Adding a generator. Is bigger better?

[rich4]

Just wondering about the best sizing of a generator for adding to a solar + battery system to reudce reliance on a grid connection.

With battery storage, it becomes possible to have a much larger generator running for less time instead of a smaller generator running for long periods of no sun.

Are there merits for doing this? Does a big generator work out more efficient? Big gensets are certainly cheaper per kVA, when buying used and obviously less annoying if they only run briefly to pump a load of amps into the batteries. I'd assume a lot less wear and tear on the engine too.

Obviously there would be a practical limit due to max charge current of the battery system and I assume higher charge rates might reduce battery lifetime.

 

[pollenface]

I'd go as big as my wife would let me personally.

 

[DIYrich]

Best to run a generator at 50-75% of load for efficiency and wear and tear. Size for . 5c charge rate plus load. I assume this is for backup when grid is down. If you need more regular use, then you need to look more into the specifics of the generator. Diesel generators are bad at low loads, so be careful about that. May need to factor in PV production while generator is running.

 

[farmhand]

Note that MOST any generator is only going to give you 120days of service before needing an overhaul. From portables to larger generacs, 120days use is equal to about 200,000miles if is was a car engine, and most generators are not matched in quality.

I would go with (or build) a belt driven MG set, then you can swap out/repair MUCH easier. This is most how all plain people that use them often are setup. Peace

 

[farmhand]

FYI,
We got a complete generac for $400. Had to remove it & reinstall, but hey for $400!

Keep a weekly eye on Facebook Marketplace. Jonesteads going into foreclosure will very frequently sell off costly appliances they couldn't afford, and this includes whole house generators. Mostly list $1000-2000, but offer them a 1/4 of it in cash & removal and you likely got a great deal.

 

[Brett V]

If you can find a used Onan 4 pole genset, those things last forever.

 

[Steve_S]

Just wondering about the best sizing of a generator for adding to a solar + battery system to reudce reliance on a grid connection.

With battery storage, it becomes possible to have a much larger generator running for less time instead of a smaller generator running for long periods of no sun.

Are there merits for doing this? Does a big generator work out more efficient? Big gensets are certainly cheaper per kVA, when buying used and obviously less annoying if they only run briefly to pump a load of amps into the batteries. I'd assume a lot less wear and tear on the engine too.

Obviously there would be a practical limit due to max charge current of the battery system and I assume higher charge rates might reduce battery lifetime.

This all depends on how you use the generator & what you do with it.
Battery Bank Charging is TYPICALLY Handled by either the Inverter/Charger or a dedicated Charger with a proper Charging Profile for the battery chemistry. DC from a Genset is not used for charging solar batteries. The most common method is to use the Inverter/Charger as the charger and to provide additional VAC input for Passthrough to home which keeps everything running. NOTE: Depending on Inverter/Charger this varies and some are not capable, so check your specs to know what your system can do.

LFP/LiFePo4 batteries typically can discharge at 1C Rate and take a max of 0.5C Charge Rate, so for a 24V/280AH Battery, that means it can discharge at a max of 280A for 1 hour or take 140A Charge for 2 hours MAX (not suggested). Most people will charge at 0.25C which is much gentler on the cells and keeps "runners" at bay.

SIZING the Generator is important.
Look at the Charger Device you have and at how many Amps it can charge at for your battery voltage and how much passthrough is used. Passthrough is the usual amount of power being used to run the home without heavy loads (like electric dryer/hot water tank, electric stove).

Example:
I have 24V Battery Bank, 1200AH in 6 packs. Inverter/Charger set to 80A Charge Rate to battery bank, this takes 120V/22-23A (it floats a little) + Passthrough power which the inverter feeds directly to my AC Side. This is roughly 2800W (120V X 23A=2760W) which is provided by the Genset L5:30 Plug (120V/30A) I am running with a Champion Power Equipment INVERTER GENERATOR 4650W Model which is very quiet, reliable, easy to maintain... I have had no issues with Champion Equipment and my Big 9200W has well over 1000 hours and purrs like a kitten, just have to change the oil and clean the filters per schedule. REF: Champion 4500W and up Inverter Generators

Pure Sine Inverter Generators are not only considerably quieter they provide clean pure sine wave form which is best for your electronics and motors (fridge, AC etc). A regular Construction/Industrial use genset is MOD-SINE (much cheaper) which the Frequency will float from 57-63 Hz and is not consistent (you'll notice lights will flicker a bit, fridge sounds louder noisier when running etc) and some electronics will misbehave.

