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..

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..??






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 test I used my smaller 400 watt inverter. This is the one that I power my laptop with 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.
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!!

[GaryHLucas]

The big reason you can't get the maximum rated efficiency from an inverter is that AC power also involves power factor, a measure of whether the current and voltage are synchronous. As the power factor falls, so does the maximum efficiency. A resistor has a power factor of 1. Drive a heating element off the inverter and you'll probably see a conversion efficiency much closer to the 10% value. If I recall correctly, a CRT type television has a rather poor power factor rating.

[Maine Sail]

Originally Posted by GaryHLucas

The big reason you can't get the maximum rated efficiency from an inverter is that AC power also involves power factor, a measure of whether the current and voltage are synchronous. As the power factor falls, so does the maximum efficiency. A resistor has a power factor of 1. Drive a heating element off the inverter and you'll probably see a conversion efficiency much closer to the 10% value. If I recall correctly, a CRT type television has a rather poor power factor rating.

Wow this is exactly why I posted this!!

This is the same mumbo-jumbo only a select few of us actually understand. For the rest of the boaters out there who mistakenly think their inverter is only 10% inefficient they are way, way off for the types of devices normally run off of 120 volts AC on a boat..!

Why is it that inverter makers can post long lists of devices the inverters will run, and the average amperage consumed by such devices, yet they totally OMIT what type of efficiency can be expected for say a toaster vs. a LCD TV ???

[badsanta]

I know I am not close as I dont have the book with me. But I remember some one telling me that it takes about 10 amps DC to make 1 amp AC or it will take 10 times the DC current to do AC current. just a thougt

[bobmcgov]

Power factor of the load is one issue, wire runs and cable sizing is another. Maybe not so important on a sailboat, but my shop is a loooong way from my batteries. So's my kitchen. Unless I wanted to run 6 gauge wire to the latter and double-aught to the shop, DC line losses would be stupendous, easily outweighing any inverter losses.

It's like batteries: lead acid batts can be up to 95% efficient; but if you cycle them hard and fast, don't expect to see that kind of performance.

[Hartley18]

It's a good experiment - you've done well.

Don't bag the inverter (the fact that both inverters show a similar current draw should indicate that you're missing something..) your problem is with the power supply on the TV.

Assuming your TV is LCD (it looks like it is), the power supply in the thing has to convert your 120 volts back down to somewhere near TTL level (5 VDC) to drive the screen. This will use (waste) a lot more energy than going from 12 volts, due to larger inductors and power transistor heating.

A good way to check this is to feel the temperature of the TV power supply with the back of your hand after it's been on a while. On 120 volts it will be hotter for sure.

Yes, 120VAC is a very inefficient way to operate the electrics in a small boat - that's because 120 volt electrics are designed for use in the home (theroetically unlimited power supply) and not running on batteries in a boat.

Cameron

[artbyjody]

Hal - I think, that they measure that efficiency from amps pulled off the AC on AC alone versus AC converted from DC. I am willing to bet that the AC amps on shorepower only are within 10% of the AC converted from from DC. b That is the one item missing from your tests as most invertor tests do not do a DC only versus AC from DC.

[PilotAlso]

I'm far from an electronics person but the my first thought was that the inefficiency may be more in line with the

12V (battery)-inverter-120V-TV plug/transformer-12V

Am I correct to think that the TV plug/transformer adds significant loss to the equation as compared to powering direct from 12V?

Would using a 120V TV without a brick transformer change the equation?

Nice discussion!

[sailingdog]

Nicely done Halekai.

Can you do the same test using various other loads, like incandescent light bulbs, to see if the type of load actually affects the efficiency.

Also, is the DVD player built into the TV??? if not, how was it powered??

[raisin56]

Hal
Cameron had it right, what you were comparing was 12vdc TV to a 120vac TV.
All TV's (not just LCD) run on DC current internally. It is a lot more efficient to regulate a battery to 12vdc & 5vdc than to convert 120vac to DC then do the regulation.
The inverter regulation efficiency is the DC power drawn from the battery (inverter power drawn) compared to the AC power drawn (device power drawn). Power is the factor of multiplying the voltage by the amperage (P=IE). Of course there is the power factor thing when dealing with AC, but that's not going to count for 15% - 20% difference.
It will always take more power from the battery since the inverter needs to run too.

Good try, keep questioning your probably right that they aren't achieving a 10% anyway.

Fair Winds
Dan