I was just thinking if you are on your computer anyway would it just use some of the excess electricity that your computer would have wasted or would it be worse than charging your phone from a charger while using your laptop separately.
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.
This would only affect the 12V rail though no? It’s not like they are beefing up the 5V rail that supplies your USB ports in excessive amounts. Picking a random PSU from pcpartpicker, the CORSAIR RM650e vs RM1200e (650W vs 1200W) both have a +5V@20A rail. There would be no need to have a larger 5V rail to support gaming cards.
Also correct me if I am wrong, most PSU’s are more efficient at 20-50% utilization, not 100%. I’m basing this off the higher ratings for 80 Plus.
I’ve previously spoken with PSU engineers for enterprise power supplies – specifically for 48-54v PoE equipment – who described to me that today’s switch mode power supplies (SMPS) tend to get more efficient with increasing load. The exception would be when the efficiency gains from higher loading start to become offset by the heating losses from higher input currents.
This graph for a TDK PSU shows that North American 120 VAC nominal (see here for the small difference between nominal and utilization voltages) will cause a small efficiency hit above 75% or so. And this is exactly why data centers – even in North America – will run with “high line” voltage, which is 200 VAC or higher (eg North American 208VAC delta supplies, British 240/415 wye, European 230/400 wye).
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/
Your computer doesn’t have “excess energy”
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.
