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Generator Selection for Off-Grid Setup

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Andersonjk4

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Mar 6, 2023
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Spangle, WA
I need help with figuring out a generator selection for my upcoming off-grid home build. Quick details: Planning on two Schneider XW Pro+ Inverters and two Conext MPPT-80A or 100A, 18-22 kW PV array, ~50kWh LiFePO4 battery bank (to be expanded later). Location is Eastern Washington, where snow is common and several (3-5) days with very little sun happens in the wintertime (but not socked in like the west side can be). I fully expect to run a generator most days during the winter when days are short for supplemental charging, but hoping that my dependancy on full generator power will be limited. My (very conservative) estimated loads are about 32kWh per day max and 28kWh average with a peak demand of about 10kW. As far as fuel type goes I am not set on any fuel type. Although I will likely have propane on the property for supplemental heating/cooking/water-heating during the low solar production times, and I also already have a 250 gallon diesel tank. Gasoline would probably be my third choice, but not totally against it.

I am having trouble deciding on a larger (15 kW+) typical backup generator that can handle prime power and charge batteries or going with a smaller portable generator in the 7kW to 9kw range that would mainly just charge the batteries (I would likely have two at all times for redundancy). It seems like with current battery technology allowing pretty quick charging that running a smaller generator to supplement battery charging may be all that is needed. But this is my first foray into off-grid living, so maybe I am being too optimistic about a small generator (or 2 small generators in parallel) being able to charge batteries.

Any insight on generator usage for This type of system will be greatly appreciated.
 
Andersonjk4 said:
I need help with figuring out a generator selection for my upcoming off-grid home build. Quick details: Planning on two Schneider XW Pro+ Inverters and two Conext MPPT-80A or 100A, 18-22 kW PV array, ~50kWh LiFePO4 battery bank (to be expanded later). Location is Eastern Washington, where snow is common and several (3-5) days with very little sun happens in the wintertime (but not socked in like the west side can be). I fully expect to run a generator most days during the winter when days are short for supplemental charging, but hoping that my dependancy on full generator power will be limited. My (very conservative) estimated loads are about 32kWh per day max and 28kWh average with a peak demand of about 10kW. As far as fuel type goes I am not set on any fuel type. Although I will likely have propane on the property for supplemental heating/cooking/water-heating during the low solar production times, and I also already have a 250 gallon diesel tank. Gasoline would probably be my third choice, but not totally against it.

I am having trouble deciding on a larger (15 kW+) typical backup generator that can handle prime power and charge batteries or going with a smaller portable generator in the 7kW to 9kw range that would mainly just charge the batteries (I would likely have two at all times for redundancy). It seems like with current battery technology allowing pretty quick charging that running a smaller generator to supplement battery charging may be all that is needed. But this is my first foray into off-grid living, so maybe I am being too optimistic about a small generator (or 2 small generators in parallel) being able to charge batteries.

Any insight on generator usage for This type of system will be greatly appreciated.
Click to expand...

I have two 5kW Victrons in parallel split phase and a puny 3850W Predator that currently only feeds ONE phase (about 1600W). I can tell you, the reduced power is very restricting during charging. If Victron didn't have the power assist function, it would be maddening. Clearly a worst case situation compared to your proposed scenario. I need to get the second leg connected to the other inverter.

In any case, it really depends on what your baseline loads are, and what you can tolerate in a "mostly charging" situation.

For efficiency of a traditional generator, you want to run at about 70-80% rated. If it's an inverter generator, it matters much less.

Since I'm too cheap for an inverter generator, I think what I'm going to do is evaluate my typical "background" load (about 300-500W), add my peak charge power to that and then divide by 0.65 and select a generator around that size (I've factored in the charger efficiency in the 0.65 number).
 
The XW Pro inverters do have a Gen Support feature so something in the 7 to 10kW range should work since you can limit the gen current and it doesn't have to support the full load. If you have that much diesel, a Kubota GL-7000 would be really sweet if the budget allows for it. Alternatively, an open frame Champion Inverter generator model 100520 7000W would be a good value choice. EDIT: You can add a dual fuel kit to the champion if you prefer propane rather than gasoline.
 
As far as I'm aware, only the generac ecogen and the Kohler 14rca are warrantied for sole source production. I'd start looking there.
 
