formula for selecting a generator?

elesaver

I'm beginning to review generators. So far, I have learned that the Honda brand seems to be recommended. Here's the question: I want to know what sized generator to select. I have a 24V system with 8 ea Trojan 125 batteries. This is an off-grid system. If (or better...when) there is not enough sunlight to keep the batteries charged, I would like to have the option of using a generator to charge the batteries. This could happen in the event of an ice storm/snow storm that lasts for 3-4 days. I have an Outback VFX3524 which can accept a generator hook up but I don't think I've ever read any threads or posts about calculating how long it would take to charge up a battery bank which had been discharged at a 25-30% rate. Is there a formula for this kind of calculation? My guess is that it hinges on the amp output of the generator and the amp acceptance through the inverter/charge controller, but then, what do I know? Perhaps someone would be kind enough to help walk me through this concept?

Also, I have seen converters for the Honda generators which will allow propane to be used as a fuel. Propane seems better for the use I have mentioned above since this would probably not be something that is used commonly. I understand that the generator would need to be powered up every once in a while to keep it in shape but perhaps not as much as with gasoline which in time can gum up the carburetor. These converters seem to run about $150 but other than that, I don't know much. Is this a good idea or would it be better to buy a generator already fitted for propane?

May I also add that this is a wonderful place for learning and I thank all of you who are willing to take your time and probably tire of saying the same things over and over as new people come with the same questions. If there are threads that discuss what I've asked, simply give me the link and I'll look into it. I've tried to search but search results seem to bring up all kinds of things that I don't want. Anyway, thanks to all....and to all, a good night. :-)

westbranch

Re: formula for selecting a generator?

My guess is that it hinges on the amp output of the generator

It depends on the Wattage of your generator and the Voltage of your battery bank and the max charge (A) rate you want to put in.. Blackcherry 4 should be around soon and he can fill you in on the setup for the VFX inverter charger.

hth

mike95490

Re: formula for selecting a generator?

Batteries have a max charge rate. (usually about c/15)

Generators have a max sustained output, about 20% lower than their "model" indicates (a model 3K can sustain 2.5 - 2.8Kw rate)

Chargers have a Power Factor (PF) which is sort of an efficiency rating, your genset needs to be capable of running the charger & PF losses.

Does your inverter have integral Gen Support software ?

Does your genset voltage 120 or 240 match what your inverter's charger needs ?

For fuel efficiency, the genset is run for the BULK phase of the charge cycle, and maybe some into the Absorb, but you don't want to run a 4Kw genset just trickling 3 amps into the batteries.

So, if your battery bank is 125Ah, 15% is 18.75 amps @ 27V, = 506 watts into the batteries. A 1,000 w genset should do be fine. (Or I may have mised something)

Mountain Don

Re: formula for selecting a generator?

Propane generator... I like propane as a generator fuel because it eliminates the gummy carburetor issue. No need to drain the gasoline, also no need to store gasoline and no concern about the useful life of the fuel. No need for gasoline stabilizers. Engine life is also increased. But the downside is that propane does not have the same energy content of gasoline so the generator output is down rated. That also causes increased fuel consumption. If a large on ground propane tank is available already that is less of an issue. Just don't count of using 20 lb cylinders unless you have several or can make a lot of trips to the propane dealer.

As for converting vs buying ready to go... depends on you and how much work you want to do. These guys sell lots of converted generators; Honda, Yamaha, and more.

BB.

Re: formula for selecting a generator?

OK--Here are some details to get you started (of course, verify everything--I am not an expert on Outback products--Or any products for that matter).

The Outback can support 82 amps DC output for charging.

The Trojan 125 is a 6 volt @ 240 AH battery. You have 2 strings of 4 batteries each, for a 24 volt @ 480 AH battery bank.

We suggest for solar a 5% to 13% rate of charge... For an AC charger, you usually can justify 10% to ~20% rate of charge (note, above ~13% rate of charge, you need to monitor battery bank temperature and/or use a remote battery temperature sensor to ensure the battery bank does not overheat at high charging rates).

