Should power affect the speed of external SSDs?

One of the great advantages of using fast external SSDs is that you don’t need a large and expensive enclosure with its own power supply, as they can draw the power they need over the Thunderbolt or USB-C cable from your Mac, or from a powered hub. Can’t they? Comments to my previous article about external SSDs have pointed out that concerns over power drawn could be a reason for limiting the speed of Thunderbolt 3 enclosures. As almost all the power supplied is converted to heat, SSDs drawing a lot of power will also be those that may have problems staying cool in use. This article therefore looks at how much power may be used by external SSDs, and how that might affect their performance.

Power supply

The standard requirements for Thunderbolt 3 and 4 connections are the clearest: according to official articles and a technical briefing, Thunderbolt 3 hosts like a Mac are expected to be able to deliver 15 W of power on at least one port for external devices requiring bus power.

As expected, USB4 isn’t as clear as that. While a USB4 source must be able to provide 7.5 W of power per port, that available with USB Power Delivery can supply up to 240 W for ‘Extended power range’, or 100 W for ‘Standard power range’, although those are primarily intended for recharging devices such as phones. Those USB4 sources intended to be compatible with Thunderbolt 3 are expected to be able to supply at least 15 W of power, just as with Thunderbolt 3.

Thus, Thunderbolt 3 should be able to provide an external SSD with 15 W; USB4 would be the same if intended to be compatible with that, but at an absolute minimum should be able to provide 7.5 W, and potentially more than ten times that.

Power required

Given the limits on power available from Thunderbolt and USB4 ports, you might have thought that vendors of external SSDs would be falling over themselves to give information about their products’ power requirements. Not a bit of it: so far I have been unable to find a single vendor who gives figures for power consumption, at least not for their ready-assembled SSDs.

Some of the better manufacturers of SSD modules do give power figures. For example, OWC gives the following for its fastest PCIe 4.0 product, the Aura Ultra IV: they require up to 9.1 W maximum during sustained writes, and a mere 0.3 W when idle.

Several of the better hardware reviewers also provide their own power measurements on SSDs under test. My own favourite, Samsung 990 Pro, drew a maximum of 6.2 W in its 4 TB version, and other SSDs of the same size drew up to 8.7 W maximum. The highest measured power use that I have seen recorded in recent years is 10.7 W for an old Corsair.

It thus appears most unlikely that any modern, reputable external SSD would require more than 11 W maximum power, and most should be under 9 W.

Power dissipated

If the power available over Thunderbolt 3 and USB4 should be sufficient, the other question is whether the case or enclosure of the SSD is capable of dissipating that much heat. Almost all the power supplied to a peripheral solid-state device will be converted to heat; for the SSD to remain at a suitable working temperature, that in turn requires that its enclosure is able to dissipate that amount of heat.

As most bus-powered cases and enclosures are cooled passively, without the aid of a fan, the rate of heat dissipation is largely determined by their surface area, hence the deep fins provided on most cases and enclosures. Modelling such passive heat loss is complex, but some idea could be gained by comparison with other devices such as MacBook Air computers, which have neither fans nor fins, but a larger surface area to help them cool.

Measuring power consumption of any notebook isn’t easy, as it isn’t the same as the power drawn by its mains adaptor, which is primarily used to charge the battery. However, it’s likely that at idle, power consumption is below 5 W, rising to over 20 W when heavily loaded. This suggests that, to keep pace with heat production, SSD cases and enclosures do need more than minimal finning, and the deep fins of models such as OWC’s Express 1M2 may well be necessary when sustained writes are expected.

Clearly, more attention is needed to this issue, as a hot SSD should protect itself by undergoing thermal throttling, resulting in dramatic reduction in performance. But permanently limiting transfer speeds to ensure that power drawn remains within the 15 W provided doesn’t appear to make sense.