell, the cell's internal chemistry and its construction) is typically lower than that of an alkaline cell - even when the NiCd or NiMH cell is significantly discharged.