Sizing Kubota Lowboy Generator

[eastvt]

I'm looking at replacing my generator, currently gas powered with the ubiquitous Honda G390 on a Husqvarna generatore. That engine I guess puts out 11.7 HP, and my generator is rated 6500 watts. I seriously doubt it is really able to produce that. Maybe something is a little bit wrong with it? Maybe the specs are lying?

This is hooked up to a switch panel with a sub-set of breakers. It bogs down if we're not careful. Run too much stuff and the voltage will drop. I've got a plug in LED voltage meter, and if I run the electric tea kettle or something it will drop right down to 110V or below, LED bulbs might flicker or go out. Engine is not obviously slowing or bogging down, but I can see the voltage dropping. If we're careful and pull some plugs and don't power multiple heavy loads at the same time it can keep 115 to 120V on the line. It can run the well pump, but I don't like to do that on the generator unless it's a serious outage.

So the house has an electricity monitor on the mains and we can see our usage when the power is not out. Normally we rarely use over 2000 watts, extremely rare to use 5000 watts -- like if the oven and dryer are both on at the same time. With two heat pumps running we rarely use over 1500 watts in normal conditions. Right now I've got a fish tank heater on, multiple lights on, two heat pumps running for heat, and it's 960 watts (which will climb a bit with the heat pumps' cycles).

Assuming we won't even run the heat pumps on the generator, though in a heat wave some AC from one of them would be nice if we had a power outage when it was hot.

If the Kubota 7k generator can really crank out a steady 7k, then it would be fine. On the other hand the 10 HP of the Kubota is less horsepower than the Honda engine has (11.7 or "13" HP), and 6500 watts supposedly delivered by the current gas generator is just barely enough if we are super careful.

The Kubota 11k generator would clearly be fine, but it would use more fuel I assume and cost like $1600 more or something, which I'd obviously rather not spend.

Opinions on whether the 7k Kubota diesel would beat the Honda gas engine, or would it drop voltage too? Should I spring for the bigger one?

We have no propane or natural gas. The goal is to have an auto start generator that can power a bit more than the gas generator is doing, but not a crazy amount more.

Recently I was away from home and the power went out and my wife wasn't able to pull the honda to life, so we're thinking electric/auto start.

 

[NCL4701]

General “rule of thumb” is 2HP on the PTO per kilowatt. So if you have 10HP on the PTO, you’re not going to get more than about 5kW continuous, even if the generator is capable of more if connected to a more powerful engine. If you have a bigger tractor than the one in your profile and it will put 22HP or more out the PTO shaft I’d go with the 11kW.

Based on your numbers above it sounds like your 6.5kW generator is 6.5 surge, not continuous. Some of the portable generators have the surge (sometimes referred to as starting) watts plastered all over in big font and continuous watt rating buried in the fine print. For starting a motor, such as your well pump or heat pump, surge capacity is important. For something like an oven or dryer, continuous watts are more important (although a dryer does have a motor spinning the drum).

Bottom line I doubt you’ll be able to run a generator off your BX 1880 PTO significantly more capable than the portable gasser you’re working on replacing.

And I don’t say that from the perspective of one of the “never power a generator with your tractor under any circumstance” folks (who are likely to show up any minute now). I have a PTO generator. It is not a good solution for all people in all scenarios. It has proved to be a good solution for me and my wife in our specific circumstance.

I will also add the disclaimer, I’m not an electrician, electrical engineer, power company lineman, etc. I’m a guy with a tractor and PTO generator that I successfully use to power my house when needed and provide significant portable power in remote off-road areas of the property when either necessary or convenient. If someone more knowledgeable than I knows how to squeeze significantly more than 5kW continuous out of a 10HP PTO, I am very willing to learn.

 

[eastvt]

General “rule of thumb” is 2HP on the PTO per kilowatt. So if you have 10HP on the PTO, you’re not going to get more than about 5kW continuous, even if the generator is capable of more if connected to a more powerful engine. If you have a bigger tractor than the one in your profile and it will put 22HP or more out the PTO shaft I’d go with the 11kW.

Interesting. I wonder how that translates to directly powered generators. The Kubota "Lowboy" I'm talking about is a Kubota/Yamaha dedicated standalone generator that comes in 7k, 11k and 14k flavors. The 7k is two cylinder diesel, the 11k is 3 cylinders. Water cooled. Kubota makes the engines, Yamaha makes the generator. Painted orange it's branded Kubota, Blue and it's Yamaha.

My current gas pull start generator is specced at 6500 watts continuous, 8500 surge. So it does handle the well pump alright.

 

[NCL4701]

My bad. I totally misread your intent, thus resulting in a mostly useless answer.

