# Power Inverter vs. Generator



## vickers (Jan 16, 2013)

Wanted to get everyone's thoughts.... For small use applications (2000 watts and below) would it be better to use a small generator or a power inverver directly wired to an automotive battery. Knowing that when SHTF, gasoline will be sparse, im more talking about an extended power outage due to a weather related event.

From my research, it appears that running a car uses less gas than running a generator. However, my primary concern is at what point can the alternator no longer keep up with the demands of the inverter? Is it a simple "at 1000 watts, you begin to pull from the battery and the alternator can only keep up with an 800 watt constant pull". Is this car dependent, or are all alternators basically the same? 

Thank you.


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## ZoomZoom (Dec 18, 2009)

I would definitely go the route of the generator.

For a 2KW genset, I don't see how it uses more fuel then a car. A small generator will run for probably 8+ hours on a gallon of gas.


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## cnsper (Sep 20, 2012)

Not if there is a heavy load. I have a 3500 watt generator and I can run nearly 24 hours on 3 gallons but if I add more than just the satellite and laptop then that time shortens quickly.


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## vickers (Jan 16, 2013)

Interesting.. I have a 4500 watt Generator that burns through 5 gallons in about 6 hours under heavy load.


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## ZoomZoom (Dec 18, 2009)

OK, since you disclosed you already have a generator, here's a thought and something I do.

During the day, run your existing generator as needed to get the fridge, freezer... back to where it should be. At the same time, connect a battery charger to the generator and charge a stand-alone battery. The battery should be a deep-cycle marine battery since it's made for this kind of application (discharge, re-charge, repeat process over and over & it has a huge reserve). Put the battery in a marine box and strap an inverter to the top.

You can then use that battery/inverter setup when you don't need a lot of power or need really portable power. I use it for immediate power when there's an outage to keep the fans running on the woodstove (so it doesn't overheat). I then use it at night for lights and such.

Picture "borrowed" from the internet but here's what my systems look like.


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## cnsper (Sep 20, 2012)

vickersja said:


> Interesting.. I have a 4500 watt Generator that burns through 5 gallons in about 6 hours under heavy load.


That is a heavy load. At about 1/2 load I will burn through 3 gallons in 8 hours.


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## Caribou (Aug 18, 2012)

I have a Honda 2Kw generator, a 1500W and a couple 400W inverters. I still need to get the twin to my generator and a battery bank. Honda makes a cable that will connect two, of this model, generators together. With both the generators and the inverters you are most efficient when running at near capacity. When I am finished I will have redundancy and flexibility. If all I want is a my boiler and a couple of lights the 400W inverter will suffice. If the batteries are up and I need to cool down the fridge I'll switch to the larger inverter. When it is time to charge the batteries I'll fire up the generator. If I have a heavy load I'll use both generators. With two generators that can be tied together I not only have flexibility but redundancy. Mechanical things break. When one generator quits I can fall back on the second. When the second one breaks hopefully I can salvage parts from one to repair the other.

If I had a 4Kw genset I would have as much power but I would not have the redundancy and it would be less efficient when I was running with a low load. Also this particular generator sips fuel and is quieter than my wife. It is not A vs. B, it is designing a system that suits your needs while remembering that your needs are not static. Right now I have three from column A and one from column B, at some point I'll add something from column C.


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## ZoomZoom (Dec 18, 2009)

Hi, I'm Zoomzoom and I'm a generatorohaulic.

[Crowd - "_Hi Zoomzoom_"]

I'm big on right-sizing and redundancy as well. I have generators ranging from 1.5KW to 200KW (yes, the latter is a monster turbine driven genset that will run around 20 houses but burns 44 gallons-per-hour under full load so it doesn't get used much...).

Anyway, I digress.

If you go the stand-alone battery/inverter route, also consider a small solar panel for charging if you don't want to rely on a generator to recharge it.


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## cowboyhermit (Nov 10, 2012)

Charging a battery bank and or using an inverter with a car is not efficient, at all, however it is a great source of occasional or emergency power. As for how long until you are drawing on the batteries a 100amp alternator, as an example is 1200 watts but that is at peak (engine revving) and there is inefficiencies in every inverter.
I use inverters all the time around the farm even if I have a generator on the service truck it is easier and quieter to plug into the inverter if it can handle the load.
The best setup in my opinion is a (hybrid) system. You have a generator and a big battery charger, some inverters have one built in. Then you have a battery bank of what ever size, you can charge the bank with solar, wind, a car with booster cables, grid power, and of course the generator.


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## hiwall (Jun 15, 2012)

Most car alternators only put out about 600 watts at peak.


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## cowboyhermit (Nov 10, 2012)

Not sure about "most", many vehicle alternators are 130 or 150A.


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## vickers (Jan 16, 2013)

Excellent information. Thank you all. Ill be getting this system to get my feet wet with solar. I have a coupon for $139.00. http://www.harborfreight.com/solar-panel-kit-45-watt-68751.html


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## hiwall (Jun 15, 2012)

cowboyhermit said:


> Not sure about "most", many vehicle alternators are 130 or 150A.


