Understanding Dell laptops and USB-C power sources

(UPDATE: Since writing the post below, it seems Dell may be changing its 65W max power draw limit from non-Dell USB-C power sources.  I've seen reports from owners of the XPS 15 9500 (released in 2020) indicating that it will draw at least 90W from non-Dell sources.  But what I wrote below would still be true for most of the Dell laptops in use as of this update in July 2020.  Original post below.)

I decided to write this post because there seems to be a fair amount of confusion around Dell laptops and USB-C power sources, particularly third-party USB-C power sources, including how to determine what wattage the system is drawing from them and why power sources are identified a certain way.  Hopefully this will help Dell laptop users understand how to learn more about how their setup is working and why it's working a certain way.

First off, most if not all recent Dell systems will display an AC adapter wattage readout in their BIOS Setup interface under the Battery Information section, specifically below the battery graphic.  You can access your BIOS Setup by pressing F2 during initial startup.  When using non-USB-C power sources, i.e. Dell-specific AC adapters that use a barrel-style connector, the wattage shown there will be the detected wattage of the AC adapter, even if it's more wattage than the system will actually use.  So for example if I plug a Dell 130W AC adapter into an XPS 13 9350 that's only designed for 45W, it will still identify the power source as 130W, even though it will never actually USE that much wattage.  Using a Dell adapter that can provide more wattage than the system needs will not damage the system in any way.  I've kept a Dell 130W AC adapter in my living room for years and have used it with Dell systems designed for anywhere from 45-130W and it's been fine.

With USB-C power sources, the wattage readout you see there is a bit more complicated.  The reported wattage will be the maximum wattage that particular system will actually draw from that particular power source.  That might sound simple enough, but it isn't quite so simple, partly because of a restriction around USB-C charging that I've found implemented on multiple Dell systems.  Basically, as of this writing, multiple Dell laptops I've tested will not draw more than 65W from a non-Dell USB-C power source, even if the power source can provide more wattage and the system is designed for more wattage and would therefore benefit from drawing it.  In addition, the USB-C cable you use can matter.  USB-C cables that support USB Power Delivery are available in two versions: up to 60W (sometimes written as 3A, or amps) and up to 100W (sometimes written as 5A).  The latter are typically thicker in order to carry the extra power safely.  The relationship between watts and amps here is based on the current 20V max of the USB PD spec.  Citing amps is technically more precise because the higher amperage requirement is what drives the need for thicker wiring, but most customers think in terms of wattage.  Anyhow, if you use an "up to 60W" cable, you will never get more than 60W, even if you have a USB-C power source that can provide more wattage and a system that would otherwise draw more.

So let's look at some examples.  All of these assume USB-C power sources (as opposed to the barrel-style Dell AC adapters) and that you're using an appropriate cable for the wattage involved.  They also assume that you're using a USB-C source that can supply 20 volts.  Some lower wattage sources only supply power up to 15V, and although some laptops will work with such sources, others will not.  I didn't test such chargers.

Case #1: An XPS 9350, which is designed for 45W.  If you connect a 30W power source, the system will show 30W since that's all that's being offered.  If you connect a 60W power source, it will show 45W because that's the most the system is designed to draw.

Case #2: A Latitude 7480, which is designed for 65W but will take advantage of 90W when available.  If you connect a 30W source, it will show 30W.  If you connect it to a 65W source, it will show 65W.  If you connect it to a Dell dock that can provide up to 90W, it will show 90W.  But if you connect it to a non-Dell 90W power source, it will show 65W, even though other non-Dell systems will correctly show that power source as 90W.

Case #3: An XPS 15 9570, which is designed for 130W.  That's actually more than the official 100W max of the USB Power Delivery spec, but Dell has done something proprietary on this and some other 130W systems and certain docks (like the WD19TB) to stretch that spec to allow carrying 130W over USB-C.  They also make a 130W USB-C power adapter.  If you connect this system to one of those devices, it will show 130W.  But if you connect it to a non-Dell 90W power source, it will still only show 65W.  This is frustrating because this particular system would actually perform better if it drew 90W rather than 65W and is still refusing to do so.  In addition, I believe (but have not confirmed independently) that the XPS 15 models will not work at all with any USB-C sources that supply less than 60W -- although even a 60W source would be inadvisable overall.  Battery charging will be slow to non-existent and CPU and GPU performance throttling would be significant as the system attempted to operate within the constraints of a power supply that offered less than half the wattage it the system was designed to use.  A 60W source would be fine to use purely for battery charging while the system was asleep or off, but would not be a great choice while actually using the system.

Hopefully this helps people understand how Dell systems identify and work with USB-C power sources!