Email of the day: Generator VS Alternator 1-23-13

Email received:


I have acquired a Lister/Petter 12hp one lunger. My mechanic assures me that it sips fuel at 800RPM(2/3pint to 1 pint/hour) and will produce 4KW-5KW. So I am on my way so to speak. 

My question is this, which of the two is more efficient….alternator vs generator? I know most alternators work at about 60% efficiency. You recommend the generator.

Thanks Darden

George’s Reply:

“You recommend the Generator”, I should have asked you what you mean by this, but in my rambling style I charge on! Don Quixote De La Mancha! I favor the AC dynamo in many cases, as I believe it gives you backup to the inverter IF it goes bad.. Others say that’s not likely, and they rely on one source of AC. But this generator VS Alternator thing, what is the difference, and does the world have the same definition? One definition is where the armature is fixed or rotating, others suggest that if you have a commutator, it’s a generator. I correctly or incorrectly think that all rotating machines are AC machines first. And.. with the note of how languages change, the term alternator and generator are often used to reference the same machine.

I am most annoyed when people use the term ‘motor’ in reference to an engine, but I expect and accept the fact that the Alternator and Generator are often considered appropriate terms for the same machine.          

If you are making reference to DC and AC approaches, there are two basic models in people’s minds. One uses a Dynamo that is designed for lower voltage output, typically 12 or 24 volts DC.  Some think it is best that this be their only dynamo, and that any AC they make WILL come from their inverter. It seems they sometimes think that since the dynamo was designed for charging batteries, it will do the better job. I suggest that measuring your (fuel/KWH) numbers is the best possible way to compare.

Generator VS Alternator is an old debate, but there’s also a lot of confusion as to what one is, VS the other. Maybe we get that straight first?

I like to think of all Dynamo machines (rotating machines) as being AC machines first, and at that point some might say they’re ‘alternators’ first… every single one of them! Rotating machines don’t make DC, we need adjuncts that follow as a second stage that dissect our wave form, and rearrange it to look more like DC..

In the case of a so-called DC generator , we follow this AC producing section with a commutator or rectifier to turn this AC wave form into a rough form of DC. We look at it with a scope, and we can generally tell how smooth it is, and we can generally guess how it got that way.  The half wave rectified form might be easiest to spot, but there are many different machines that have been made for more than 100 years, so maybe we focus on the more typical choices of today?

Before we do, we should note that anytime you rectify, there are losses.. diodes create these losses, and some are lower loss, so we consider them WHEN we burn fuel to make a KWH. A typical automotive dynamo of today is a three phase AC machine followed by six diodes that rectify the AC to a form ‘closer’ to what a battery delivers, that be DC with a good amount of ripple.    

Car alternators (generators) are a huge compromise, as they need to be! They need survive a wide range of RPMs. The cost of manufacture is a high priority, size matters, weight matters, and these factors seem to grow in importance. Automotive Engineers struggle with compromise everyday.

The efficiency of a car alternator (generator) can not be given a high priority, as there are other critical requirements. One of the factors is the Automotive Dynamo’s rotor, it must stand up to a whipping, as RPMs change violently. Hidden inside most Auto dynamos are necessary elements to assure survival of the Alternator. The engineer can’t control what you decide to add to the load, maybe a trailer, or extra head lights.  Add high under the hood temps, and the Dynamo might be forced into survival mode! Sometimes the Designer(s) use a thermistor output to lower the duty cycle, when temps reach a threshold.  Other designs just kill the output all together till it cools down.

People with their Jiggawatt sound systems have called triple A to be towed in. The sound system sucked the batteries dry, and the Dynamo literally refused to make power, later the owner noted the Alternator was just fine, and returned to work once the load was reduced, and the alternator had time to cool..

We can go on, but we need focus on your goal. If it is to make a KWH on as little fuel as reasonable, why would you pick a Dynamo that gives efficiency such a low priority?

You are already committed to loading your engine at about 80% or more to get the work done at the more reasonable cost in fuel. Running at half load or less will produce a far more expensive KWH regardless of the efficiency of the alternator.

With this said, it becomes important to consider your runs and the work you might do.. Charging batteries might be one load, pumping water, running the lights and other loads in the house at the same time might be the better idea.. Consider managing the work you need done, and load the engine at the better ration of  (fuel/work done). 

In my opinion, a 12hp engine driving a typical car type dynamo is going to be an expensive way to make a KWH,

We need consider a decent quality AC Alternator around 5-6 KW continuous output rating, and know the advertised efficiency. Too big an alternator generally creates additional overhead per KWH of load, and we need consider giving the machine good air flow where it can run in a narrower band of temperatures, this helps with longevity, reliability, etc.

At the end of the day, the AC machine gives us back up for the inverter we may have, and using the AC alternator to power the charging section built into your Inverter >MIGHT< be the more efficient way to charge batteries over an Automotive dynamo.         

At the end of the day, it’s all so easy for the off gridder to know which is more efficient, as he learns to know the time and even the sound the power plant makes while charging.        

It’s all about the fuel/KWH ratio for an off gridder, why would he choose a compact alternator than had cooling troubles designed in? One that was build with less efficiency because the production cost had to be very low?   

