For this post I wanted to test the inefficiency of inverters when powering an LCD TV. It is stated by many companies that inverters are about 10% inefficient when converting from 12 volts DC to 120 volts AC.
This can be true when running them at max output and with inductive loads but is far from true when powering items such as computers & LCD TV's. These efficiency losses are obviously not all from the inverter but stem from operating devices that normally take 120 volts and then convert it to DC voltage inside the unit, or in the AC cord with an in-line converter. If your device already runs internally on DC then on a boat you're converting from DC to AC then back to DC again. By doing this you are only adding to whatever inefficiencies your inverter already has.
To accomplish this test I used our 19" Polaroid TV that runs on either 12 volts or 120 volts. What I really wanted 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. Please be aware that not all devices that internally run on 12V can run on variable 12V meaning they may do okay on 12.00V but may toast themselves when fed 14.6V. Please be sure your 12V internal voltage device can be run on a wide voltage range at least 10.5V to 15.5V, before using it on a boats DC system fed directly off the 12V system.
To make sure this 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.
The on screen shot is showing the TV paused and in this case it is running off of 12 volts DC. A little known fact is that many items you would choose to use on a boat, such as an LCD TV or a laptop computer, already run on DC internally. By using an inverter and the "wall wort" or AC/DC converter box, that usually resides in the middle of the power cord, you are getting horrible inefficiencies because you are converting from DC to AC then back to DC again. Many devices will already run on 12 DC without an inverter and the AC/DC wall wort. You can simply read the output specs of the "wall wort", or AC/DC converter, to determine if it is a 12v output.
24-SEP-2008
Amp Consumption At 12 Volts DC
This photo is of the TV on pause while plugged into the boats 12 volts system.
The TV when running the DVD, paused, on 12 volts, consumes 3.9 amps or 3.9 ampere hours per hour of use.
It should be noted that I left it paused for over 5 minutes and the voltage & current never fluctuated. I made sure to hold voltage steady in all tests so as to get as accurate a data shot as I could.
24-SEP-2008
TV Plugged Into A 2000 Watt Inverter
This photo shows the DVD paused in almost the exact same spot though this time it's running on 120 volts AC through a 2000 watt Freedom inverter. Take a look at the next photo to see how inefficient it is compared to 12 volts DC.
24-SEP-2008
TV Running On A 2000 Watt Inverter
There is no trickery here and the Xantrex battery monitor is not lying. Running this same exact TV on 120 volts AC through a 2000 watt Freedom marine inverter uses 5.8 amps or 5.8 ampere hours for every hour of run time.
When compared to running this SAME EXACT TV on 12 volts DC the inverter / wall-wort combo is 32.8% less efficient or a far cry from what many would assume is a 10% inverter inefficiency.
Keep in mind the efficiency loss is not just inverter. It comes from the conversions from DC to AC in the inverter and then AC back to DC through the TV's "wall wart". This inverter also has a rather high quiescent draw when just sitting there doing absolutely nothing waiting for an AC load.. The quiescent draw of the inverter is often left out of conversion efficiency claims yet it also serves to impact the total efficiency...
24-SEP-2008
TV Running On A 400 Watt Inverter
For this next test I used a smaller 400 watt inverter.. My reason for doing this test is to show that an inverter sized closer to the load can sometimes be slightly more efficient. Of course brand and build quality have a role here too.
24-SEP-2008
TV Running On A 400 Watt Inverter
So maybe it's not that much more efficient but 6 tenths of an amp is nothing to snub your nose at. The 400 watt inverter ran the TV using only 5.2 amps or 5.2 ampere hours for every 60 minutes of run time. It was a mere 25% inefficient when compared to direct DC. Again this is a far cry from the 10% claimed by most manufacturers, and often incorrectly believed by boaters.
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.
It is quite clear that if you have the option to buy a device that will run on 12 volts DC vs. 120 volts AC, you should buy it. Inverting power from 12 volts DC to 120 volts AC, then back to DC again is a terribly inefficient way to power devices off your boats house bank of batteries..
MAY-2006
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