Voltage refers to the power consumed by a module. In the past, most DDR3 memory voltage ranged from1.5 - 1.65v. More recently, dual 1.35/1.5 voltage modules came available at Crucial.com. While the lower voltage memory has some advantages over high voltage (namely lower power consumption), your system must support it in order to realize the benefits.
Will the dual voltage 1.35V/1.5V parts work with my original memory?
If your original installed memory is 1.5V and you are adding a dual-voltage 1.35V/1.5V module to an open DIMM slot, the dual-voltage module will operate at 1.5V, not 1.35V. In order to run at 1.35V, all installed memory modules must be dual-voltage 1.35V/1.5V and the system must support DDR3L (1.35V low voltage) to enable the module to run at 1.35V.
Which is better; 1.35V/1.5V or 1.5V?
Generally, lower voltage is optimal as it consumes less power and in turn, may generate less heat. However, it is important to note that the overall system must support DDR3L (1.35V low voltage) to enable the module to run at 1.35V. If the system does not support DDR3L (1.35V low voltage), the module will run at 1.5V.
Which one should I buy; 1.35V/1.5V or 1.5V?
Purchase whichever of memory your system supports, but keep in mind the dual-voltage module will run only run at 1.35V if your specific system supports that voltage, otherwise the memory will run at 1.5V.