Look at your Inverter/Charger and/or External Charger. Determine at what Amperage you want to charge with at XX Volts. Look at the specs to work out how many Amps / Watts are required to run your Charger at those settings and add 20% at least for overhead.

There is another long extended thread on generators here and you will see a LOT of people have Champion Generators and no issues over the years... I've used Champion Products for over 20 years and no complaints.

Hope it helps, Good Luck

 

[farmhand]

This all depends on how you use the generator & what you do with it.
Battery Bank Charging is TYPICALLY Handled by either the Inverter/Charger or a dedicated Charger with a proper Charging Profile for the battery chemistry. DC from a Genset is not used for charging solar batteries. The most common method is to use the Inverter/Charger as the charger and to provide additional VAC input for Passthrough to home which keeps everything running. NOTE: Depending on Inverter/Charger this varies and some are not capable, so check your specs to know what your system can do.

LFP/LiFePo4 batteries typically can discharge at 1C Rate and take a max of 0.5C Charge Rate, so for a 24V/280AH Battery, that means it can discharge at a max of 280A for 1 hour or take 140A Charge for 2 hours MAX (not suggested). Most people will charge at 0.25C which is much gentler on the cells and keeps "runners" at bay.

SIZING the Generator is important.
Look at the Charger Device you have and at how many Amps it can charge at for your battery voltage and how much passthrough is used. Passthrough is the usual amount of power being used to run the home without heavy loads (like electric dryer/hot water tank, electric stove).

Example:
I have 24V Battery Bank, 1200AH in 6 packs. Inverter/Charger set to 80A Charge Rate to battery bank, this takes 120V/22-23A (it floats a little) + Passthrough power which the inverter feeds directly to my AC Side. This is roughly 2800W (120V X 23A=2760W) which is provided by the Genset L5:30 Plug (120V/30A) I am running with a Champion Power Equipment INVERTER GENERATOR 4650W Model which is very quiet, reliable, easy to maintain... I have had no issues with Champion Equipment and my Big 9200W has well over 1000 hours and purrs like a kitten, just have to change the oil and clean the filters per schedule. REF: Champion 4500W and up Inverter Generators

Pure Sine Inverter Generators are not only considerably quieter they provide clean pure sine wave form which is best for your electronics and motors (fridge, AC etc). A regular Construction/Industrial use genset is MOD-SINE (much cheaper) which the Frequency will float from 57-63 Hz and is not consistent (you'll notice lights will flicker a bit, fridge sounds louder noisier when running etc) and some electronics will misbehave.

Look at your Inverter/Charger and/or External Charger. Determine at what Amperage you want to charge with at XX Volts. Look at the specs to work out how many Amps / Watts are required to run your Charger at those settings and add 20% at least for overhead.

There is another long extended thread on generators here and you will see a LOT of people have Champion Generators and no issues over the years... I've used Champion Products for over 20 years and no complaints.

Hope it helps, Good Luck

Agree, Champions are unknown tanks. I see them get used in several applications in Ohio, and oddly never heard a complaint. A group of Beachy run these and can set you up a custom Champion for what you need. Just search out Grabers in Glenford Ohio. Champion does make this pure sine wave gasoline invertor, I think Grabers can make it propane or no... Good luck!

 

[Mattb4]

Besides being able to handle the load you also need to look at amount of usage. Potable small gensets (engine mated to generator) are typically for short periods of run time. They often have rather cheap single cylinder engines without things like oil filters and also simplistic governors.

If you are going to use a genset for emergency standby you will want a unit with a more dependable engine mated to the gen head. These are stationary units for the most part (you do find some trailer mounted) and designed for longer run time. The engines are either multi cylinder gas or diesel.

If your needs are day in and day out 24/7 operation than you go for a commercial genset rated for Continuous duty. These are mostly diesel with 3 phase gen heads and higher voltage, though you can order them with 240v/120v output. You will not find these in the Big box stores but custom order them from a supplier.

