In this article:

A comparison of different types of standby generators.

Standby generator cost vs what you get for ~$5000 (ie “what’s in the box”).

Why a used diesel standby generator can be the better choice, especially for rural areas.

Why if you have a heat pump, you need to be very concerned with “locked rotor amps” when selecting any standby generator.

First off.

I want to be very clear on something.  This is not at all a hit piece on Kohler or Generac.  Both are respected companies and make whole home generators to fit the needs of a segment of the market and do a great job selling.  Period.

Here’s the thing: Depreciation is our friend.  For $5,000, you are comparing that new ‘big box special’ to a used diesel industrial generator that would have cost $40,000+ new.

That’s a whole lot of machine for the money.

Different types of standby generators

Propane/Natural Gas Standby Generators

For the residential or farm side of things, you’re primarily going to see LP/NG (liquid propane or natural gas) standby generators.  It’s what Lowe’s and Home Depot sell, it’s what you see marketed all over the place, and it’s what is practical for many, many homeowners, especially those who have smaller lots.

The two big players are Generac and Kohler, which, if you’ve landed on my humble site, you probably already know.  They do well because they make standby generators an easy decision: pick a size at your favorite big box, here’s your authorized installer, have at it.

(There’s actually a lot more to it, say, if you actually want to run a heat pump, by the way.  You’ll see me talk about it later on in this article, and I’ll be posting more on it because it’s a make-or-break kind of decision.)

There is a lot of private labeling and branding in the residential LP/NG standby generator market, as you see a lot of shared parts and architectures if you dig around.  They also have programs for contractors, so it’s an easy upsell when building a home.

Diesel Standby Generators 

You really don’t see a lot of marketing around residential diesel standby generators from the manufacturers, because most of them are more concerned with the industrial and commercial market.  There might be a distributor near you, but you won’t see them being sold at big boxes.

The biggest player you’ll usually see in the 60-80kw range is Cummins Onan, though Katolight, MTU (Detroit Diesel), Generac, Kohler, Kubota, also play here.  When you get into huge diesel generators, you see a lot of Caterpillar and MTU.  (My generator is a 2007 60kw Katolight built around a Deere 5 cylinder turbodiesel.  MTU purchased the family-owned, Minnesota-based Katolight in 2007.)

One thing I want to point out is that Generac and Kohler residential and industrial standby generators are not at all similar, which is why they are kept completely separate on their websites.

To give you some idea of the difference in use between the two types of standby generators, my electrical sub in rural PA had installed many of the little Generac/Kohlers, one 40kw Generac Guardian in a multi-million dollar mansion, and my diesel was the first they had ever wired.

Here’s the thing: People can argue quality and brand preference and durability all they want, but one thing I want to make very, very clear is that there is no argument in the difference between a LP/NG small engine residential standby generator and an industrial diesel standby generator.

The $5000 Standby Generator Cost Comparison Challenge

Let’s imagine you live on a couple acres and have around ~$5000 to purchase a standby generator, not including automatic transfer switch or install.  Let’s see what you get for the money.

So, our matchup will be the natural gas Generac 20kw Guardian Series Standby Generator (about $5400 at Amazon) and I’m going to compare it to my diesel 60kw Katolight.   I paid $5500 for my unit, and it’s pretty typical of what you see out there in the 60kw class.

Engine

The Generac G-Force 999c.c. v twin engine as used in the Generac Guardian whole home generator series
A shot of the Generac G-Force 999c.c. v twin engine

Generac Engine

  • 999cc (.999 liter) G-Force Engine, an overhead valve v-twin. It also features “full pressure lubrication.”
  • Air-cooled, oil cooler about the size of a deck of cards
  • Aluminum block with iron liners
  • 40 hp and 60 lb/ft of torque
  • 1.9 quart oil capacity
  • Runs at 3600 rpm
  • Built in the USA using domestic and foreign parts
  • 301 cu. ft./hr of natural gas to run at full electrical output
john deere industrial turbodiesel engine as used in Katolight 60kw standby generator

Katolight Engine (John Deere OE engine)

  • 3.0L John Deere Industrial turbodiesel inline 5 cylinder
  • Liquid cooled with a large radiator, and the turbo features a large air-to-air intercooler (about 12″ x 32″)
  • Iron block with iron heads
  • 99 hp and 300ft/lb of torque
  • 11 quart oil capacity
  • Runs at 1800 rpm
  • Built in Mexico
  • About 1 gal of diesel per hour to run at the same electrical output as the Generac

The pictures don’t do it justice.  Pop the hood of your riding lawnmower, that’s the Generac.  Pop the hood of a backhoe, that’s the Katolight.


Electrical output

A summary graphic of a 20kw generac standby generator's ratings - 20kw, 75 amps at 240 volts

Generac

  • 20kw (actually, 18kw if you’re running on natural gas)
  • Fine print on the spec sheet: “Maximum kilovolt amps and current are subject to and limited by such factors at fuel Btu/megajoule content, ambient temperature, altitude, engine power and condition, etc.”
  • 75 amp maximum continuous load at 240v
  • Fine print on the spec sheet: “No overload capability is available for this rating.”
  • Detailed spec sheet
A summary graphic of a 60kw katolight diesel standby generator's ratings - 60kw, 250 amps at 240 volts

Katolight

  • 60kw
  • 250 amp continuous load at 240v
  • Most industrial generators are rated to handle 300% of rated current for 10 seconds
  • To give you an idea on power quality, I use a Panamax power line conditioner for audio equipment, and the voltage indicator is more stable when I running from generator power than when I am on the grid

I highlighted the amps for a very, very important reason.  I am going to write a full article on the finer points of generator sizing, but here’s what you need to know:

If you have a heat pump, you are going to be in a world of hurt unless you pay attention to LRA as listed on the data plate of your heat pump.

