Inverter Inefficiency - Myth vs. Reality?

[Maine Sail]

Hi All,

It is stated by many companies, and correct using their lab testing methods, that inverters are about 10% inefficient when converting from 12 volts DC to 120 volts AC. I was curious, and had a sinking feeling, that 10% was a very optimistic number when using the general every day items one might choose to use on a boat..

This questions was posed a few days ago in another post so I did a little photo experiment to help show off SBO's new multiple photo features and just how inefficient inverters really are in the "real world"..

Showing off the new "quote" capabilities here:

Just wondering which one waste more 12 volts using a 12 tv with a converter or
a 120 TV plugged into a 2500 watt inverter built into the boat.
Nick

To accomplish this test I used our 19" Polaroid boat TV that runs on either 12 volts or 120 volts. What I really wanted to achieve was less "lab" or "theoretical" numbers and more "real world" on a device that can operate on both DC and AC and one that would give a steady output from which to measure. I could have used a laptop computer but any one who's used one knows that the current draw is highly variable and NOT at all steady. My Macbook Pro uses between 3.1 amps (with the lid closed) to 7+ amps depending on the software and program it's running. So a laptop was not used because I could not get a steady current draw from it.


To make sure my TV was consuming a fixed amperage I loaded a DVD into it and then paused it at exactly the same spot in the "Elmo" disc of my daughters. All other devices and charging sources were turned off including my solar panel. The Xantrex XBM battery monitor read 0.0 amps before turning anything on.


The on screen shot is showing the TV paused and running off of 12 volts DC:

Here is the Amp Load at 12 volts DC:

This photo shows the DVD paused in almost the exact same spot though this time it's running on 120 volts AC through my larger 1200 watt inverter.

And the amp draw! There is no trickery here and the Xantrex battery meter is not lying. Running this same exact TV on 120 volts AC through a 1200 watt inverter uses 5.8 amps per hour vs. 3.9 amps per hour!!

When compared to running this SAME EXACT TV on 12 volts DC the inverter has a 32.8% efficiency loss or a far cry from the 10% inverter inefficiency claimed by most manufacturers using very non-real word lab numbers..??

Yes, if you're wondering, I do have a lot of junk on the boat! Big deal... It does make for some interesting experiments though.

For this second test I used my smaller 400 watt inverter. This is the one that I power my laptop and camera chargers with. My reason for doing this test is to show that an inverter sized closer to the load can sometimes be slightly more efficient and that perhaps different brands of inverters can offer slight advantages in efficiency.

Once again the screen shot:

And the result!

So maybe it's not that much more efficient but 6 tenths of an amp hour is nothing to snub your nose at. The 400 watt inverter ran the TV using only 5.2 amps per hour and was a mere 25% inefficient. Again, this is a far cry from the 10% claimed by many manufacturers.

This certainly was NOT a very scientific experiment but rather a REAL WORLD experiment designed to show what one item, a TV designed to run on both 120 volts AC and 12 volts DC, will do run both ways.

As far as I know I have never seen an item designed to run on both AC and DC tested in the real world on a boat with a real system and not in some theoretical BS mathematical equation that shows a misleading 10% inefficiency!

It is clear to me that if you have the option to buy a device that will run on 12 volts DO IT!!

Inverting power from 12 volts DC to 120 volts AC is a terribly inefficient way to power devices off your boats house bank of batteries! I'm sure inverters vary in efficiency as my two inverters do but I'd be very surprised to see an inverter meets the claims of 10%, in the real world, using the same device on both 120 volts AC and 12 volts DC..


Sorry for the rant!!

 

[BenH]

Very interesting...

To see the real world results. Thanks for taking the time to run the experiment and post the results. I've always been curious about the amp difference a device would draw that could run on 12v or 120v.

 

[GuyT]

Basic understanding of inverters

Maine Sail although it is true that inverters are 90 percent effecient it does not mean that they are always 90 percent efficient. Its like saying that a car gets 30 MPG and you find that you get 20 MPG under your driving conditions because you idle your car for a half hour before you start to drive.
All inverters have a quiescent power. This power is just to keep the stuff in the inverter powered up.
The smaller the inverter, the less the quiescent power - that is why your other inverter faired better in your test.

If I use a 18 wheeler to get to work, does that mean that the rig is inefficient - ya it is the way I am using it. Now if I pull 40,000 pounds while I'm going to work it kind of makes it look better now doesnt it.

My point is that if you run an inverter at a fraction of what it is capable of, your effeciency is not going to be good.
If you want to run a TV that draws little power, get a little inverter.

Your point is valid though - anytime you have a conversion, you have a power loss. Best way to eliminate power loss is to eliminate the conversions.

 

[Charisma]

Thanks . . .

Mainsail, you are truly amazing! This was so timely, i have always wondered how much difference it really is. We were having this same discussion last week while at anchor watching the Broncos. Thanks for your many wise and wonderful info on sailing. Keep it up

 

[Maine Sail]

Thanks Guy!

Maine Sail although it is true that inverters are 90 percent effecient it does not mean that they are always 90 percent efficient.