BentleyJ said:
The XW Pro inverters do have a Gen Support feature so something in the 7 to 10kW range should work since you can limit the gen current and it doesn't have to support the full load. If you have that much diesel, a Kubota GL-7000 would be really sweet if the budget allows for it. Alternatively, an open frame Champion Inverter generator model 100520 7000W would be a good value choice. EDIT: You can add a dual fuel kit to the champion if you prefer propane rather than gasoline.
Click to expand...
That Kubota generator does look pretty nice, but spendy. I was just looking at the Champion 100520. I have a Champion 3500 open frame inverter that I have been using for our RV and it works well.
 
Andersonjk4 said:
I need help with figuring out a generator selection for my upcoming off-grid home build. Quick details: Planning on two Schneider XW Pro+ Inverters and two Conext MPPT-80A or 100A, 18-22 kW PV array, ~50kWh LiFePO4 battery bank (to be expanded later). Location is Eastern Washington, where snow is common and several (3-5) days with very little sun happens in the wintertime (but not socked in like the west side can be). I fully expect to run a generator most days during the winter when days are short for supplemental charging, but hoping that my dependancy on full generator power will be limited. My (very conservative) estimated loads are about 32kWh per day max and 28kWh average with a peak demand of about 10kW. As far as fuel type goes I am not set on any fuel type. Although I will likely have propane on the property for supplemental heating/cooking/water-heating during the low solar production times, and I also already have a 250 gallon diesel tank. Gasoline would probably be my third choice, but not totally against it.

I am having trouble deciding on a larger (15 kW+) typical backup generator that can handle prime power and charge batteries or going with a smaller portable generator in the 7kW to 9kw range that would mainly just charge the batteries (I would likely have two at all times for redundancy). It seems like with current battery technology allowing pretty quick charging that running a smaller generator to supplement battery charging may be all that is needed. But this is my first foray into off-grid living, so maybe I am being too optimistic about a small generator (or 2 small generators in parallel) being able to charge batteries.

Any insight on generator usage for This type of system will be greatly appreciated.
Click to expand...
I did this…


…and it’s working great.

It’s smart turns itself off when completely charged and throws 75A to my batteries from a Harbor Freight 7,000w 240V generator. Both of with cost less than a grand.
 
I would stay away from the "standby generator" lines (Generac, Kohler, etc), as the target market is someone having to run the standby unit to power the whole house for some hours during a grid outage only. These are higher priced units, $5k or thereabouts, plus as much as another $5k to get installed, and expenses mount up after that. When something breaks, it is major effort and cost to get them fixed.

This makes them not well suited for more heavy-duty use in off-grid, very periodic charging scenarios.

I run two Westinghouse wgen9500df's (one is a spare), open-frame units, on propane. The price ($1k or thereabouts) and feature set (remote start, smart port, etc) is what has me on these units. I tend to get about 2000 hours or so out of a "portable" generator (these units in this size are wheeled), and when one finally dies with a major component failure (engine, rotor/stator), I just feed all the remaining parts into the next unit, of the same model.

My Magnum 4024 eats the power from these just fine, but if you are worried about the power from these, either use a "chargeverter" (relatively new product, mentioned above), or switch to an inverter-gen line of portables, although now the price is roughly doubled, on up to $5k or so for top of the line like Honda's.

Hope this helps ...
 
You want to size the generator so it runs at about 75% of its capacity to charge the batteries.
 
The tow behind light tower generators are rated at 4 kw , 6KW or 8Kw depending on the model. They typically have a ~ 1 liter liquid cooled 3 cyl diesel that runs at 1800 rpm. I see them for sale used some in the $1500 - $3500 range depending on the hrs. I've thought about getting one since my oil furnace would run on diesel and could use the heat off the coolant and exhaust to heat my home and domestic hot water if I had to run one in during an outage.
 
Are you in a location where a little wind generation would be possible. The nice thing about wind is it typically is the opposite of solar, day to day. If solar generation is bad, wind is up, and vice versa. It wouldn't be enough to replace a dinosaur-fueled generator, but it could save fuel.
 
My experience with over 30 years off grid...

Generators,
CNG or Propane. Buy gas in the summer, as much storage tank as you can get the supplier to plunk down for free with exclusive contract.

Gasoline or diesel DOES go bad, it DOES require fuel filter changes, they WILL need fuel systems maintained/replaced.

When operating a generator, the initial cost is nothing compared to fuel, maintance costs. Lubrication oil, oil filters, fuel filters, air filters, etc.
From bad fuel (CNG/LPG never goes bad), to rodents working on the wiring to corrosion over time... MAINTIANCE.

Then there is the spare parts issues. This is your 'Emergency Back-Up' that absloutely HAS TO BE NO FAIL.
Spark plugs, starter motors, starter relays, starting batteries, any ONE failure point without spare parts and you are screwed.

