The other answers have touched upon the relative efficiencies between a phone charger and a desktop computer's PSU. But I want to also mention that the comparison may be apples-to-oranges if we're considering modern smartphones that are capable of USB Power Delivery (USB PD).

Without any version of USB PD -- or its competitors like Quick Charge -- the original USB specification only guaranteed 5 V and up to 500 mA. That's 2.5 W, which was enough for USB keyboards and mice, but is pretty awful to charge a phone with. But even an early 2000s motherboard would provide this amount, required by the spec.

The USB Battery Charging (USB BC) spec brought the limit up to 1500 mA, but that's still only 7.5 W. And even in 2024, there are still (exceedingly) cheap battery banks that don't even support USB BC rates. Motherboards are also a mixed bag, unless they specifically say what they support.

So if you're comparing, for example, the included phone charger with a Samsung S20 (last smartphone era that shipped a charger with the phone) is capable of 25 W charging, and so is the phone. Unless you bought the S20 Ultra, which has the same charger but the phone can support 45 W charging.

Charging the S20 Ultra on a 2004-era computer will definitely be slower than the stock charger. But charging with a 2024-era phone charger would be faster than the included charger. And then your latest-gen laptop might support 60 W charging, but because the phone maxes out at 45 W, it makes no difference.

You might think that faster and faster charging should always be less and less efficient, but it's more complex since all charging beyond ~15 Watts will use higher voltages on the USB cable. This is allowable because even the thinnest wire insulation in a USB cable can still tolerate 9 volts or even 20 volts just fine. Higher voltage reduces current, which reduces resistive losses.

The gist is: charging is a patchwork of compatibility, so blanket statements on efficiency are few and far between.

There is no such thing as 'excess electricity' in a modern (switching) power supply unit. They use as much power as is needed. There is a few percent of loss in the device, no big deal.

Some desktop computers are less efficient because they have too strong psu's (lots of reserve for your future "gaming" graphics card) built in.

It'll depend on how efficient your phone charger is vs your PC PSU. Looking at some charts, it's a very close battle but generally the phone charger seems to win out. Probably because it's more optimized for its max power output, whereas the PSU needs to support a wider range of loads.

https://silentpcreview.com/power-lost-a-better-way-to-compare-psu-efficiency/

https://www.electronicdesign.com/technologies/power/whitepaper/21129264/gan-fulfills-promise-of-efficiency-and-simplified-power-adapter-design

Your computer doesn't have "excess energy"

Charging at higher voltage is more efficient (at least on electric vehicles) but also your phone is such a relatively small battery it barely matters. Avoid wireless charging as it is extremely inefficient.

If you're bothered about overall waste, consider that some batteries degrade slower if you charge slower. I tend to prefer a slow charger when i can.

excess electricity that your computer would have wasted

Ya this is not how electricity or computers work.

Your computer doesn't "waste" electricity, power usage is on-demand. A PSU generally has 3 "rails"; a 12V (this powers most of the devices), a 5V (for peripherals/USB) and 3.3V (iirc memory modules use this). Modern PSUs are called Switched-mode power supplies that use a Switching voltage regulator which is more efficient than traditional linear regulators.

The efficiency of the PSU/transformer would be what determines if one or the other is more wasteful. Most PSUs (I would argue any PSU of quality) will have a 80 Plus rating that defines how efficiently it can convert power. I am not familiar enough with modern wall chargers to know what their testing at.. I could see the low-end wall chargers using more wasteful designs, but a high quality rapid wall charger is probably close to if not on par with a PC PSU. Hopefully someone with more knowledge of these can weigh in on this.

Gallium Nitride based modern phone chargers are 95% efficient.

The very best, most expensive PC power supplies on 115v AC will only reach 94% at the very specific 50% load of power supply rated wattage. So if you have a 500 watt power supply, and aren't using almost exactly 250 watts, you aren't getting that 94% efficiency. Regular power supplies under normal variable load conditions are going to be somewhere in the 80% efficient range. If the PC is idle, that efficiency can drop to 20% (but it's fine because it's only a few watts).

https://forum.level1techs.com/t/super-high-efficiency-300-400w-psu/184589/2

So using a modern Gallium Nitride stand alone charger will be more efficient. It will be extremely more efficient if you use that stand alone charger instead of charging off your PC while your PC is idle.

It reaaaaaally doesn’t matter. If you charged your phone from empty to full every day it might cost you a dollar. Per year.

I'd like to remind everyone of the "vampire effect" of wall-wart chargers - if you just leave them plugged into the wall waiting for you to connect a device, you're constantly wasting a bit of electricity. That should also be involved in the efficiency decision of using the already plugged in computer or laptop.

It is a little bit more efficient, because your PCs Power Supply is not very efficient when you dont use much power. By using it more of it, it becomes more efficient, peaking somewhere at 50%. But: we are talking about very little differences here and only for Desktop PCs. When you use a Laptop, you Powersupply is way less powerful, so you use more of it by just using the Laptop. So in that case, i would rather use the charger. To be perfectly honest with you: All that is not realy worth thinking about. Its like opening your Fridge for only 6 Secounds instead of 8. Yes, its saves power, but there are petter ways to do so. Doing only 1 kilometer less with your electric car ist about the same as charging your phone 10 times.

IF you've a high-efficiency charger, then I'd say it's probably more-efficient to use that charger.

The warmer you run your computer, the less-efficient it becomes, & the shorter the lifespan of the hottest chips in it ( this effect shouldn't be significant )

e.g. increasing a CPU by 10Celsius should cut its lifespan in half.

by having more heat-generating-stuff going on in your computer, you impair the cooling of your CPU & GPU ( slightly, probably ), & that may affect your computer's time-to-failure.

Fan-bearings may dry-out sooner, too.

hth, eh?

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