In this case it’s using 1.49 amps, at 1.2 volts = 1.8 watts of power draw.

This can be seen by placing the system under a normal load, using the excellent iStats menus to see exactly how many amps the RAM is currently using, then using a simple calculation to get the wattage being drawn by the RAM. That would be about 20% of the overall system power usage to use DDR4 RAM, vs a bit less than 10% of overall power usage by LPDDR. The short and very rough answer is that under normal conditions the RAM uses 1.5 watts as it is, and if it were using DDR4 memory it would use about 3-5 watts. So then the question would be: How much power are they really saving by using LPDDR3E memory over the DDR4 memory they could have used which is supported by the very same CPU? So desktop class memory (plainly spoken DDR4) would be the only option if they wanted to go past 16GB. LPDDR4 is not supported by Intel’s CPU, and the DDR4L (another low voltage RAM type) standard is not finished yet. All DDR3 memory types are restricted to 16GB. How is this possible? That is because there yet another RAM standard, known as LPDDR3E (E for enhanced) RAM, that does go up to 1866Mhz. However, LPDDR3 only goes up to 1600 megahertz in speed, but Apple uses 1866Mhz 2133MHz RAM. The Intel chipset being used supports the following kinds of RAM: DDR3 RAM, Low Power DDR3 RAM (LPDDR), and DDR4 RAM. While this gives a good indication of the reasoning, let’s dig deeper: I hope you check out this new generation MacBook Pro, it really is an incredible system. To put more than 16GB of fast RAM into a notebook design at this time would require a memory system that consumes much more power and wouldn’t be efficient enough for a notebook. The community reaction was strong enough to elicit a response from Phil Schiller: Then to much disillusionment its biggest limitation became clear: that it has a 16GB RAM ceiling on all of its models. To much fanfare the MacBook Pro was released.

8gb vs 16gb ram on macbook pro