A saying when it's 'NO FAIL',
"One is None and Two is One..."

Now...
All things considered, panels are cheaper (not cheap).

Panel string -> charge controller -> battery.

Batteries (plural) En Banc. Power supply redundancy. You will NEVER loose all panels at once, you will never loose all charge controller at once, you will never loose all batteries at once.

Stand alone charge controller are so cheap I wire two in parallel with transfer switches... Flip the switch, back in business and I can replace rhe failed unit at my leisure.

Same with inverters. Parallel inverters, flip the switch and back in business, even if it's half power, the beer & bologna stays cold, the lights stay on.

Besides redundancy, advantages include upgrading/expanding any part of the system as new equipment becomes available. From PMW chargers (hate those things) to switch from high loss transformer inverters to high efficiency semiconductor inverters, from lead/acid to Lithium batteries, everything changes.

If you have an 'All In One' box, all propritary, NOTHING CHANGES, and in a few years (or less) your 'BOX' won't be supported anymore making servicr/repair damned near impossible to completely impossible.

For instance, my switch from lead/acid to Lithium batteries. A inexpensive charge controller swap to perfectly support the battery chemestry change, tailor the charge to the battery for longest life, AND another layer of safety for my expensive batteries...

While I *Could* use the built in charger from the inverter, I choose to use what exactly, and specifically feeds each battery En Banc the best.

The 'Trick' is to get enough panels to charge the batteries on the shortest day of the year...
Expensive, but not hateful, about the same as a small generator and necessary charge controller/battery chargers.
Remember, generators produce in AC, your batteries are DC, that's a rectifier/converter AND a battery voltage regulator (charger).

The next 'Trick' is enough battery to power for those 'No Sun' days that can and do happen (sorry 'Idealists' its just a fact of life).

This plagued me for 30 years... The switchover to Lithium completed in 2020 FINALLY gave me enough reserve storage we don't have to watch every Watt.

While you WILL have substantial money in 3 days of storage, the size of your batteries EN Banc will make your batteries live MUCH longer. Return on investment... Redundancy AND much longer life when you aren't working the batteries 80% or 90% capacity every day.
 
Probably the best bang for the buck on Amazon, it’s made by the same company that makes the Predator and DuroMax inverters generators. This one is only $2.299, dual fuel, remote or dry contact start. Cheap enough to be replaced several times over when it wears out. 10,000 watts surge might be a stretch based on cid. This probably doesn’t have an oil filter so fill with conventional oil, run for 10 minutes, drain and refill with conventional oil, run reduce load for a few hours, then replace oil with pennzoil ultra and follow recommendations there after. This unit has a nice clean sine wave so the inverters accept it;
AIVOLT Dual Fuel Portable Inverter Generator- 10000 Watts Gas or Propane Powered Home Back Up Electric Start Inverter Generator https://a.co/d/9vCvz1Z

IMG_0715.jpeg
 
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My opinion on the best way to figure generator sizing is to have enough generator power to conservatively charge your batteries at a good rate and have enough generator wattage to run your house if inverter is down.

I would not run a generator for more than three or four hours at greater than 75-80% rated load. Maximum fuel efficiency is typically between 25% and 75% of rated load on generator.

If you go for an external fixed installation on generator, better to buy a liquid cooled diesel, propane, or natural gas engine model. If you have natural gas feed to your house the decision is pretty easy. You can always back it up with propane if natural gas utility goes down.

Get one with electronic governor control with stepper motor throttle control.

If you are going for two XWpro units, a 12 kW to 15 kW generator would be about right size.

I have a 15kW generator and a 3 kW inverter-generator for 'trickle' charging backup.
 
RCinFLA said:
My opinion on the best way to figure generator sizing is to have enough generator power to conservatively charge your batteries at a good rate and have enough generator wattage to run your house if inverter is down.

I would not run a generator for more than three or four hours at greater than 75-80% rated load. Maximum fuel efficiency is typically between 25% and 75% of rated load on generator.

If you go for an external fixed installation on generator, better to buy a liquid cooled diesel, propane, or natural gas engine model. If you have natural gas feed to your house the decision is pretty easy. You can always back it up with propane if natural gas utility goes down.

Get one with electronic governor control with stepper motor throttle control.

If you are going for two XWpro units, a 12 kW to 15 kW generator would be about right size.

I have a 15kW generator and a 3 kW inverter-generator for 'trickle' charging backup.
Click to expand...
The worst thing for fuel economy is to not have the generator doing reasonable work while running, set the charge to get it done around the 65% of the continuous rated generator load. Be careful of what loads you put through at the same time because many inverters add this on top of the charging power.
 