 

[pvdude]

Before I discovered this amazing forum and learned about DIY solar,
we ran the property with one 21kw Perkins Diesel generator during grid outages.
1800 RPM, very durable, but noisy & smoky & required fuel storage and treatment.
Years later, we have enough solar to avoid using the generator much.

Our configuration is a bit different than most, as we had the generator first, added solar later.
Each power source (generator, solar) is treated as separate.

When the grid is down, either solar OR generator runs the property.

The generator makes too many amps (87A) for the relay that is used for generator input to the Schneider XW Pro inverters (60A)

It is not possible to charge the solar batteries from the generator w/o some (temporary) wiring changes.
So far, have not required a wiring change, the Sun comes out eventually to charge the batteries.

 

[rich4]

Great info guys!

I see that some hybrid interters have generator inputs which makes the setup a bit easier I guess.

The max charging current of LiFePo4 is the big issue isn't it. 0.5C on a 48v 304Ah battery means I can only put in about 7kw per hour so getting the battery up to half capacity in an hour. I was kinda hoping for a solution where the generator would only run 30mins or less to cover a days usage of say 14kwh, given that quite big gensets can be had used for not a lot of £££. Keeping the run time down to aviod lots of rebuilds/services is obviously beneficial. No idea what the service intervals are but I guess 500 hours would be typical? A service life before rebuild might be 5,000 hours?

So next question is, what if I want to run the generator on used (but dried and filtered) veg oil? Will only ancient gensets run that?

 

[Delmar]

Bigger is not necessarily better. With a gas engine low loading can cause carbon buildup. With a diesel engine low loading will not produce enough heat for combustion and create lots of problems. Reference link.

For my moms house I installed a 14KW Generac NG standby that runs her 3-ton AC unit. Generac recommended a 20KW unit, and the 14KW grunts a bit but starts the AC compressor just fine.

 

[Mattb4]

...

The max charging current of LiFePo4 is the big issue isn't it. ...

While obviously you want to be below max there is another consideration. Batteries do better when they are gradually charged back up to full. Fast charging (max charge rate) is generally limited to about 80% of full to protect the battery. They get hot when charging at high current.

 

[50ShadesOfDirt]

We utilize Westinghouse wgen9500df (propane) open-frame gens ... their main purpose is to recharge battery bank when solar isn't cooperating. This is pretty much their only function, so it is a nice, smooth load for the gen. At most, 4 hours per day. This model line-up typically has electric start, auto-choke, smart-port (ATS), and most importantly for us, it is propane (we have a site tank) ... no fiddling with messy fuels.

We get about 2000 hours out of a gen, or about 2 years or so. Then, by buying the same model, we recycle the old one into the new one (all parts transfer over) ... saves big bucks over after-market parts pricing. Open-frame means they are easy to work on, compared to the tight spaces and complexity of an inverter-gen. We actually have two such gens at any one time, as one is the fallback for the other. The reliability of a $5000 generac, at a fraction of its costs (or those of an inverter-gen).

THD, after a decade of operation, has never been a problem for us ... no devices die an early death. We did install SPD's, as I believe that it is surges that kill things, not THD.

Hope this helps ...

 

[rich4]

Bigger is not necessarily better. With a gas engine low loading can cause carbon buildup. With a diesel engine low loading will not produce enough heat for combustion and create lots of problems. Reference link.

For my moms house I installed a 14KW Generac NG standby that runs her 3-ton AC unit. Generac recommended a 20KW unit, and the 14KW grunts a bit but starts the AC compressor just fine.

The low loading issue should be addressed by charging the batteries at 7kw for an hour rather than trying to run the house at 1000W for 7 hours, but I know what you mean. Trying to use a 100kVA for charging a 14kWh LiFePo4 battery isn't a good idea! 30kVA genset might be ok?

 

[rich4]

We utilize Westinghouse wgen9500df (propane) open-frame gens ... their main purpose is to recharge battery bank when solar isn't cooperating. This is pretty much their only function, so it is a nice, smooth load for the gen. At most, 4 hours per day. This model line-up typically has electric start, auto-choke, smart-port (ATS), and most importantly for us, it is propane (we have a site tank) ... no fiddling with messy fuels.

