Watts are Volts x Amps.
The higher the Volts the more chance electrons accidentally jump between wires, known as a spark. To reduce this, thickness of the insulation between wires increases. Thicker plastic typically.
The higher the Amps the more heat from resistance in the wire, which reduces efficiency, ages the wire faster, and ultimately causes wire to melt. Resistance is lower in thicker wires, the metal inside the plastic.
So in general you’re seeking ideally the shortest wire for purpose, as less total distance for resistance, and thicker cables.
Each cable design has been tested by reputable manufacturers to a stated voltage (thickness of insulation) and Watts (Volts x Amps, thickness of wires).
USB-C comes in versions designed for differents Volts and Watts. The highest is 20V 5A 100W, cables for that are quite thick.
Your device, phone for example, will have a min and max Voltage it supports. You really need to focus on the Volts mostly and seek cables and chargers which overlap those Volts the most.
I notice you only seem interested in Watts, while if you really want to make the best decisions you must research the Volts supported as input to your device, overlap as much as possible the Volts of output of your charger. If you dont you really risk something not working. For example a 60W charger can be made which is 20V 3A but if you connected to a phone only capable of input at 5V and 9V then the phone will refuse the charger.
The energy transfer rate of electricity, measured in Power Watts, can be made into any Voltage using induction , known as step up or step down, which is being done inside the charger. As energy lost in wires is a function of Amps x distance, good engineering is to use a high voltage, so lower Amps, over long distances. This is why electrical distribution is to your wall at 110V-240V.
But your phone would need much thicker insulation if it were designed for 110V-240V, it is usually 5V to keep it thinner. So the charger steps-down 110V-240V to 5V-20V. Then the USBC cable between your charger and phone.
So in general you want to avoid wall chargers and long USB cables, but go for power extension to near your phone and short USB cables.
The Lithium cells in your device are designed for a specific input (charging) and output (discharging) Voltage. Typically in phones is around 3.7V, tablets 7.8V and laptops 18V. To get energy into a cell you must push at a higher voltage than it outputs, so say a phone 3.8V cell output needs 4.2V input.
Step up/down converters lose energy, inefficiency, usually as heat, as a function of the ratio of step. So wall sockets steping down from 110V-240V to 5V create more heat than your phone stepping down 5V to 4.2V.
So what means is a phone, trying to avoid heat so it performs well as a phone while being charged, really wants a Voltage as low as possible nearer to 5V, so you’d often see phone chargers supporting the lower USBC Voltages of 5V 9V. 9V 2A is 18W for example. So say a 60W charger which output only 12V, 15V, 20V would not work in the phone.
A laptop with 18V cells really wants 20V and to make itself generate least heat in its step-up/down converter, will refuse the lowest USB C voltages like 5V, 9V, 12V. So say one 45W charger of 20V would work, another of 15V would not.
So to reduce the chance of chargers not working, seek chargers with the maximum range and diversity of Voltages, so 5V,9V,12V,15V,20V.
So really, if you want to research and make the least likely wrong buying decisions, you must not look at Watts, but instead look up the Volts of everything, the device, cable and charger.
Portable chargers have a fixed amount of stored energy, the smaller they are the more relatively important it is efficiently consumed. So actually in portable chargers in general higher Wattage is worse than lower Wattage, but it very specific. So say if Anker connected 3.8V cells in serial to make 7.6V, it can equally make 5V and 9V. But if they had 3.8V in parallel to be 3.8V then even if it could offer 9V you’d want 5V. My phone has 3.8V cells so I carry a 5V Powercore, but my tablet has 7.8V cells so I carry a 9V Powercore. Notice the attention to Voltage, not Watts.
Here endeth the lesson.
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I wish USB consortium would have more mandatory specs so all to that standard must support all the voltages in the standard, so you can be sure all USB type X work interoperate so the geeks can step back and less falling on Amazon renewed returns to circulate the accidental bad fit purchases.
I wish phone engineers would make phones work with any voltage, particularly flagship ones as the $ extra cost is noise.
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