# Unconnected wall wart power consumption

• Electrical
• noname12345

#### noname12345

TL;DR Summary
How much power (if any) does a plugged in but unconnected wall wart (transformer/rectifier) consume?
Hi.

Not homework. I left that behind me 40yrs ag

How much power (if any) does a plugged in but unconnected wall wart (transformer/rectifier) consume?

Eg. I have a phone charger labelled as:

Input: 100-240V~50/60Hz 0.3A.
____________
Output: 5V - - - - - - 2.0A

So, how much does it cost me if I forget to unplug it between uses?

Thanks.

So, how much does it cost me if I forget to unplug it between uses?
No way to know for sure. I'd check with a wattmeter if you have one.

DaveE
No way to know for sure. I'd check with a wattmeter if you have one.
Russ is right. We would need more details.

However, 10 years ago those things were terrible. At one point, they were projected to become one of the major uses of electricity. But before that happened, the manufacturers made new ones much more efficient so that they don't use much power plugged in but disconnected.

You can get an idea by just putting your hand on the plugged in wall wart. If it doesn't feel very warm to your touch, it is not consuming much energy.

Spinnor
No way to know for sure. I'd check with a wattmeter if you have one.

This isn't about either this one specific wallwart; but rather a desire to understand the principles involved.

What are the limitations on knowing? Have multimeter, can measure.

If the input was DC, then with no load on the output, the steady state current draw would simply be the input voltage / the primary coil resistance; and from that power consumption.

With AC, thing are complicated by induction effects, but with an open circuit secondary coil not so complicated?

If it can be measured, can it not be calculated?

Russ is right. We would need more details.
What more details?

This isn't about either this one specific wallwart; but rather a desire to understand the principles involved.

What are the limitations on knowing? Have multimeter, can measure.

If the input was DC, then with no load on the output, the steady state current draw would simply be the input voltage / the primary coil resistance; and from that power consumption.

With AC, thing are complicated by induction effects, but with an open circuit secondary coil not so complicated?

If it can be measured, can it not be calculated?
It can't be calculated because they aren't necessarily simple circuits; they may have control and communications circuitry in them as well.

Anyway, I just tested four different phone chargers with a Kill-A-Watt that reads out in tenths of a Watt, and all registered zero with no phone plugged in. Charging wattages ranged from 5.8 to 18.2W.

Spinnor and phinds
What more details?
The exact make & model so that we could try to look it up.

Does it feel warm to your hand? Like hot water?

It can't be calculated because they aren't necessarily simple circuits; they may have control and communications circuitry in them as well.

I've been assuming that any circuitry would be limited to a diode bridge and RC smoothing stage connected on the secondary side and thus open circuit when nothing is connected.

Now you got me wondering who my wall warts are talking to and what they are saying about me :)

Anyway, I just tested four different phone chargers with a Kill-A-Watt that reads out in tenths of a Watt, and all registered zero with no phone plugged in. Charging wattages ranged from 5.8 to 18.2W.

That's really intriguing. If your meter is truly capable of measuring down to 1/10th Watt; that implies that the primary coil is being open-circuited when nothing is being charged. (Otherwise, 0.1W @ 240V means the primary coil would have to have a 576kΩ or higher resistance. Or 121kΩ at 110VAC)

But as any control circuitry must inevitably be powered from the mains input (via the transformer&inverter), how would it detect when something was plugged in, so that it could close a switch in the primary coil?

I know some of the latest high power "fast" chargers, do have some Constant Current circuitry in them and detect when the battery voltage has reached its "full charge" level in order to stop the charging; but the ones I'm talking about are nowhere near that sophisticated.

Now you got me wondering who my wall warts are talking to and what they are saying about me :)
Don't worry, they are probably just talking to your phone and each other.
That's really intriguing. If your meter is truly capable of measuring down to 1/10th Watt; that implies that the primary coil is being open-circuited when nothing is being charged. (Otherwise, 0.1W @ 240V means the primary coil would have to have a 576kΩ or higher resistance. Or 121kΩ at 110VAC)

But as any control circuitry must inevitably be powered from the mains input (via the transformer&inverter), how would it detect when something was plugged in, so that it could close a switch in the primary coil?
Agreed. So, whatever control they have runs on less than 0.5 watts (assuming the meter is accurate).
I know some of the latest high power "fast" chargers, do have some Constant Current circuitry in them and detect when the battery voltage has reached its "full charge" level in order to stop the charging; but the ones I'm talking about are nowhere near that sophisticated.
That would surprise me -- I assumed all the charging circuitry/logic was in the phone.