Assuming "nominal" AC battery charger characteristics of 80% charging efficiency, 0.95 Power Factor (some disagreement here about many chargers), and you are charging at 29 volts (absorb voltage set point). With an 80% derating of the genset for continuous battery charging, the math would look something like:

• 480 AH * 0.10 rate of charge = 48 amps
• 480 AH * 0.20 rate of charge = 96 amps
• 480 AH * 29 volts charging * 1/0.80 charger eff * 1/0.95 PF * 1/0.80 Genset Derate * 0.10 rate of charge = 1,832 VA rated genset minimum
• 480 AH * 29 volts charging * 1/0.80 charger eff * 1/0.95 PF * 1/0.80 Genset Derate * 0.20 rate of charge = 3,663 Watt genset rated minimum

So, your Outback would charge at ~82 amps (if you have a large enough genset):

• 82 amps * 29.0 volts charging * 1/0.80 eff * 1/0.95 PF * 1/110 VAC = 3,129 VA input
• 82 amps * 29.0 volts charging * 1/0.80 eff * 1/0.95 PF = 28.4 Amps AC

In general, a Lead Acid battery bank is near 100% efficient (current) when below ~80% state of charge. So, for charging from 50% to ~80%:

• 80-50% SOC = 30% battery capacity
• 0.30 * 480 AH = 144 AH
• 144 AH / 48 amps (aka 10% rate of charge) = ~3 Hours for "Bulk" charging
• and roughly 2-4 hours of "Absorb" charging (holding at ~29 volts with declining rate of charge)
• End charge rate will be under ~1-2% rate of charge (less than ~9.6 to 4.8 amps or less)

For many people, they will stop charging when the battery bank is back in the ~80-90% state of charge... At that point, you are burning a lot of gasoline for little increase in battery state of charge or increase in battery life.

If you are in bad weather or had your solar charging system fail, you may once a week wish to charge to 90%+ State of Charge to help mix the electrolyte and reduce chance of sulfation (another long set of posts/discussions).

Anyway, that is where it starts. Of course, your mix of solar charging and AC loads will have a bearing too. If you run a lot of AC loads during the day (and just from batteries at night), the genset will have to support both battery charging and AC loads too at the same time.

Most gensets are reasonably efficient at over ~50% of rated output power. For gasoline/propane powered genset, as the output power falls below ~50% rated output, the fuel flow remains the roughly the same (i.e., at 25% of rated output, the fuel flow is the same as when running a 50% rated output load).

So, most of the time, you want to avoid a large genset running small AC loads.

And many people end up with two gensets. One small/fuel efficient (charging batteries, running smaller AC loads) and a second large genset (running well pump, shop tools, cooking in evening, backup for small genset) for larger loads.

-Bill

CDN_VT

Re: formula for selecting a generator?

Mountain Don wrote: »

Propane generator... I like propane as a generator fuel because it eliminates the gummy carburetor issue. No need to drain the gasoline, also no need to store gasoline and no concern about the useful life of the fuel. No need for gasoline stabilizers. .

Just went into 3 different Dealers of Honda & Yamaha to see if they had any propane units , all said the same stuff , the valves get tight , pounded in . We have a toyata fork lifts that are just fine after my conversions. Anyone else hear this stuff , because I'm thinking they don't sell them so say there are about to fail from Valves pounding into seats , I thought all Aluminum engines had stellite seats , Valves not so much , defiantly not sodium in small engines .

VT

icarus

Re: formula for selecting a generator?

All things consided, (price, fuel economy, conversions costs, storeability of fuel etc) for a small genny, I would simply stick with gasoline.

Tony

stmar

Re: formula for selecting a generator?

I bought the Honda EU2000 because of the reviews and I wanted a reliable unit that would work when I needed it. A friend of mine just got one of these: DuroMax XP4400EH 7 HP Dual Fuel Propane/Gas Powered, it is about half of what the Honda cost. He has not had it very long but likes the way it works. There was another forum member that bought a DuroMax so you might do a search and contact him to see what his experience is with the brand.

elesaver

Re: formula for selecting a generator?

Thank you all for the input. It is now time for mulling and more searching. Yes, I remember the post about the DuroMax generator and have looked into that. I'm a little concerned about noise and its longer term durability. I've looked at reviews but sometimes those are not really reviews by people who have used or own the item. These days, they do anything to make a sale, even giving false reviews. It's a good idea to check with someone who owns/uses one. Now, why I think of longer term, I couldn't say! I don't plan to use it too much but who knows? The more I learn about this stuff and the more I use my system, the more I know that I need to know! It is a great pleasure to have put together something that works. Expensive, of course. But regardless of that, it has been worth it. I'll say that even more emphatically if/when I need the power.