If you build a gen set for backup, maybe efficiency is as low on your list of importance as efficiency is to an automotive engineer as per the electrical system itself?  He loses his head if the alternator burns up, and it must fit in a tight place..  outright efficiency… well the lack of is not likely to get him transferred to marketing or some other form of hell…

An interesting note on the PMG, we gave the ability of this alternator to make 127 volts at no load a high priority, and at 3KW load, it would be above 120vac..  To compare.. There are durable and well thought of Honda Generators that make 110 volts at 60HZ, and we can see droop from the. I have a customer who is far off grid, and in the clouds much of the winter, he is one of those off gridders who had a lot of experience charging batteries, and instantly noticed a great reduction in the power plants run time to charge his batteries via the huge built in charger section in his popular Inverter. The difference was the voltage, and  the fact the AC powered Charger could produce far more charging power into the large battery he has. If you consider an alternator with adjustable field power, you might be able to charge your batteries far more efficiently, I think this customer cut his fuel bill 20% by choosing the alternator with a higher output voltage. We best not attribute efficiency to the PMG, as we mainly save only field power with the design, we should see near identical saving in such a situation having the higher voltage,      

You can think about it all you want, but measuring it is best.. one advantage wiht the big flywheel engines, it’s easy to have both AC and DC dynamos hand on the same machine.. IF you have an efficient charger powered by AC, and a DC Dynamo, you can compare which charging system puts your battery back to full charge more efficiently..   

Following are things people might mention:

There are a few big truck alternators that are claimed to be efficient, and might make a good chunk of the power you need,  

Lowering the RPM of your engine to 750 RPMs may be possible, you need see how a softer spring might work with the Governor, and if it will do a decent job of holding RPM. This could make  your (fuel/KWH)  ratio more favorable WHEN you are making small power.




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8 Responses to Email of the day: Generator VS Alternator 1-23-13

  1. Russell Furzer says:

    That is awesome fuel consumption – 1 pint per hour for 4kw is (reaching for the back of an envelope and stub of pencil) 0.576 litres at 830g per litre= 478 grams per hour for 4kw is 119 g per kwh.

    Doesnt pass the sniff test

  2. George B. says:

    It was a statement, not a question:-) I haven’t visited fuel/kwh figures for a bit, but I think our community here in North America thought .125 gallons per kwh of electrical power out of the typical AC generator head with a 6/1 with a decent load was in the ball park. Remember our USA Gallon is what I reference, the Canadians use the Imperial Gallon.. course you likely know that 🙂

    • Russell Furzer says:


      .125 US gal = 0.475L = 390g per kwh.

      JKSON claim 268g per kwh at the flywheel on the website, so add 15% marketing bulldust correction factor = 308 then correct for the 80% gen efficiency and you get 385 g per kwh – right on your rule of thumb.

      Darden is talkin 0.67 to 1 PINT per hour for 4 0r 5 kW. ie 119 g per kwh- about a third of the real world.

      BTW the energy density of the fuel is about 30 MJ/kg ie 8.3 kwh/kg or 120g per kwh. That lister/petter would have to be 100% efficient! must have some magnets or sumpfink!

      BTW I didnt know that the Canadians used the imp gallon. That must cause some confusion 🙂


  3. George B. says:

    Good Post Russ,

    I think you’re right on the mark 🙂

    Cleaning up a little waste motor and running a bit of it with fuel oil can help make a cheaper KWH.

  4. george mraz says:

    I have 12 volt nos generator from the 50 s 55 amp that I can power it with small diesel my battery bank is a 48 volt forklift altered to 12 volt what is the pros and cons good or not I also a 6 solar panels and 2 diesel generators 6 and 4 kw we are off grid by 4 miles thanks.

    • George B. says:

      George, I’m not sure I know your question, but I’ll attempt an answer. there are advantages to high voltage batteries, and inverters, staying away from cells in parallel is generally considered a good idea. To make a 48 volt fork lift battery into a 12 volt source, one can center tap, or even re config the battery in to 4 sets of 12 volt stings parallel. Parallel strings nearly always against each other to some degree, this isn’t an issue in a battery when all cells are in series. High voltages help tame wire losses, and can reduce heat in inverters and controllers. But there will always be people who will go 12 volt no matter what the penalty might be.

  5. JUSTON says:

    What a great thread! I have a off grid setup and currently use a yanmar 7.5 Hp to run a PMA that creates 120 amps at 1200 RPM at 12 volts. Use this to charge my 700 amp battery bank. My question is, is it better to run a higher RPM with more amps into the battery or lower RPM with less amps and longer run time. I’ve played with this a little. At higher RPMs my rectifier will heat up. It’s external.

    • George B. says:

      It’s always best to state your goals, and then to prioritize them. Many of us put battery life near the top, they’re often expensive, and we want to maximize the return on investment. with that said, the battery manufacture will provide a maximum charge rate in Amps. During your initial charge of a lead acid battery (when cool), you can pour the amps to it, in order to do this, your dynamo could spin at more RPM with a higher voltage in order to produce a higher current flow into the batteries. One of the cool things about using a dynamo to charge is you can often set the RPM at the current (amps) the battery manufacturer recommends, and then as the battery is charged, it’s increasing voltage allows the charge to taper off and a full charge to be achieved. There’s lot of info on the WEB about the proper methods to charge batteries..

      Rectifiers heating up? Yes.. they do that, and that’s why most people assure a proper heat sink for their rectifiers. You might research diodes, some are more efficient than others, in the old days, we had to tolerate Selenium rectifiers, they were more problematic, and had a very memorable smell when they burned up. BTW, a great place to mount a rectifier is in the path of cooling air, most dynamos have a fan, make use of the air flow, and do mount your prime mover and dynamo in a place where the air can slip through, even a slight breeze can help carry the heat off a machine. One more note, running a dynamo hot can eventually break down the varnish insulation on windings, voltage regulators can push current high in a field winding, PMAs can run hot in the stator windings too. One you exceed the temp the varnish can handle, the varnish is on it’s way to failure, this could happen in the same day, or over several years.

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