btw, if my phone's mah rating is at 5V (doubtful), it would be 20 W-h. If it were to be charged fully once a day, that's 7.3 kWh per year or at my electric rate, about $1.24. That's really intriguing. If your meter is truly capable of measuring down to 1/10th Watt; that implies that the primary coil is being open-circuited when nothing is being charged. Many modern wall warts have SMPS PS which can sleep at very low power consumption. They wake up occasionally to check if anything has discharged their output, or have some sort of interrupt circuit to wake them up. 100uA is really a lot for a MOS circuit to detect activity. I think it's the only way to get low standby power. AFAIK, no one would turn a 60Hz transformer on/off. russ_watters Anyway, I just tested four different phone chargers with a Kill-A-Watt that reads out in tenths of a Watt, and all registered zero with no phone plugged in. Charging wattages ranged from 5.8 to 18.2W. Do you have any of the old adapters that are just a transformer? Do you have any of the old adapters that are just a transformer? If there is such a thing, I don't own one. The oldest of the ones I tested are at most probably 5 years old. If there is such a thing, I don't own one. The oldest of the ones I tested are at most probably 5 years old. Old folks have big boxes of those, with various voltage, wattage and connectors (but never the combination they need). Edit: Oh, and I forgot polarity. Last edited: NTL2009, hutchphd and anorlunda Old folks have big boxes of those, with various voltage, wattage and connectors (but never the combination they need). Are we still talking about USB cell phone chargers? Because if we're going beyond that, yeah, I can provide test results from a bunch of them... There are existing energy efficiency standards for these things and more on the way. One little wall wart may not seem significant, but what if everyone in your country has 10-20 of them plugged in all the time. That adds up to lots of wasted energy if they are still old school designs. Here's a summary from a PS manufacturer. Since no one pays me for this anymore, I'm not really going to read the details, you can. But it looks like a standby power consumption limit of 100mW is a common requirement. The days of 50/60Hz transformers are practically history. Keith_McClary and russ_watters I've been assuming that any circuitry would be limited to a diode bridge and RC smoothing stage connected on the secondary side and thus open circuit when nothing is connected. Correct. If the wall wart contains a mains frequency transformer, then the magnetising current will continue to flow when there is no output current. But the magnetising current is in quadrature with the voltage, so it circulates and does not constitute real power. Expect magnetising currents between 0.25% and 5% of the full load current. While the transformer is connected you must pay for the heating of the transformer by core hysteresis and the resistive losses in the primary winding only. But those losses will not be high. Expect a core loss current of 1% of the full load current. If the circuit includes a regulator then you can expect a maximum of about 20 mA at 12 V = 240 mW. That holds also for switching converters, that have no mains transformer. That would surprise me -- I assumed all the charging circuitry/logic was in the phone. Wouldn't that mean it couldn't be charged if powered off; and letting your battery get completely flat would effectively brick it? There are existing energy efficiency standards for these things and more on the way. One little wall wart may not seem significant, but what if everyone in your country has 10-20 of them plugged in all the time. That adds up to lots of wasted energy if they are still old school designs. Indeed. I was wandering around my house looking at all the devices that use wall warts and are designed to be plugged in permanently: Including: cordless vac; shavers; toothbrushes; hairclippers; PVRs; TV sticks; cordless phones; wifi hub; wifi extenders; LED desk lamp; LED monitors; alarm clocks; clock display/timers on conventional & microwave ovens; video games. And then there are all the transient ones that often get left plugged in when not in use: phone chargers; tablet chargers; laptop chargers; camera battery charger; satnav charger; LED torch charger; car battery charger; ebike charger; cordless tools(screw driver, drill, dremel, etc.). And several of them are well old -- 20yrs+ in a few cases -- so almost certainly linear tranformers. It just got me wondering how much all that convenience is costing per annum. Correct. If the wall wart contains a mains frequency transformer, then the magnetising current will continue to flow when there is no output current. But the magnetising current is in quadrature with the voltage, so it circulates and does not constitute real power. Expect magnetising currents between 0.25% and 5% of the full load current. While the transformer is connected you must pay for the heating of the transformer by core hysteresis and the resistive losses in the primary winding only. But those losses will not be high. Expect a core loss current of 1% of the full load current. If the circuit includes a regulator then you can expect a maximum of about 20 mA at 12 V = 240 mW. That holds also for switching converters, that have no mains transformer. Thanks. That reassures me about most of the modern devices with SMPSs. I still need to understand the costs of the older devices with mains frequency transformers. My understanding of But the magnetising current is in quadrature with the voltage, so it circulates and does not constitute real power.. leaves something to be desired. As far as I know, "quadrature" simply means the secondary is 90° out of phase from the primary; so I don't understand how that translates into "does not constitute real power"? As far as I know, "quadrature" simply means the secondary is 90° out of phase from the primary; so I don't understand how that translates into "does not constitute real power"? The power is the product of volts and in-phase amps. If there is a 90° phase shift of current to voltage, the energy flows backwards and forwards = circulates. It is called Volts Amps Reactive, VAR, and does no real work. Think of the integral of Sin(x) * Cos(x); https://en.wikipedia.org/wiki/AC_power#Reactive_power The power is the product of volts and in-phase amps. If there is a 90° phase shift of current to voltage, the energy flows backwards and forwards = circulates. It is called Volts Amps Reactive, VAR, and does no real work. Think of the integral of Sin(x) * Cos(x); https://en.wikipedia.org/wiki/AC_power#Reactive_power Okay. I read the wiki article (and several others linked from it); this is what I think I got from it: Because the primary coil is reactive as well as resistive, the current flow lags the (AC) voltage rise and fall. And because both voltage and current keep reversing, the net current flow over a cycle is 0. Thus, the reactive nature of the coil effectively negates the resistance of the coil and no net power is drawn from the source, if the secondary coil is open and we ignore parasitic losses (eddy currents in the core). Did I understand? Because the primary coil is reactive as well as resistive, the current flow lags the (AC) voltage rise and fall. The concept of rise and fall confuses it. Think simply of phase shift. The reactive current flow lags voltage by 90°, the resistive current flows in phase. The sum of those two gives the current an intermediate phase that can be resolved into a resistive and a reactive component. And because both voltage and current keep reversing, the net current flow over a cycle is 0. Not quite, if voltage and resistive current change in phase together, the product would always have the same sign, so real power would always flow in the same direction. Since the product of voltage and reactive current keeps reversing, the direction of energy flow alternates and there is no net energy flow, so no real power. Thus, the reactive nature of the coil effectively negates the resistance of the coil ... The reactive magnetising current still generates heat in the resistance of the primary. With no secondary load, the phase of the current in the primary is not quite 90°, because of the primary winding series resistance. Wouldn't that mean it couldn't be charged if powered off; and letting your battery get completely flat would effectively brick it? No, not if "powered off" doesn't really mean powered off. Remember, USB is primarily a data transfer interface, and a 15 year old computer isn't going to have a database of phone/battery specs in it to regulate their charging. Indeed. I was wandering around my house looking at all the devices that use wall warts and are designed to be plugged in permanently: Including: cordless vac; shavers; toothbrushes; hairclippers; PVRs; TV sticks; cordless phones; wifi hub; wifi extenders; LED desk lamp; LED monitors; alarm clocks; clock display/timers on conventional & microwave ovens; video games. And then there are all the transient ones that often get left plugged in when not in use: phone chargers; tablet chargers; laptop chargers; camera battery charger; satnav charger; LED torch charger; car battery charger; ebike charger; cordless tools(screw driver, drill, dremel, etc.). And several of them are well old -- 20yrs+ in a few cases -- so almost certainly linear tranformers. It just got me wondering how much all that convenience is costing per annum. Ok, re-reading your OP, I see it was a generic wall wart description even though the example was a phone charger. The variation is likely vast. I'll test a few non phone chargers tonight. I will say that I have a whole-house meter and my minimum usage is about 250 watts. This includes a few devices that are truly powered on, such my network system and dvr. Last edited: The concept of rise and fall confuses it. Think simply of phase shift. The reactive current flow lags voltage by 90°, the resistive current flows in phase. The sum of those two gives the current an intermediate phase that can be resolved into a resistive and a reactive component. Ooo-kay. But... :) Reactance results from self-inductance. And inductance is due to a change (ie. rise or fall) of voltage... Not quite, if voltage and resistive current change in phase together, the product would always have the same sign, so real power would always flow in the same direction. Since the product of voltage and reactive current keeps reversing, the direction of energy flow alternates and there is no net energy flow, so no real power. I think this (from: https://en.wikipedia.org/wiki/Electrical_reactance) explains what you are saying: "Reactance is similar to electric resistance in this respect, but differs in that reactance does not lead to dissipation of electrical energy as heat. Instead, energy is stored in the reactance, and later returned to the circuit whereas a resistance continuously loses energy." The reactive magnetising current still generates heat in the resistance of the primary. With no secondary load, the phase of the current in the primary is not quite 90°, because of the primary winding series resistance. So, a mains frequency transformer's primary winding will always consume power due to its resistance, regardless of whether the secondary winding is drawing current. Your OP did not put a time limit on devices. I had a wall wart for my first laptop circa 1989. Things were much different back then. Here's a longer story from Wikipedia. It echos my simple advice. Just put your hand on suspected devices. If its not hot, it is efficient. But efficiency is in the eye of the beholder. What number means "efficient" to you? https://en.wikipedia.org/wiki/AC_adapter#Efficiency The issue of inefficiency of some power supplies has become well known, with U.S. president George W. Bush referring in 2001 to such devices as "Energy Vampires".