I went with this :


$945 free shipping.

I haven't actually had to rely on it and I've only ran it to get thru the breakin period and change the oil so no bragging on how awesome it did powering the house or anything.

I did run some heavy power tools on it during the break in and it did fine.

Where it shines is it being dual fuel out of the box and for $50 I added natural gas to its fuel list so its tri-fuel now. There is a company that makes a remote start system that allows full control of it start/stop, exercise, etc for $200 so now its a generac for pennies on the dollar.

There is a guy in hawaii that sells plans for sound proofing kits for generators that I highly recommend looking into because the larger generators are god aweful loud. I think the plans were $75 or something but its easy to follow and works amazing.
 

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The question of which generator is impossible to answer in isolation. You've told us a lot about your system, which is very helpful, but you haven't told us how you feel about going without power and you haven't said anything about budgetary constraints.

JeepHammer makes several important points, the most important of which is the need for redundancy in your system. Right up with that point is using natural gas/CNG/propane to fuel your generator. Also key - extra panels are cheap these days. JeepMaster skips over the relative safety of propane/NG vs pouring gasoline into a hot generator that you're trying to keep running when the tank has run dry.

When I built my initial system, panels cost $4/W (not total system cost, just the panels themselves.) As a consequence, a 3kW system with 40kWh of lead-acid batteries and generator backup seemed a good balance. We live at high elevation in NM, so the solar input is about as good as it gets. We averaged about 200 hours of generator use over the next 15 years but watched our usage like a hawk. Our initial generator was a Kohler 15RYG - an 1800RPM, water-cooled generator with a 4-cyl GM Vortec engine. (I'd kill to get a low-hours one today). We initially ran it on propane and eventually connected to the natural gas pipeline that flows from the wells north of our house. In spite of this being unprocessed, raw well-head gas, we had essentially no problems with filters, lubricating oil, or the like. In fact, when I change the oil each year it is still the honey color of new oil. We did have an SCR go out after 15 years and of course that was in the middle of a blizzard. We replaced the whole PV side of things 18 months ago with 13kW of panels and 40kWh of LiFePO4 batteries. We've run the generator 4 hours in that time, other than weekly exercise cycles. Like JeepMaster, we no longer watch every Wh. Heck, we even ran the dishwasher twice in one day when we had a lot of company and were preparing messy dishes. Having gotten 15+ years out of my generator with two failures (the SCR and one mystery one - the service guy claimed we had ice between the brushes and the commutator rings), both of which occurred during blizzards, we decided to add a second generator - a Kohler 14RCA, mentioned above as the only small Kohler generator that can be used with off-grid solar without voiding the warranty. Other than an annual oil change and an air filter every five years or so, these generators are essentially maintenance free. Both can run on either propane or natural gas, and mine are plumbed to both. I can switch fuel in under five minutes. The are wired to the inverter for auto start and exercising. They will run whether I'm there or not, or more importantly, if I'm not there and my wife IS there. Peace in the family was easily worth the price of a second generator.

A brand new Kohler 14RCA is $4250 delivered. That includes a sound-attenuating enclosure so the noise won't drive you crazy (or piss off your neighbors, if you have them). You can pour the pad in an afternoon. The cost for the balance of installation depends on your skills and how far it is from the propane and the inverter. The wire for a 14kW generator gets expensive fast. If your budget allows, a liquid cooled, 1800 rpm generator is the way to go. They last longer, run quieter. But they cost probably twice what an air-cooled, 3600 rpm generator of the same rating costs. Don't forget you have to derate for altitude. Google tells me you're at 2400', so the derate isn't too bad - 2-3% per thousand feet is the rule of thumb. I'm at 7500', so it gets noticable. And the listed output rating is usually for propane, natural gas is about 15% lower.

I do have one key disagreement with JeepMaster. You want to buy your propane tank, you do not want a "free" tank from the supplier in exchange for an exclusive contract. That exclusive contract means you have to buy from that one supplier, and they know it. (I think there are some states that have limited protection against price gouging - check this). I pay at least 35% less than my neighbors who don't own their tanks. When I need a fill I first call a company I've never actually used. Then I call the one that is more convenient. When they quote me a price I then tell them (honestly) what their competitor is offering. They usually beat that by a dime. Of course, ever since I connected to the pipeline for heating and the generators, I only use less than 50 gallons of propane a year for water heating and the cooktop/oven, so it's not going to break the bank either way. With a 1000 gallon tank (I didn't know I was going to be able to connect to the pipeline when we built), it takes a couple of years to reach the minimum delivery level.
 
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