The 45W Label Is a Ceiling. A 9-Volt Device Stops at 27 Watts.

A phone sits on a 45W charger overnight and the battery readout crawls. The same phone on a cheap 20W brick seems no slower. Nothing is broken, no cable is frayed, and no setting was changed. The number printed on the adapter simply does not mean what it looks like it means.

Wattage on a USB-C charger is not a tank of power that gets poured into whatever is plugged in. It is the highest point on a short list of voltage and current pairs that the adapter is willing to supply. The device picks exactly one entry off that list. If the entry it picks sits well below the top of the list, the rest of the adapter's rating stays unused, and no amount of waiting changes that.

A Wattage Number Is a Ceiling, Not a Pool

A USB Power Delivery adapter advertises its fixed capability as discrete lines. Each line is a fixed voltage paired with a maximum current. The device attached to the port reads that list, decides which line its own charging circuit can accept, and asks for it. Delivery is the product of that one line's voltage and current, not the number stamped on the plastic.

Google publishes the list for its own 45W adapter. The fixed lines are 5V/3A, 9V/3A, 15V/3A, 20V/2.25A, with a maximum rating of 45W. Multiply each one out and the shape of the problem appears immediately. The 5-volt line is worth 15 watts. The 9-volt line is worth 27 watts. Only the 15-volt and 20-volt lines reach the full 45 watts the label promises.

A device whose charging circuit tops out at 9 volts therefore takes 27 watts from a 45-watt adapter. The other 18 watts are not throttled, not lost to heat, and not held back by a policy. They live on voltage lines this device does not request.

Two Ports Change Both Numbers at Once

Dual-port adapters add a second rule on top of the first. The per-port ceiling and the combined ceiling are separate published values, and they are not the same number.

Google states it plainly for the Pixel Flex Dual Port 67W charger: "When only one device is connected, the maximum output on either port is 67W." Plug in a second device and the published figures change to a maximum of 60W on either port and a maximum combined output of 67.5W across both. Those two ceilings do arithmetic on each other. If one device is drawing the full 60 watts, the combined ceiling of 67.5 watts leaves at most 7.5 watts for whatever is in the other port.

Apple publishes the pairings themselves rather than a single per-port ceiling. For the 35W Dual USB-C Port Power Adapter, the documentation says "If you connect a Mac laptop and an iPhone or iPad, each device receives up to 17.5W", and separately that a Mac laptop or iPhone paired with an Apple Watch or AirPods results in the larger device receiving up to 27.5W while the watch or earbuds receive up to 7.5W. Both pairs add back to exactly 35 watts.

Apple also states the remedy in one sentence: "If either device needs more power, unplug the other device and its charging cable from your power adapter." That is the whole fix for a laptop that charges slowly on a shared brick. The adapter is behaving exactly as documented.

PPS and AVS Are Ranges, and the Rating Still Wins

Fixed lines are only part of a modern adapter's list. Programmable Power Supply mode lets a device request a voltage in fine steps within a range instead of jumping between fixed rungs, which keeps the charging circuit efficient and the battery cooler. USB-IF describes the related adjustable voltage supply mode as giving the powered device "an ability to request intermediate voltages between 15V and up to the maximum available fixed voltage".

Ranges look generous on a spec sheet and are still bounded by the adapter's rating. The 45W Google adapter lists its PPS capability as up to 11V/3A, 16V/3A, and 21V/2.25A, and the line ends with "max 45W". That trailing clause is doing real work. The second range multiplies out to 48 watts and the third to 47.25 watts, both above the adapter's 45-watt rating, so the rating truncates them. The Pixel Flex 67W behaves the same way, listing PPS up to 11V/3A, 16V/4A, and 21V/3.35A with "max 67W", where 16 volts at 4 amps is 64 watts and 21 volts at 3.35 amps would be 70.35 watts.

This matters for buying decisions because vendors set different thresholds depending on which mode a charger supports. Google's guidance for Pixel phones names two separate qualifying categories: "Any Programmable Power Supply (PPS) power adapter rated for 30W or more", or "Other USB Power Delivery (PD) adapters rated for 15W or more." A 30-watt PPS adapter and a 30-watt non-PPS adapter are not interchangeable in that sentence.

The Cable Carries a Limit of Its Own

The third participant in the negotiation is the cable, and it declares its own ceiling independently of both ends. USB-IF requires that "all USB-C® to USB-C cables categories must be labelled with either a power capability of 60W or 240W". USB-IF gives the worked example itself, requiring that "a USB 20Gbps USB-C – USB-C cable that supports 20V at 3A must be marked with the Combined Performance and Power 20Gbps/60W logo." Twenty volts at 3 amps is 60 watts, which is the lower of the two labels. Cables built with an electronic marker chip are handled separately, and USB-IF directs that "For cables implemented with an E-marker refer to the USB Power Delivery Compliance Plan for cable tests."

The ceiling above that has moved. USB-IF notes that "Prior to this update, USB PD was limited to 100W using a solution based on 20V using USB Type-C cables rated at 5A", and that the revision added "28V, 36V, and 48V fixed voltages enable up to 140W, 180W and 240W power levels, respectively". Divide each of those power levels by its own voltage and the result is 5 amps. The connector specification was revised in step, "updated to Release 2.1 to define 240W cable requirements".