LRA stands for “Locked Rotor Amps,” which is what your heat pump REQUIRES to start.

For example, my new, ultra-efficient Bryant pulls 96.0 amps (LRA) to start.

The Generac is rated at 75 amps maximum load, and the spec sheet notes, “No overload capability is available for this rating.”

Guess what? Either A) the generator will overload and instantly shut down when my heat pump would kick on, or, B) strain its way through that half a second of huge LRA load a few times before doing serious damage to itself.


Sound level

A summary graphic showing that the generac standby generator measured 67 decibels from its quietest side, at 23 feet away, and while under load

Generac

  • 67 decibels at “23 feet with generator operating at normal load”
  • Fine print on spec sheet: “Sound levels are taken from the front of the generator.  Sound levels taken from other sides of the generator may be higher depending on installation parameters”
  • Fire up a riding lawn mower to full revs and stand next to it, that’s the noise/note you’ll hear at load
A summary graphic showing that the katolight diesel standby generator measured 64 decibels from its quietest side, at 10 feet away, and while under load

Katolight

  • 64 decibels 10 feet from the side of the shed
  • 63 decibels just outside closed double doors
  • Maximum of 77 db when 5 feet directly in front of exhaust shutter
  • The construction of the shed is basic 2″x6″ walls with 5/8″ Type X drywall and fiberglass insulation
  • Majority of the noise is actually from the radiator fan
  • Noise is low in tone (almost a low “white noise”)

Maintenence and service

Generac 999c.c. standby generator maintenance kit as recommended by Generac for 20kw generator maintenance

Generac: $58 per year/100 hours*

  • Owner could easily replace maintenance items like oil and air filter, oil, etc
  • Oil change interval is 200 hrs or 2 years, whichever comes first**
  • **It then states, “Change sooner when operating under heavy load…”  For a point of comparison, the Kohler 20kw recommends 150 hrs or annually, and I would personally tend to go with that.
  • Oddly, SAE 30 is recommended for “above 32 degrees”… and 10w-30 between “-10 and 40 degrees.”  In PA, I would need to run two viscosities annually
  • Scheduled maintenance kit is around $60 on Amazon
  • The manual recommends setting valve clearances at 6 months, which I would think would be a service call for most.
  • It appears there are about 250 Generac residential backup generator service locations within 150 miles of my home in rural PA
  • Limited ability to repair and rebuild the engine
  • Built similarly to a riding lawnmower engine
John Deere Plus 50 II 0w-40 diesel engine oil as recommended by Katolight for 60kw generator maintenance

Katolight: $26 per year/100 hours*

  • Owner could easily replace maintenance items like oil and air filter, oil, etc
  • Oil change interval is 375 hours, which is conservative
  • Air filter has an indicator to tell you when needs to be changed
  • 0w-40 is rated to cover -40 to +122 degrees
  • 3 gallons of John Deere synthetic oil and OE filter is about $85 total
  • I have 15 John Deere Industrial service dealers within 60 miles, and almost any diesel equipment mechanic could work on it (for example, equipment rental locations service diesel generators.)
  • In many ways, the more rural you are, the greater the chance there is a John Deere dealer nearby
  • Full ability to repair/rebuild the engine
  • Built similarly to a small diesel truck or heavy equipment

*Generac basic maintenance: $28 for 2 oil changes per year (no filter) to abide by the oil recommendation, $30 prorated annual cost of maintenance kit.

*Katolight basic maintenance: $23 prorated oil/filter, $3 prorated air filter.

If you look only at the cost of scheduled maintenance, assuming 100 hours of use per year, that little Generac is actually more expensive.


Expected lifespan/durability

Graphic showing the expected total lifespan of a Generac 20kw standby generator engine as 3000 hours, as stated by Generac

Generac

  • 3600 rpm = fairly high RPM for any engine
  • Higher load and stressing due to the engine working harder to meet demand
  • Small engines typically can not be rebored, etc
  • Rebuildability unknown
  • Life according to Generac: “Because Generac engines were developed specifically for generators and designed to provide approximately 3,000 hours of use, with typical usage and proper maintenance an automatic standby generator can provide in excess of 30 years of service.”
  • I can’t find the direct replacement cost of the 999c.c. G-Force, but to give you some idea, a comparable Briggs 993c.c. is $2600+
Graphic showing the expected rebuild life of 20000 hours of a Deere 5030 diesel engine as used in 60kw katolight standby generator

Katolight

  • 1800 rpm = half the wear
  • Lower load and stressing due to the engine not working as hard
  • Diesel everything is built more heavily and for low maintenance
  • Consensus rebuild life on the 5030 is ~20,000 hours… again this is time before a REBUILD
  • By the time the Generac is dead, the Deere is barely broken in

So, the math is that you would need SEVEN Generac LP/NG engines to make it to the rebuild interval of the Katolight diesel.

That’s the difference in durability you’re looking at here.

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