I have a bunch of 2.5" HDDs. Their power requirements vary but it's usually in the range of 5V 0.6A – 0.85A (so 3 to 4.25 watts). RPM speed doesn't seem to play a huge factor nor does physical size, surprisingly (even 3.5" HDDs have similar power requirements). In fact, it seems like the biggest factor is capacity but idk.
2.5" SSDs, in contrast, have power requirements more along the lines of 5V 1.7A (or 8.5 watts). A NVM PCIe SSD that I have is 3.3V 2.8A (or 9.24 watts).
Overall, it seems like SSDs just have higher power requirements than HDDs and this seems counter intuitive to me.
Any ideas as to why this would be the case?
Best Answer
This is almost certainly a question of peak power draw versus average power draw.
The figures on the outside of the drives are going to be the maximum power drawn at the worst case.
For the HDD this will be during motor startup, every time the drive spins up and down, and as you may have noticed this can take anywhere from half a second to a couple of seconds. After that there will be a constant power draw required to keep the drive spinning. Once the drive decides it is not going to be used for a while then the drive will park the heads, the motor will turn off and you will be left with the power draw for the controller electronics.
For the SSD there may be the briefest of high currents on startup, as capacitors charge and electronics does its startup busy work, but it will be a much shorter time than the HDD. This should be a lot quicker than HDD startup times. After that it will fall back to its "idle" current draw which, due to the larger amount of electronic chips, be slightly higher than the HDD when it has parked the heads and turned the motor off, but not significantly higher.
The worst time for an SSD will be during drive block erasure and then sustained writes to the drive. Erases require a very large voltage to pull electrons off of the NAND cells to reset them, and a higher drive strength is required to write them. Reading will require a bit more power than idle, but not a massive amount.
But for writing the speed of an SSD works to its benefit. It can write a lot faster than an HDD and so spend a lot less time drawing higher power. It might briefly require a lot more current (the number printed on the label) but it will require that current for a massively reduced time.
The other factor is that a mechanical 2.5" drive in a laptop will potentially never spin down due to tiny reads happening here or there so it will be a constant power drain. The SSD has no large constant power drain.
So an SSD at average is probably in the same ballpark for power drawn as an HDD that has gone to sleep, perhaps a little higher but certainly not order of magnitude different. A HDD that is ready to work will definitely have a higher average power draw.
As a result SSDs can, despite having a higher peak current draw, have a much lower average current draw.
In short P=VA is only a useful equation when you know for certain that the current draw is constant. Most labelling will state peak draw because it determines what kind of PSU you get, but average draw is not mentioned.