[5] Legislation is being enacted in the EU and a number of U.S. states, to reduce the level of energy wasted by some of these devices. Such initiatives include standby power and the One Watt Initiative. But others[who?] have argued that these inefficient devices are low-powered, e.g., devices that are used for small battery chargers, so even if they have a low efficiency, the amount of energy they waste is less than 1% of household consumption of electric energy.[citation needed] Considering the total efficiency of power supplies for small electronic equipment, the older mains-frequency linear transformer-based power supply was found in a 2002 report to have efficiencies from 20–75%, and have considerable energy loss even when powered up but not supplying power. Switched-mode power supplies (SMPSs) are much more efficient; a good design can be 80–90% efficient, and is also much smaller and lighter. In 2002 most external plug-in "wall wart" power adapters commonly used for low-power consumer electronics devices were of linear design, as well as supplies built into some equipment. External supplies are usually left plugged in even when not in use, and consume from a few watts to 35 watts of power in that state. The report concluded that about 32 billion kilowatt-hours (kWh) per year, about 1% of total electrical energy consumption, could be saved in the United States by replacing all linear power supplies (average efficiency 40–50%) with advanced switching designs (efficiency 80–90%), by replacing older switching supplies (efficiencies of less than 70%) with advanced designs (efficiency of at least 80%), and by reducing standby consumption of supplies to not more than 1 watt.[6] Since the report was published, SMPSs have indeed replaced linear supplies to a great extent, even in wall warts. The 2002 report estimated that 6% of electrical energy used in the U.S. "flows through" power supplies (not counting only the wall warts). The website where the report was published said in 2010 that despite the spread of SMPSs, "today's power supplies consume at least 2% of all U.S. electricity production. More efficient power supply designs could cut that usage in half".[7] Since wasted electrical energy is released as heat, an inefficient power supply is hot to the touch, as is one that wastes power without an electrical load. This waste heat is itself a problem in warm weather, since it may require additional air conditioning to prevent overheating, and even to remove the unwanted heat from large supplies. berkeman, russ_watters and noname12345 No, not if "powered off" doesn't really mean powered off. Remember, USB is primarily a data transfer interface, and a 15 year old computer isn't going to have a database of phone/battery specs in it to regulate their charging. Hm. If the battery is flat. It's off! And a 15y/o computer is neither a wall wart nor a fast charger. And USB plugs used for charging often (if not always) only have the power pins connected. No communication is possible. Ok, re-reading your OP, I see it was a generic wall wart description even though the example was a phone charger. The variation is likely vast. I'll test a few non phone chargers tonight. I will say that I have a whole-house meter and my minimum usage is about 250 watts. This includes a few devices that are truly powered on, such my network system and dvr. Main draws in my house -- in roughly the order of consumption most to least -- are the electric kettle, electric oven, dishwasher, washing machine, two freezers and fridge. But all of those are high but intermittent power draw. Ie only when in use. It's all the (20+) devices that are ostensibly off but none-the-less drawing standby current 24 hrs a day that add up I think. I'd love to see the results of your testing of none-phone stuff. Though I think I am going to purchase a power meter, if I can find one that explicitly states it is accurate at very low power draw and not too expensive. Your OP did not put a time limit on devices. I had a wall wart for my first laptop circa 1989. Things were much different back then. Here's a longer story from Wikipedia. It echos my simple advice. Just put your hand on suspected devices. If its not hot, it is efficient. But efficiency is in the eye of the beholder. What number means "efficient" to you? You got me walking round the house putting my hands on all the devices trying to detect heat :) So far, the only one that is detectably warn to touch is my ancient rechargeable shaver. May be time for a new one. berkeman and anorlunda Ooo-kay. But... :) Reactance results from self-inductance. And inductance is due to a change (ie. rise or fall) of voltage... No, a rising or falling current, di/dt. For an inductor; V = L * di/dt. Inductance has a positive reactance, capacitance has a negative reactance. Hm. If the battery is flat. It's off! I feel like we're talking in circles. I just powered-off my pone, then plugged it into a charger. The screen lit up and an animated counter of the charge status appeared. So clearly it has some circuitry active even when powered off. Presumably even if the battery is completely dead this circuitry will still activate when the phone is plugged in, to control the charging and display the status. And a 15y/o computer is neither a wall wart nor a fast charger. And USB plugs used for charging often (if not always) only have the power pins connected. No communication is possible. Agreed; variety. Note though, that google tells me iphones require communication (identification) to work with a charger and will refuse to charge if they don't get it: https://www.ifixit.com/Answers/View/418299/iPhone+6+will+not+charge+with+certain+chargers Anyway, here's some results: • Microwave: 3.3W • Weather Station: 0.5W • Shark Cordless Vac: 6.2W. Charger only: 0 • Receive-only DVR: On: 20.7W. Off: 20.7W • Network printer on: 9.7W. Standby: 5.2W • Multi-voltage DC PSU: 0W The cable box/DVR is the most annoying. If left on all the time, that's about$31 a year. The one I tested I keep off from a power strip except when using it a few hours a week.