90cummins

Re: formula for selecting a generator?

I have 2 Hondas a 650?? & EU2000i Honda, 2 Generac XL4000's, 1 Yanmar L70 Diesel mated to a 5kw gen head, 1 Generac 25kw pto unit and 1 6kw Chinese Yanmar clone.
Portable Generators by design must run at full rpm 3600 unless it is an inverter and if they are an emergency power source you do not want to save a buck here.
The last generator I mentioned is a 6kw Chinese clone that I bought new thinking how bad could it be.
That winter we had a sever ice storm and I was selling and renting my generators out.
The Chinese generator failed at 41 hours due to low oil pressure.
When I did a teardown I found the camshaft needle bearing had worn into the cam bearing journal which contaminated the engine with metal.
The camshaft was soft and the quality of the other components was poor.

That Chinese generator I mentioned failed 1/2 thru the ice storm, you don't want that!
The Honda EU series is a really nice fuel efficient dependable machine and if I could sell a couple of mine I would get the new EU 7000i.
My 2 cent but it's your money!
90cummins

elesaver

Re: formula for selecting a generator?

Good to know, 90cummins. And, that's the problem...quality. And, certainly we are all learning, you should get what you pay for...sometimes you do, sometimes you don't.

Thanks for the input.

oil pan 4

Re: formula for selecting a generator?

Do your self a favor and don't get a Yamaha.
It will cost as much as a Honda and will fall apart like any other chicom POS and parts will be very expensive.

The carburetor will only gum up if you leave it sitting with hydrated ethanol gas in the tank.

90cummins

Re: formula for selecting a generator?

I'd like to mention that my Honda eu2000i has a fuel drain on the carburetor which helps prevent carb issues.
Another nice feature is that it has a fuel pump and can draw fuel from another fuel tank for greatly extended run times between refueling.
90cummins

oil pan 4

Re: formula for selecting a generator?

mike95490 wrote: »

Generators have a max sustained output, about 20% lower than their "model" indicates (a model 3K can sustain 2.5 - 2.8Kw rate)

Chargers have a Power Factor (PF) which is sort of an efficiency rating, your genset needs to be capable of running the charger & PF losses.

The sustained 80% output goes for any fixed wiring too. In fixed wiring you don't want to run more than 80% load for more than 2 or 3 hours.
Power factor is current draw phase distortion and current draw falling out of alignment with voltage generation. I don't like running low power factor so I power factor corrected all my welding machines and air compressors. When you are running a power factor of 1 your voltage sine wave and your amps build and peak then fall at the same time. With low power factor due to inductive loads or solid state switching circuits current rushes in and peaks before your volts peak. These are VoltAmps kind of like watts, but totally different.
Generator are sold by "watt ratings" but if you are running any kind of motor or inductive load are ultimately limited by VoltAmps.

CDN_VT

Re: formula for selecting a generator?

Nice info Gearheads from Oil Pan & Hummin Cummins , +2

VT

706jim

Re: formula for selecting a generator?

oil pan 4 wrote: »

Do your self a favor and don't get a Yamaha.
It will cost as much as a Honda and will fall apart like any other chicom POS and parts will be very expensive.

Funny to read your comment here.

Before I bought my Honda EU1000i, I looked at the Yamaha equivalent from the dealer where I had purchased an expensive boat.

They told me that they had three Yamaha generator failures and quietly suggested buying the Honda. It has been wonderful.

Hill_Country

Re: formula for selecting a generator?

We purchased a Yamaha EF2000iS (2000 watts peak, 1600 watts running) back in April of 2014 and have used it extensively for the interior finishing of our house (examples: 13 amp Skil saw, small DeWalt air compressor for a nail gun, construction lighting, etc.). We even use it for watching movies when we want to take some time off.

We have had no issues with our Yamaha whatsoever, and the generator has performed flawlessly. The econo-mode is great and only sips gas. As I'm sure you're already aware, you can find Honda vs. Yamaha generator threads galore that extol/bash either brand of generator. I did extensive research and pretty much it was a toss-up...it just depends on what features are most important to you. You can find drawbacks to both the Honda and Yamaha as well. You really can't go wrong with either one and they are definitely worth the money versus the big box/chain store generators that run only a couple hundred dollars.

Hope this helps!