I know, this is exactly why I posted this! The manufacturers will NOT tell you this but your battery meter will if you get creative.. Sure, if you were to run an electric heating element through an inverter it may come in at 90% but most other items boaters will actually be using in the "real world" will not be at a 10% efficiency loss.

All inverters have a quiescent power. This power is just to keep the stuff in the inverter powered up.
The smaller the inverter, the less the quiescent power - that is why your other inverter faired better in your test.

True my 1200 watt inverter uses .2 amps in standby and my 400 watt uses .1 amp in stand by mode. The numbers still don't quite match though. If I take away the standby loss of the 400 W of .1 amps I get a 5.1 amp draw. If I take away the stand by loss of the 1200 watt inverter of .2 amps I get 5.6 amps. That is still a .5 amp per hour difference. I guess we can chalk that up to different brands...??

If I use a 18 wheeler to get to work, does that mean that the rig is inefficient - ya it is the way I am using it. Now if I pull 40,000 pounds while I'm going to work it kind of makes it look better now doesnt it.

This is exactly why I also used my 400 watt inverter as it is sized much closer to the actual load of the TV and in comparison to the efficiency model of your 18 wheeler towing 40,000 pounds. It still gives a 25% inefficiency loss when compared to the same exact device running off of 12 volts...

My point is that if you run an inverter at a fraction of what it is capable of, your effeciency is not going to be good.
If you want to run a TV that draws little power, get a little inverter.

I guess this would be the difference between the 1200 watt inverter at 5.8 amps and the 400 watt at 5.2 amps or about a 7.8% difference in efficiency.

The problem here is that many, many boaters are bamboozled into buying HUGE 2500+ watt inverters then only charging cell phones with them or running a small TV... This is exactly why I have two inverters on board one for small items and one for larger items like power tools or my vacuum...

Your point is valid though - anytime you have a conversion, you have a power loss. Best way to eliminate power loss is to eliminate the conversions.

Amen! It is now possible, in many devices, to find 12v items that work equally as well as their 120 volt counterparts. If anyone finds a 12 volt vacuum worth a darn please let me know..



More use of the new quote system!! Cool stuff!!!:dance::dance:

 

[RanAweigh]

Good thinking, the plan was fine but the test may be flawed.

I think that the television you used as the device under test is designed as a 12 volt DC device. That is to say it really runs on 12~14 volts. In order to operate it on line voltage 110~120 VAC there is an internal power supply to reduce the input voltage to the operating voltage and then rectify it. Thus converting the line's AC to 12 volts DC. This conversion costs a good deal power as you are taking 12 volt DC power converting it to pulsed DC then stepping up to 110 volts (in the inverter) then stepping it back down to 12 volts and rectifying it back to DC (in the TV power supply). I am surprised that that all of this conversion cost less than 23 watts and that your TV only consumes 45.6 watts that's pretty darned good.

Inverters, gen-sets, alternators and the like are great to have aboard in order that we may operate devices that will only run effectively from AC lines. However if the device is really designed to be operated at 12~14 volts DC then that's going to be the more energy efficient way to use it.

Not as much energy is lost in the inverter as your test would lead you to believe. Another contributor suggests that the bigger the inverter the more the loss. This is somewhat true in that the larger the current handling capacity the larger the capacity of the internal components must be. It will consume more to operate these components.

Think of the inverter as a necessary evil to allow you to use the microwave and a hair dryer. Devices that are designed at 12 volts DC do not need the intervention of the inverter.

Sorry to babble on

Ran

 

[Maine Sail]

Exactly

Good thinking, the plan was fine but the test may be flawed.

Remember this was a test of what sailors might actually come across in the real world not a scientific test. I never disputed that inverters won't run at the claimed 10% efficiency loss only that the devices most sailors will buy and use on a inverter will not usually see this 10% efficiency loss and it will likely be more.

I think that the television you used as the device under test is designed as a 12 volt DC device.

Surprisingly most small LCD TV's actually run on anywhere between 10 and 36 volts DC. They then use a "wall wort" or an internal voltage converter to step down from 120 volts AC to 10, 12, 18, 24, 36 etc volts DC. You can't avoid this in the real world on many devices including laptops, TV's stereos and the like!

That is to say it really runs on 12~14 volts. In order to operate it on line voltage 110~120 VAC there is an internal power supply to reduce the input voltage to the operating voltage and then rectify it. Thus converting the line's AC to 12 volts DC. This conversion costs a good deal power as you are taking 12 volt DC power converting it to pulsed DC then stepping up to 110 volts (in the inverter) then stepping it back down to 12 volts and rectifying it back to DC (in the TV power supply).

Exactly! this is why you should search for and buy a TV or other device that can be run on 12v DC directly!

Inverters, gen-sets, alternators and the like are great to have aboard in order that we may operate devices that will only run effectively from AC lines. However if the device is really designed to be operated at 12~14 volts DC then that's going to be the more energy efficient way to use it.

yep that's why I have two inverters on board they are great when you need them. Like you my point is to try and buy items designed for 12 volts DC first and when you can't find them then use your inverter..

