I recently asked a question regarding a sustainable way to generate white noise. I received a wonderfully helpful comment from Móż (thank you!) in which Móż recommended ensuring that the power source (a charger, in this case) is efficient.

That comment prompts this question:

How does one ensure that their electronic chargers are energy efficient?

I do have a digital multimeter and electrical leads.

  • Does your multimeter measure ac current, and are you comfortable connecting it to mains voltage?
    – LShaver
    Commented Feb 22, 2021 at 15:18
  • @LShaver I'm almost positive that it measures AC current, but I'll double-check as this one replaced a broken analog one that I used for years. Regarding connecting to mains voltage, the leads are insulated but not heavy duty. Also, the multimeter was not expensive (meaning it's not industrial quality). I can follow instructions carefully as long as they are written clearly (and stop if they aren't!). I'm not an electricity expert by any means, but I have a very healthy respect for it. Given those responses, do you think it's reasonably safe to connect the multimeter to mains voltage? Commented Feb 22, 2021 at 17:45
  • Turns out there are some standards around these things, marked on most chargers -- see my answer. Could be a better option than testing on your own.
    – LShaver
    Commented Feb 23, 2021 at 5:03
  • 2
    Quick way: pick up the charger. If it feels lightweight, it's efficient; if it feels like a solid block of metal, it isn't. The efficiency difference between modern switch-mode power supplies and older transformer-based ones dwarfs any minor variation between switch-mode designs.
    – Mark
    Commented Feb 23, 2021 at 6:28
  • 1
    Also, it's not worth trying to put a multimeter in series with mains to measure current. The people who know enough to do that almost always buy a proper mains current or power meter instead specifically because it's so dangerous not to. A plug in power meter (search online for that, you'll find ones wityh local plugs) is about $20 in most places and while those are almost always garbage for loads under 10W they're much better than bare multimeter probes.
    – Móż
    Commented Feb 24, 2021 at 5:12

2 Answers 2


Power ratings -- choose level VI

Take a look at any external power supply (EPS) you have lying around -- you should notice, along with all the regulatory markings, a roman numeral I through VI in a circle. Here are four that I just dug up in my house:

enter image description here

These indicate energy efficiency standards for EPS starting with the California Energy Commission in 2004 (levels I through III), through the U.S. Department of Energy and the European Union Ecodesign in 2020 (level VI), according to this timeline.

Level VI is the current highest standard, required for every EPS sold in the U.S., Canada, Australia, and the E.U. This pdf from a manufacturer includes a table explaining the level VI requirements:

Basic Voltage: nameplate output voltage ≥ 6 V

Nameplate power (Pno) Min avg efficiency in active mode Max no-load power (W)
≤1 watt ≥0.5 x Pno + 0.160 ≤0.100
>1 to ≤49 watts ≥0.071 x Ln(Pno) - 0.0014 x Pno + 0.67 ≤0.100
>49 to ≤250 watts ≥0.880 ≤0.210
>250 watts ≥0.875 ≤0.500

Low voltage: nameplate output voltage < 6 V and nameplate output current ≥550 mA

Nameplate power (Pno) Min avg efficiency in active mode Max no-load power (W)
≤1 watt ≥0.517 x Pno + 0.087 ≤0.100
>1 to ≤49 watts ≥0.0834 x Ln(Pno) - 0.0014 x Pno+ 0.609 ≤0.100
>49 to ≤250 watts ≥0.870 ≤0.210
>250 watts ≥0.875 ≤0.500

You can do the math to determine the specific minimum requirement for any charger you may have, but reading the chart we can see some general rules. Voltage will be a function of the device you need to power, but you may have some leeway to select the nominal power:

  • Choosing an EPS with the lowest nameplate power that meets your need will reduce no-load power consumption -- this would apply to anything that you will leave plugged in (speakers, light, etc).
  • Choosing the highest power rating will increase active power efficiency -- this would apply to anything mobile (phone or laptop charger in your knapsack).

Nominal power won't be listed on each EPS, but the voltage (V) and current (A or mA) should be, so you can compute it by multiplying these two values.

Choose a known brand, and be wary of counterfeits

In searching for info, I came across this blog where the author did detailed testing of 12 chargers. They don't talk about the EPS rating, but you can see in the photos that most of these are level V chargers (which makes sense, since the post is from 2012).

They include this chart comparing efficiency and "vampire" (no load) power:

Efficiency of 12 chargers compared

While it's possible that things have changed since then, in general the advice probably still holds true -- buy a brand-name charger from a trusted source to get the most efficient (and safest) EPS. There are ways to check or test for counterfeits, but buying direct from the source or from a reputable reseller is probably the best insurance.

Choose switching power supplies over linear (transformer type)

When comparing power supplies that don't have a printed rating, look for a switching (or switched mode) power supply, vs a linear (or transformer type) power supply. Linear supplies can be identified because they use a large, heavy transformer to step down the mains voltage (100 to 230 VAC at 50 or 60 Hz) to a lower voltage before converting to dc current. Switching power supplies use transistors to first increase the frequency of the mains voltage, so that a smaller transformer can be used. Wikipedia offers this comparison of their efficiency:

Linear power supply Switching power supply
"[E]fficiency largely depends on voltage difference between input and output; output voltage is regulated by dissipating excess power as heat resulting in a typical efficiency of 30–40%." "Output is regulated using duty cycle control; the transistors are switched fully on or fully off, so very little resistive losses between input and the load. The only heat generated is in the non-ideal aspects of the components and quiescent current in the control circuitry."
  • 1
    Wow. This is an amazingly helpful answer. Thank you. What about EPS made before 2004? I hesitate to use them because of potential inefficiency, but I hesitate to get rid of them due to environmental waste combined with the resources required to create new ones. Commented Feb 23, 2021 at 11:22
  • @RockPaperLz-MaskitorCasket I added the last section for one other way to estimate power supply efficiency.
    – LShaver
    Commented Feb 23, 2021 at 18:33
  • Thank you for the added information! Commented Feb 23, 2021 at 18:35

How does one ensure that their electronic chargers are energy efficient?

They already are.

Do a simple experiment.

Connect a laptop to a charger. Place full CPU load on the laptop. Can you hear the fan spinning on the laptop? Yes you can.

But can yo hear the fan spinning on the power supply? No you can't.

A power supply doesn't have a fan precisely because it's very efficient, above 90% efficient.

So if you're purchasing a power supply producing any significant quantity of power, you simply select a power supply that doesn't have a fan. It would self-destruct if it wasn't energy efficient.

Of course with very large powers, multiple kilowatts, a fan is needed even in relatively efficient power supplies but that's a different story.

In the small power supply segment, you won't find poorly efficient power supplies of fans at all because they can't compete with more efficient fanless supplies.

  • Some would say that these days simply being "above 90%" is not "very efficient". It's also not a fair comparison; computers do not convert power, they consume power. Every time you charge the gate of a CMOS FET you do work that can't be recovered. It's not the same "consume" as a space heater who's job it is to convert all the electrical power it draws into heat, but it's thermodynamically inevitable nonetheless. Even the power of the light emitted by the display and EM waves radiated by the bluetooth and WiFi and the power delivered to headphones and speakers nearly all ends up as heat.
    – uhoh
    Commented Apr 3, 2022 at 2:49
  • For AC/DC power converters you can see 90, 95 and even 98% efficiency values discussed, and that represents a factor of five difference in wasted energy.
    – uhoh
    Commented Apr 3, 2022 at 2:51

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