We get about 2000 hours out of a gen, or about 2 years or so. Then, by buying the same model, we recycle the old one into the new one (all parts transfer over) ... saves big bucks over after-market parts pricing. Open-frame means they are easy to work on, compared to the tight spaces and complexity of an inverter-gen. We actually have two such gens at any one time, as one is the fallback for the other. The reliability of a $5000 generac, at a fraction of its costs (or those of an inverter-gen).

THD, after a decade of operation, has never been a problem for us ... no devices die an early death. We did install SPD's, as I believe that it is surges that kill things, not THD.

Hope this helps ...

2000 hours and it's dead? I guess that's like driving a car 120k miles.... What happens to them, just general wear and tear on bores and bearings?

So there's no point in rebuilding them? Given the cost of a rebore these days, I guess its just not worth it.

 

[Ampster]

The generator makes too many amps (87A) for the relay that is used for generator input to the Schneider XW Pro inverters (60A)

Unless the Schneider can pull that many Amps without tripping its own Breaker I don't see why that is an issue.
In the context of the thread title, it is just a waste of capacity because that capacity will be unused.

 

[C&K]

The generator makes too many amps (87A) for the relay that is used for generator input to the Schneider XW Pro inverters (60A)

We have a 25kVA Caterpillar 3304 prime-rated genset that inputs power to twin Schneider/Xantrex XW6048's without any problems, charging a 1700ah forklift battery for our off-grid system @ 180 amps. 60A is just the input limit of what the inverter can accept. You can hook that inverter to a 2.25MW Cat 3516 if you want.

In generators there is two types - prime rated and standby rated. Prime rated generators are very expensive. Prime power generators can run an unlimited number of hours per year at full rated load and to have a prime rating they must be able to deliver emergency power at 110% of their prime rating for 1 hour out of every 12 while maintaining AC freq @ 60Hz +/-0.1Hz. These generators have a life expectancy based on fuel burned, not hours. Example: a 2.25MW Cat 3516 is rated for 4.2 million gallons of fuel burned to overhaul, which equates to about 36,000 hrs at full rated load. On the other end of the scale our little Cat 3304 is rated for 90,000 gallons of fuel burned to overhaul, which equates to about 31,500 hrs at full rated load. Prime power generators are not designed to be operated below 70% rated load. They are almost exclusively 4-pole, 1,800 rpm for 60Hz power if they are powered by a piston engine.

Prime power generators are a one-on-one replacement for grid power, and they are many times use to supplement the grid when they are installed in applications where a utility pays a customer to run a prime generator during peak load times. Entry cost is usually $30,000 and up.

Standby rated generators are what most home and portables are. They are designed to run at no more than 50% rated load continuous, typically have a life expectancy of 500 - 2,000 hrs for spark ignition units, 5,000-7,000 hrs for diesels. They come in everything from super cheap with poorly wound gen heads that put out a lot of THD, to megawatt class sets that provide standby power for hospitals and such. Standby units have "surge ratings" where they can put out maybe 150% of their nameplate to get inductive loads turning after a power outage while maintaining a reasonable range of frequency and voltage that falls far out of the range of what prime power sets are required to deliver.

Standy generators are only designed to provide intermittent power if the grid goes down. They are not certified to supplement the grid like prime power units are. Entry cost can be as low as $500 for portables.

When it comes to using generators on inverters, your XW-series inverters likely won't even qualify a cheap Harbor Freight generator. The power isn't clean enough. In standby rated sets you get what you pay for. Cheap might be "good enough" to run power tools or whatever, but expect your ceiling fans to hum and your inverter to be unhappy charging batteries with it.

 

[MurphyGuy]

You want a generator that will charge your batteries while running at 50% to 75% of its rated load capacity. Any more than that will shorten the life of the generator, any less is just a waste of energy.

 

[dcg9381]

Do you have a good indication of actual load data? I'd want to know my peak KW load over the last 12 months and I'd buy a generator that has a capacity of 130% of that peak load.

Assuming you're going "large" - most are non-inverter generators and they can't throttle back at all, so if you're using 20% of their load, you're burning a lot of fuel unnecessarily.

Our home has a 20KW generator. Our normal power use is between 2-5KW with an absolute high peak at about 11KW on the home.