Not as much energy is lost in the inverter as your test would lead you to believe.

Again this was a "real world" situation not a theoritical one that MANY, MANY sailors encounter and often use in the real world on boar their boats. The inefficiency is what it is on a TV, or laptop for example, which many people use on board running through an inverter..

Another contributor suggests that the bigger the inverter the more the loss. This is somewhat true in that the larger the current handling capacity the larger the capacity of the internal components must be. It will consume more to operate these components.

Again we fully agree. Size your inverter to the loads and use a smaller one for smaller loads. You can buy a 200-400 watt inverter these days for about $30.00 to $60.00..

Think of the inverter as a necessary evil to allow you to use the microwave and a hair dryer. Devices that are designed at 12 volts DC do not need the intervention of the inverter.

Yep!!

 

[TimCup]

Maine Sail, I get your point...

and I appreciate it's "real life" value. I'd always "heard" that it was less efficient using inverters, and converters, etc., but had no idea what the COST of the inversion was. 10% doesn't sound too bad...

25% sounds terrible!

So I get that while inverters CAN be 10 % innefficient, I'm probably overspending on my power needs by 25 to 30%. Makes a big difference.

I still can't understand how you find the time to do these things, but I appreciate it... keep it up!

cup

 

[Ross]

Re: Maine Sail, I get your point...

There is a good reason for those cooling fins on inverters. I am quite sure that if you held your hand on them when they are pulling a load, they would be warm.

 

[Salish Sea Cruiser]

Nice writeup Mainsail. Here are a few observations from my own testing and research. I have been searching desparately for a 20" DC powered LCD computer monitor at non marine price. They used to all be made to power from a brick or wall wort, but now it seems they are all powered directly from AC.

Most higher end inverters will list in the specs a no load (not the same as the search mode) draw. It will be substantially higher for a pure sine inverter than a modified sign one. My 2000w pure sine inverter lists a no load draw of 2 amps and that is what my testing shows. This makes it very inefficient to use for low power devices such as a PC monitor.

If I have a device that is powered from DC at other than 12v, I always try to get a dc/dc converter for it. They seem to be very efficient. I even use them for 12v devices that don't specifically list the acceptable voltage range. I am always afraid of blowing it out when the charger hits 14.4 volts.

For my PC monitor, I think I am going to have to break down and use a 120v powered one and use a very small dedicated inverter for it.

 

[Maine Sail]

Consider..

I have been searching desparately for a 20" DC powered LCD computer monitor at non marine price. They used to all be made to power from a brick or wall wort, but now it seems they are all powered directly from AC.

Due to consumer complaints, mostly around wall mounting, many manufacturers have moved the "brick" or "wall wort" inside the monitor. They may still run on 10-36 volts DC but you'd need schematics to figure that out. It is possible to bypass the step down device internally but I'm guessing that more than you're willing to do.

Have you considered a TV that can also be a monitor??

 

[Salish Sea Cruiser]

Re: Consider..

I actually took my monitor apart hoping that the power supply would be isolated and I could easily bypass it. Unfortunately the AC wires went directly to the main board and I wasn't about to try messing with it.

A TV woud work, but I havn't been able to find one that met my requirements: minimum resolution of 1680X1050, no larger than 21" and DC powerable.

I will undoubtedly have to relax some of my requirements. The new all-in-one TV/PCs from Sony look really interesting and are powered by a 19.5v brick which I already have in a 12v version. But the dimensions are a little big and I much prefer a more modular PC and I already have a mini-itx one.

 

[dvideohd]

Outstanding Post!!!

I am working on some similar electronic products and you hit the NAIL ON THE HEAD with this one...

It seems that ratings are posted for only the MOST OPTIMAL condition - not the average - normal - or even occasional use condition...

It seems that a lot of inverters are made for the occasional car trip - and might not even last 10 hours in normal use....!!!!

It is NOT POSSIBLE to tell from any rating. UL listings have even been faked on a lot of these products!!!!

A good review - and posting - is a GREAT thing to do!!!!

 

[Paul Brinkley]

Re: Outstanding Post!!!

Main Sail- thanks for your work and a good thread. The obvious lessons appear to be that (1) we should use inverters as little as possible, only for what we can't get in 12-v, and (2) we definitely don't want to use an inverter to get 120v and then reconvert to 12-v.
This past summer, my wife and I were going camping for a week, and taking the boat. We wanted to be able to keep our cell phones charged and we looked for car chargers for our phones. Could not find a car charger for either phone (different ones required for each phone) at WalMart or anywhere else cheap, figured we would have to go spend much more than $10 each at the phone store. However, I saw a very small 100-watt inverter at W-M for $18. That let us use our regular house 120-v chargers in the truck, and if we had to, on the boat, and cost less than buying two car chargers. Since we don't use anything else 120v on the boat, the little inverter meets our needs well, and I won't have to buy another one whenever I get a new phone. (Isn't it interesting that every time you get a new phone, none of the old accessories fit so that you have to buy all new ones?) Keep up the good work. -Paul