The practical consequence is that a 240-watt-class adapter paired with a 60-watt-labelled cable is a 60-watt system. Apple's fast-charge documentation shows the same dependency from the product side, listing for the 16-inch MacBook Pro a 140W adapter combined with a MagSafe 3 cable, and separately a "140W USB-C Power Adapter + 240W USB-C Charge Cable for MacBook Pro (16-inch, Nov 2023) or later". The adapter alone is not the specification. The pair is.

Readers who have chased a dock that quietly downgraded a connection will recognise the pattern from a different resource on the same connector, covered in how four DisplayPort lanes leave a dock's USB ports stuck at USB 2.0. And before replacing a cable that tests fine, it is worth ruling out the software-side causes described in the case where a Windows update, not the cable, left USB audio devices at Code 10.

What Each Vendor Requires for Its Own Fast Charge

Vendors define fast charging against specific adapter figures and specific time windows, which makes their documentation the fastest way to tell whether a given brick qualifies.

Apple's iPhone guidance separates generations. For iPhone 17, iPhone 17 Pro, and iPhone 17 Pro Max, the stated result is "up to 50 percent battery in around 20 minutes with a 40W adapter or higher". For iPhone 17e and iPhone Air, the requirement drops and the window lengthens: "use a 20W adapter or higher to charge up to 50 percent battery in around 30 minutes." Older generations are documented against an "Apple 18W or greater USB-C Power Adapter". On the Mac side, the 14-inch MacBook Pro is listed for fast charging with a 140W or a 96W adapter, while a 67W adapter appears only under MacBook Air, and there only for "13.6-inch models only".

A lower-rated adapter is not forbidden. Apple is explicit that "If your Mac uses USB-C to charge, you can charge your Mac with any USB-C power adapter or display, even if it provides higher or lower wattage than the adapter recommended for your Mac." The same page recommends at least the minimum wattage for the best experience. Slow charging on an undersized brick is the documented outcome, not a defect.

Reading Your Own Setup Without Buying Anything

Three checks separate an under-specified charger from a genuine fault, and none of them require a meter.

• Read the adapter's own label. Apple's instruction is to "look for its certification label, which lists the wattage (W)". On better adapters the label prints the individual voltage and current pairs, which is the list the device will choose from.
• Ask the Mac what it is receiving. The documented path is to "press and hold the Option key while choosing Apple menu > System Information. Select Power in the sidebar, then scroll down on the right to find the wattage listed under AC Charger Information". A figure well below the adapter's printed rating usually means a shared port or a limiting cable.
• Remove the second device. On any dual-port adapter, unplugging the smaller device and watching the larger one is the single fastest diagnostic, because it directly tests the sharing rule the vendor published.

If the numbers still do not line up after those three checks, the escalation is specific rather than vague. Quote the adapter's published voltage and current lines, the cable's 60W or 240W marking, and the figure the operating system reports. That combination tells a support agent which of the three limits is binding, which is far more actionable than reporting that a device charges slowly.

When This Doesn't Apply

Negotiated power explains a stable, repeatable figure. It does not explain a figure that moves, and several documented behaviours override the negotiation entirely.

Temperature is the most common override. Google states that "some Pixel phones may automatically adjust their charging to manage device and battery temperatures" and that "Sometimes, this can cause slower charging." Apple goes further and suspends charging outright, noting that "With iOS 16, iPadOS 16, and later, charging is put on hold when your device becomes too hot or too cold", with the on-screen message "Charging On Hold. Charging will resume when [device] returns to normal temperature." A device that charged quickly this morning and slowly this afternoon in a hot car is not negotiating differently.

Battery state is a second override. Charging tapers as a cell approaches full regardless of what the adapter can supply, which is why vendor claims are written as a percentage reached within a window rather than a sustained wattage. Every Apple figure quoted above is a 50-percent claim for that reason.

Battery age is a third. Google notes that "your Pixel phone's battery performance may decline because of how you use it, the temperature, and its age." A three-year-old phone on a correct adapter may simply not accept what it once did.

Finally, wireless charging is a separate system with separate numbers and none of the fixed-line arithmetic above applies to it. Google notes that "Older Qi chargers like Qi1.x will still work but they'll charge your phone slower at up to 5W."

What to Do With the Charger You Already Own

The useful reframing is to stop reading the wattage as a total and start reading it as the top of a list. A 45-watt adapter that publishes 5V/3A, 9V/3A, 15V/3A, and 20V/2.25A is a 27-watt adapter for anything that stops at 9 volts, and that is a property of the pairing rather than a fault in either part.

Before replacing hardware, check the three published limits in order: which lines the adapter offers, whether the cable is marked 60W or 240W, and which line the device is documented to request. Then check whether a second device is sharing the brick, because on Apple's 35W dual adapter the documented remedy is simply to unplug it. Behaviour that looks like intermittent failure is worth separating from behaviour that is documented and constant, a distinction covered in detail in the case of a 5 GHz Wi-Fi band vanishing on a documented timer with nothing broken.

Primary documentation for the figures above: Google's 45W charger tech specs, the Pixel Flex Dual Port 67W specifications, Apple's 35W Dual USB-C Port Power Adapter page, the USB-IF USB Power Delivery overview, the USB-IF cable labelling requirements, and Apple's fast-charge matrix for MacBook Air and MacBook Pro.