Having used a 6500 watt continuous portable for several years and then moving to our current 16KW continuous, I would go with the 11KW for an extra $1600. It doesn’t seem like much and you still have to think about what you’re doing but it’s still a good bit closer to “normal” to be able to run the well pump or air conditioning and a couple other things at the same time.

 

[InTheWoods]

I have a decent amount of experience with standby gensets. The only way to 'properly' size one is to balance cost/fuel consumption concerns against the loads you need to run.

The GL11000 burns about 1.1 GPH at full load vs. the GL7000 at 0.7 GPH. At lighter loads, the consumption will drop, but for a given load the GL7000 will use less fuel - about 30% less, I'd estimate. For standy use, the fuel cost rarely matters - My backup set rarely runs more than 30 hours a year, including excercise times, so fuel cost just doesn't amount to much. OTOH, if you feel the need to keep fuel on-hand for a zombie apocolypse, the set that uses less fuel will become more desirable.

The initial cost is up to you - no way can anyone else advise on the 'pain' of the extra $1600.

So that leaves the loads you need to run. You don't provide enough detail for me to advise. For some folks, a heat pump alone drives the genset size due to the required LRA (locked rotor amps) or surge current. For others, maybe they have an electric water heater, and at 4500W alone it represents the biggest load. A well pump? For me, that is the single most important load I have.

Some people don't want to be bothered 'managing' loads during an outage, or have an ATS (automatic transfer switch) so the genset must be sized to pick up something close to a worst-case load scenerio.

All that said, as much as I like Kubota, I've pulled back from a Lowboy set, since they run at 3600RPM. I use an old Onan set running at 1800 RPM. Onan also had some very nice sets using Kubota engines running at 1800 RPM, such at the 7.5DKD. But these are all old, and maybe you'd prefer shiny and new, which is understandable...

The Kubota Lowboy manual doesn't show the voltage or frequency regulation specs, but I'd be a bit surprised if the sets didn't make a full 7 (or 11) kW with pretty negligible voltage and freq drop.

 

[mcmxi]

Interesting. I wonder how that translates to directly powered generators. The Kubota "Lowboy" I'm talking about is a Kubota/Yamaha dedicated standalone generator that comes in 7k, 11k and 14k flavors. The 7k is two cylinder diesel, the 11k is 3 cylinders. Water cooled. Kubota makes the engines, Yamaha makes the generator. Painted orange it's branded Kubota, Blue and it's Yamaha.

My current gas pull start generator is specced at 6500 watts continuous, 8500 surge. So it does handle the well pump alright.

I have the 7kW model but still haven't got around to connecting it to my house via the fancy switch system I bought. I've used it for a couple of extended power outages, and only to run essential stuff. One thing with the Kubota generators is that they run flat out all the time i.e. no governor, and according to the local dealer, you'll get a lot of blow by if they're not working hard.

I need to get into the habit of running mine at least every other month. It sits quietly in the garage waiting for the chance to save the day.

Here it is the day I bought it. Somehow the toy trucks and track hoe didn't make it to my house.

 

[wp6529]

1hp=745.7W
11.7hp=8724.69W
For a 6500W rated unit I think an 11.7hp engine is appropriately sized and nothing is exagerated in the specs.

For PTO driven generators, definitely more PTO HP per generator W due to the losses stepping up the 540RPM PTO to 1800 or 3600RPM for the generator head.

Sizing a standby generator properly to a house is a pretty complex task. If there are heat pumps you have to look at what kind they are as older fixed speed ones do indeed have a huge startup load that needs to be accounted for, while newer variable speed inverter types have a "soft start" and very little extra load on startup vs. running. Load diversity is also a factor and some loads while not able to run at the same time will handle sharing capacity quite well without manual intervention.

As an example, here we have four inverter mini-split heat pumps (3x 18k and 1x 9k), 2x regular A/C ~6k, 1 refrigerator/freezer, 4 chest freezers, well pump, aerobic septic air and spray pump, rather a lot of computers, lights, etc. All of this runs just fine on a 7.5KW gas generator during the spring and fall seasons. In the peak of summer or winter the cooling or heating loads are higher and I switch to a large diesel generator where the load tops out around 12KW.

Interestingly the gas generator consumes around 1gal/hr and the oversized 50KW diesel also consumes around 1gal/hr so the efficiency of an oversized generator is not always as bad as you might expect.

 

[eastvt]

I have a decent amount of experience with standby gensets. The only way to 'properly' size one is to balance cost/fuel consumption concerns against the loads you need to run.

Thanks! Fuel isn't a huge issue, mostly, though pouring a bit less diesel during a long outage is better than pouring more of it. We've had outages up to about 8 days. Every year we'll have multiple outages of a few hours, but a multi day event happens once every few years.

If it can truly provide even 6500 watts and hold voltage, the 7k is probably fine, though I might go for the 11k to be sure.