Many cars have them that produce only 40 amps though the trend is towards higher amps has more electric gadgets are in cars. Very few have up to 100 and over 120 is going to most likely be a special order. Plus you can not use all this power as some has to be used to run the motor(around 25 amps or more). I freely admit my knowledge in this area is somewhat dated and things could be different in new cars.


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## cowboyhermit (Nov 10, 2012)

No, I think you are right, newer cars have bigger alternators and many older ones have been upgraded after market. When getting an alternator for an old John Deere tractor the original was only like 40A!, the new one that looks identical was 60 or 80, nothing smaller available, of course they were all mechanical so it was only for lights and starting. Bigger trucks often have big alternators, especially diesels with two batteries, even if they don't need much to run. The thing is if you are using an alternator at anywhere near it's rated capacity it is usually screaming hot and not gonna last too long.


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## vickers (Jan 16, 2013)

So I should be safe running a 1000w inverter from a running car with little or no impact to the battery?


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## cowboyhermit (Nov 10, 2012)

No, even if you have a big alternator it will only put out that much at a certain rpm, no factory setup is made to put out that much at an idle, as far as I know.
I run a 3000 watt inverter on the service truck regularly but only for short periods of time and never at full capacity (it is best to never run electrical components at their maximum rated capacity). A good battery or two will provide the power when you are using it and the alternator will keep them charged.


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## LincTex (Apr 1, 2011)

hiwall said:


> Most car alternators only put out about 600 watts at peak.


I guess it depends on what year it is from...modern alternators are pretty amazing. I replaced the bearings in a Ford 3g (1994 Taurus or Mustang) which is rated at 130 amps and tested it with a big carbon pile on it. Since it was just a spare (it was given to me for free with a bad bearing) I decided to put the screws to 'er and see how she handled are hard pull.

I first ran it for a few minutes at 130 amps to let it warm up, and to see if it got hot slowly or quickly. No worries there, so I cranked it to 150. Still no problems. Cranked to 170 amps, The engine it was attached to was laboring pretty good but it handled it for a full minute easily. I decided not to abuse it too bad (no idea what temp the solder joints were at and no way to measure) so back down to 150 amps and held for 10 minutes at 14.4 volts. Ran fine and never got "too hot".

14.4v x 150 amps = 2160 watts. That's pretty impressive.

They make a "slightly" larger case version now with "rated" 200 amps. I suppose it would put out 250 peak?


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## LincTex (Apr 1, 2011)

vickersja said:


> So I should be safe running a 1000w inverter from a running car with little or no impact to the battery?


Yes... but, as always, "it depends".

I have a 1000watt continuous/2000 watt peak inverter and have done just as you mention. However, most cars idle pretty slowly to save on gas and emissions, and at idle speed most alternators don't put out peak output. You may not get 1000 watts from your particular alternator when the car is idling. One exception is the ones used on police cars (the 200 amp one I mentioned) that sit and idle on the road side all day.

The "depends" will depend on how long you will be running it. I would always make sure the car is running first, then add the load. Most cars will keep running down below 10 volts, which is a point that any good inverter should have kicked off by that point. If you draw a full 1000 watt load and the alternator only puts out 800 watts, you might get an hour of power before the inverter says "no more". At that point, if the car is still running... leave it run so it can charge the battery back up again!

If your car is anything newer than 25 years old, it "should" have an 80 amp (worst case scenario would be 60 amps)... so at 60 amps x 12 volts (conservative) is 720 watts *IF* there is no other load at all. In this case, your 1000 watt inverter would only give about 20 minutes of power before battery voltage gets too low and the inverter shuts off.


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## LincTex (Apr 1, 2011)

cowboyhermit said:


> No, even if you have a big alternator it will only put out that much at a certain rpm, no factory setup is made to put out that much at an idle, as far as I know.


Correct.... Police cars (and ambulance) alternators do better, but even they do not put out full rated power at idle speed. (although you might get one that will do 1000 watts at idle)

The only two options in this case are:
1) faster idle speed
2) bigger alternator

Caveat: Both of these options will use more gas; there is "no free lunch"

A large alternator on a car with a smaller engine (less than 1.8 liters) would be one way around this, I suppose. You would definitely have to increase engine speed a little, though.

If you have a small Asian diesel tractor with both a large alternator and a belt driven 120VAC generator (like I have) it is the best of all worlds: 1) A very fuel miserly engine, 2) 60Hz A/C power form the generator 3) A big alternator to charge the battery bank.


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## LincTex (Apr 1, 2011)

I think it is safe to say (but verify this) that you can/could use some additional solar panel wattage to do what you need to do. Just feed all outputs into the same battery. If you need 1000 watts and the alternator is only putting out 800... and if you have 240 watts of solar and lots of sunlight... Run all wires parallel (keeping everything 12 volt) and let the inverter drink from the 1040 watt stream!


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