DaveE
Microwave: 3.3W
Really?

Edit: You mean when not cooking. What does it draw when cooking?

Really?

Edit: You mean when not cooking.
Yes, all of those were measured off or on standby except where noted. I think that's what the OP was after.
What does it draw when cooking?
1,665W give or take a couple.

Measuring an unloaded wall-wart will, I believe and as mentioned already, be totally dominated by power factor (reactive power), and I dare say full of power (both positive and 'negative' reactive) at odd harmonics too so you'd need a clever thing to measure it for sure.

Not sure where you are in the world, but making stuff for the EU (in China, most probably!) means it is probably cheaper to make ALL stuff the same way. So maybe worth considering look at some of the EU directives, or whatever accreditations are listed on it/them, to see what standards your wall warts might be designed to.

I believe there is one that says these have to be limited to less than 0.5W when not loaded, but can't find a specific detail that.

https://en.wikipedia.org/wiki/European_Ecodesign_Directive

anorlunda
I believe there is one that says these have to be limited to less than 0.5W when not loaded, but can't find a specific detail that.
The current (haha) no-load requirement is 0.1W. This only came into force in April 2020 but most suppliers (haha again) have been compliant for some time.

See page 2 of https://ec.europa.eu/energy/sites/e...019-2126_en_annexe_acte_autonome_part1_v3.pdf, linked from https://www.eceee.org/ecodesign/products/battery-chargers/.

Edit: This is only in the EU of course (and the UK where the provisions of such regulations are still effective); in the US you are probably allowed to burn coal to power an iPhone
Further edit: it seems I was wrong about the US!

Last edited:
The current (haha) no-load requirement is 0.1W. This only came into force in April 2020 but most suppliers (haha again) have been compliant for some time.

Philosophically, I dislike government regulation. But this case seems to be a big exception.

In 2001, the power consumption of wall warts plus the standby power of things like televisions was at 6% of electricity consumption in the USA. Forecasts at the time said that it would grow to 15%. But government regulations turned all that around. The designs were changed drastically. Neither the manufacturers nor the consumers suffered much pain. On the contrary, compared to other technology changes, standby power efficiency was almost trivial, but the effect on electricity consumption was major. I must concede this as a triumph of regulation.

jim mcnamara, Merlin3189, russ_watters and 1 other person
...
In 2001, the power consumption of wall warts plus the standby power of things like televisions was at 6% of electricity consumption in the USA...
Is that actually true, I mean really actually true and not an urban myth?

Is there some solid, certified published reference for this?

All the thousands of heavy industry factories sucking up juice for aluminium smelting and such, even at a scale of millions of little gadgets, I do struggle to believe that.

6% of domestic consumption? Hmmm .. I still struggle a little but that might not be quite unbelievable.

The other thing to consider is whether 'waste' power is actually wasted. I mean, sure, if it is adding to excess heat, but adding 'waste' heat to a house that uses electric powered heating isn't really adding to the electricity consumption, if you see what I mean.