I think my current Honda/Husqvarna G390/6500 watt is probably stronger on the Honda side while the Husqvarna integration may be lacking. If it provides 6500 watts, there's no way it does it at anywhere near 120 volts. I don't think it is providing 3000 watts at 110 volts.

My current research direction is that I found these Kubota Lowboys come with either just outlets, or outlets and also terminals. It seems the terminals will be far better to wire it to a switch to the house. Though it also seems there is a terminal strip add on part. I don't know, but I'm assuming that for auto start and transfer switch, the terminals will be important.

 

[GreensvilleJay]

re: if I run the electric tea kettle or something it will drop right down to 110V or below,
have to ask , what size wiring is connecting the genny to the electrical panel ?

numbers (5500/120 = +-45amps), so 3, 'normal' 120v, 15a receptacles

If turning on just a kettle drops the genny volts from 120 to 110, you have a PROBLEM !

BTW a '390cc' engine puts out about 13HP ( 'magic math' 390 cc / 30 = 13hp )
need to see real specs of the genny, most are 'peak' not continuous rated,so maybe 5000 ?

 

[wp6529]

With voltage issues the type of generator head is typically the issue if the wire sizing to the loads is correct. There are more or less three types of generator head you will find:

- Brushed Self regulated - Uses just a capacitor and has poor voltage regulation but is cheap.

- Brushed AVR - Uses an actual electronic voltage regulator with voltage sensing and brush connection to provide DC power to the rotating field windings. Provides much better voltage regulation but costs more.

- Brushless - Uses an electronic voltage regulator and uses AC coupling instead of brushes to provide AC power to a rotating diode bridge that provides the DC drive to the field windings. More expensive still but the best regulation and cleanest power.

 

[eastvt]

"- Brushed Self regulated - Uses just a capacitor and has poor voltage regulation but is cheap."

Yeah, I guess I don't even have to look up the details of this generator to guess that's what it's got. In any case we're moving on (I think I mentioned while I was away on a trip my wife wasn't able to pull the cord and start it).

 

[InTheWoods]

... I'm assuming that for auto start and transfer switch, the terminals will be important...

Not an expert on the Lowboy sets, but I always figured them for more of a 'portable' set, not designed to be used with an Automatic Transfer Switch. (Not that it'd be impossible) Are you really wanting auto transfer upon mains failure?

 

[wp6529]

"- Brushed Self regulated - Uses just a capacitor and has poor voltage regulation but is cheap."

Yeah, I guess I don't even have to look up the details of this generator to guess that's what it's got. In any case we're moving on (I think I mentioned while I was away on a trip my wife wasn't able to pull the cord and start it).

I will add that there is a lot of hype these days about "clean power" and "pure sine wave", however there are very few devices found in a typical house these days that actually care very much about the exact voltage, frequency or harmonic distortion of the waveform.

The reality is that most "sensitive electronics" these days are anything but that and are built with wide input range switch-mode power supplies for global compatibility and efficiency. These power supplies typically have input specs like "Input: AC 100V-240V 50/60Hz" (exactly that on three different devices I just looked at) and are entirely happy with anything in that range.

Modern heat pumps, especially mini-split units are similar as they are designed to operate from the 208V that is common in apartment buildings served by 120/208V three phase service, so if your generator voltage drops under load to 110V/220V they are still perfectly happy. They also do not care about exact frequency since they use inverter drives for their fans and compressors so they are in no way dependent on line frequency for proper motor speed.

 

[wp6529]

Not an expert on the Lowboy sets, but I always figured them for more of a 'portable' set, not designed to be used with an Automatic Transfer Switch. (Not that it'd be impossible) Are you really wanting auto transfer upon mains failure?

Not sure on the lowboy units, but many of the towable "construction" generators do have a terminal block for remote start/stop and can be used with an appropriate ATS.

 

[mcmxi]

With voltage issues the type of generator head is typically the issue if the wire sizing to the loads is correct. There are more or less three types of generator head you will find:

- Brushed Self regulated - Uses just a capacitor and has poor voltage regulation but is cheap.

- Brushed AVR - Uses an actual electronic voltage regulator with voltage sensing and brush connection to provide DC power to the rotating field windings. Provides much better voltage regulation but costs more.

- Brushless - Uses an electronic voltage regulator and uses AC coupling instead of brushes to provide AC power to a rotating diode bridge that provides the DC drive to the field windings. More expensive still but the best regulation and cleanest power.

The GL7000 uses an AVR system with separate and self-excitation brush.

 

[mcmxi]

Not sure on the lowboy units, but many of the towable "construction" generators do have a terminal block for remote start/stop and can be used with an appropriate ATS.

The GL7000 is compatible with automatic start/stop units.

 

[eastvt]

"Are you really wanting auto transfer upon mains failure?"

