So you're asking about efficiency, don't take this post as an intention to discourage against the motor generator, I'm merely comparing vs an inverter - there was a similar thread not more than a week ago where someone wanted to charge 12V house batteries with a 100Watt motor-generator, attached to a mains type battery charger, and that thread is very near in my mind:
I'm saying that in order to provide 400Watts of power, it needs to draw 660 Watts from the power source to do that - 280 Watts (39.4%) is lost as "heat" in the electrical-to-mechanical-to-electrical conversions steps. That waste heat that doesn't go out the 120VAC circuit, still comes from your DC power supply (battery/alternator), so it has a significant impact on run times. Also consider the heat generated - you're basically running a 300Watt light bulb where ever you put that Motor-Generator, so it needs good ventilation to keep from burning up or burning your enclosure down.
Conversely,
this 24V 600W pure sinewave inverter has a stated peak efficiency of 85%, so to output 600Watts of pure 120V AC, it draws only 706Watts from the power supply (again at peak load). To deliver 200Watts more power to the 120VAC circuit, it generates 180Watts LESS heat.
On the topic of efficiency,
Modern inverters have what is effectively a "Gear-shift" where a light load (think like a light in a refrigerator, or a clock on a microwave) doesn't need the full output of an inverter so it drops to a smaller inverter stage, once a big load kicks in (like the refer's compressor, or the microwave's magnetron) the bigger stage of the inverter kicks in. A motor generator can't slow down with a lower load, since that would directly change the frequency of the AC power, so it constantly has to overcome the friction and Back-EMF of both spinning the motor and the generator (same problem with a fixed RPM internal combustion spun generator).
Here's a calculation for you to consider:
Diesel fuel in florida currently: 2.21 - 5.99 gallon (
from this site)
1 gallon of diesel fuel = 128,488BTU/hr or 37.656kW (
from this site)
Fuel to mechanical rotation efficiency of ideal DIESEL engine is 35%, so generated mechanical power from 1 gallon of diesel is: 13.180kW
From here out we'll assume there is no friction, and no drive train parts that need to be spun (i.e. clutch, transmission input shaft, etc..) in the engine and all of the energy in the
Belt from crank to alternator ideal efficiency is 95%, so available alternator shaft power is: 12.521kW
Alternator efficiency is ideally about 85%, so DC power generated by the main engine with that gallon is: 10.643kW
Getting to this point is already "expensive" in lost efficiency, we went from about 38kW down to about 11kW just to get to where we can input to the motor-generator - that a total efficiency of about 29% to convert from diesel to 28VDC. We can extrapolate that the single gallon of diesel would get you about 16hours 40minutes of run time for your motor generator, or about 23hours, 20minutes of run time for the inverter. In the best case the Motor-Generator will cost you about 13 cents ($0.13) per hour to run, while the inverter would cost you 9.4 cents ($0.09) per hour to run - a difference of 4 cents per hour ($0.04). Obviously this is way over-simplified, unless there is storage of that extra 10.34kW of power the truck's alternator creates with the 1 gallon of diesel in an hour, the efficiency plummets to about 2% - we are also excluding the wear on the engine, filters, lube and those associated maintenance costs - if the Motor-Generator is a brushed DC motor, you have those parts and costs to account for as well (the inverter option would not likely have service/maintenance needs). We are also assuming that the devices are outputting 400Watts continuously, not intermittently as is more often the case.
Considering the cost to operate the inverter is only 70% what it costs to run the Motor-Generator, a
$200 Samlex pure sine-wave inverter that outputs 600Watts, would only need to be run for 5,000 hours before it is cheaper than the Motor-Generator. If we're talking running a light outside for 8 hours a night, that would take two years to pay back.
Just food for thought...
It's my opinion (and only an opinion) that if you are looking to power significant AC loads from a DC power supply, it's in your interest to use an inverter - they can be had cheap and are wicked simple to replace should a failure occur. A neat trick with an inverter is to hack your 120V refrigerator's thermostat switch into the On-Off switch of the inverter and hard-wiring the compressor into the inverter output. This makes the control of the inverter completely passive (no power required to control, 100% transparent to to the fridge user getting a beer), and the inverter will only run when the compressor needs to run (huge gain in efficiency since you're not running the inverter to be ready for the fridge need).
A Motor-Generator has its place with low energy HEMP/TEMPEST sensitive 120VAC equipment (most of us don't have that problem...
).