Yes, that is a real impetus. About half the power failures we have happen in the night. We both use CPAP, so it would be deluxe to keep sleeping. I've been tough enough to get up and go out into the storm to switch over to generator, but really, I'm tired of it. Especially tired of it with gasoline, where I worry about freshness and will try to use up a tank of old fuel before adding fresh fuel, so that's a pain in the middle of the night. I know diesel doesn't last forever, but potentially far more stable.

We don't travel all that much, but if we are gone it would be nice to know we are covered for at least several hours of an outage, with auto start.

 

[wp6529]

"Are you really wanting auto transfer upon mains failure?"

Yes, that is a real impetus. About half the power failures we have happen in the night. We both use CPAP, so it would be deluxe to keep sleeping. I've been tough enough to get up and go out into the storm to switch over to generator, but really, I'm tired of it. Especially tired of it with gasoline, where I worry about freshness and will try to use up a tank of old fuel before adding fresh fuel, so that's a pain in the middle of the night. I know diesel doesn't last forever, but potentially far more stable.

We don't travel all that much, but if we are gone it would be nice to know we are covered for at least several hours of an outage, with auto start.

*Important*

Do note that to work with any of the generators that provide a remote start/stop connection you need to get a "real" traditional standalone transfer switch that contains the monitoring and control electronics.

Do not accidentally get one of the available cheaper transfer switch units that go with the small home standby generators, these transfer switches do not contain the necessary monitoring and control electronics, they rely on the electronics in the standby genset (reduces cost to meet consumer price point).

Also note that you will need a UPS to power your CPAP units as a standby generator typically takes 10-30 seconds to come online and you *will* wake up when the power goes out. I have a CPAP and I am fully awake before the CPAP blower even finishes spinning down when the power goes out.

 

[fried1765]

I have a decent amount of experience with standby gensets. The only way to 'properly' size one is to balance cost/fuel consumption concerns against the loads you need to run.

The GL11000 burns about 1.1 GPH at full load vs. the GL7000 at 0.7 GPH. At lighter loads, the consumption will drop, but for a given load the GL7000 will use less fuel - about 30% less, I'd estimate. For standy use, the fuel cost rarely matters - My backup set rarely runs more than 30 hours a year, including excercise times, so fuel cost just doesn't amount to much. OTOH, if you feel the need to keep fuel on-hand for a zombie apocolypse, the set that uses less fuel will become more desirable.

The initial cost is up to you - no way can anyone else advise on the 'pain' of the extra $1600.

So that leaves the loads you need to run. You don't provide enough detail for me to advise. For some folks, a heat pump alone drives the genset size due to the required LRA (locked rotor amps) or surge current. For others, maybe they have an electric water heater, and at 4500W alone it represents the biggest load. A well pump? For me, that is the single most important load I have.

Some people don't want to be bothered 'managing' loads during an outage, or have an ATS (automatic transfer switch) so the genset must be sized to pick up something close to a worst-case load scenerio.

All that said, as much as I like Kubota, I've pulled back from a Lowboy set, since they run at 3600RPM. I use an old Onan set running at 1800 RPM. Onan also had some very nice sets using Kubota engines running at 1800 RPM, such at the 7.5DKD. But these are all old, and maybe you'd prefer shiny and new, which is understandable...

The Kubota Lowboy manual doesn't show the voltage or frequency regulation specs, but I'd be a bit surprised if the sets didn't make a full 7 (or 11) kW with pretty negligible voltage and freq drop.

The old Onan engines are wonderful, but parts can be difficult to find, and are expensive.

 

[InTheWoods]

"Are you really wanting auto transfer upon mains failure?"

Yes, that is a real impetus. ...

My suggestion would be to call a professional - whoever is going to be installing the ATS before making the Lowboy purchase.

Someone will need to get this 'system' to work and be code compliant. There's not a lot of detail in the Lowboy manual. As I noted earlier, sizing a genset that will be put on an ATS is a bit different, as it's possible the genset could start and run unattended. As such, your loads need to be 'split' in a manner that allows the genset to carry whatever is on the 'backup' bus. A 'whole house' ATS is probably not a good idea with a 7kW (or even 11 kW) set. But this depends on exactly what your loads look like.

Don't be in a rush to buy the thing - figure out who will install it, where it will live, how the exhaust will be run (this thing is designed to be run outdoors - not in your garage/basement - Carbon Monoxide kills), how the fuel system will be set up, what ATS will be used, and most importantly WHO will be doing the install.

I still say the Lowboy sets are not 'aimed' at permanent, unattended, ATS-compatible usage. I see the manual mentions an A /SS (on the LAST page), but again, not a lot of detail for your installer.

As much a I like Orange stuff, I'd keep looking if I wanted/needed an automaticlly started backup set...

(and as far as CPAP goes, just put them on decent UPS's)