Portable Diesel Generator Buying Guide

Author: Minnie

May. 06, 2024




Portable Diesel Generator Buying Guide

Power outages are on the rise in the United States. In fact, the frequency of power outages has doubled every five years since 2000. Power outages have a wide-ranging impact on our society. They can disrupt transportation, close essential businesses like grocery stores and gas stations, and present health risks. So how does one stay prepared? The answer is with a portable diesel generator.

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This is particularly important if you live in an area that is affected by long-term power outages from severe storms. Hurricanes, tornados, and blizzards are all capable of knocking out power for days and even weeks.

If you’re among those afraid of losing power and are looking for a solution we’ve assembled this portable diesel generator buying guide to help answer any questions you might have to help you buy the right generator.

Portable Gas vs Portable Diesel Generator

Everyone knows about gasoline engines. We have them in our cars, trucks, and lawnmowers, and there’s a station seemingly on every street corner. Unless you drive a tractor-trailer you probably don’t own a diesel engine.

The truth is that diesel engines are more efficient than gasoline engines, and that can be a real plus when the power goes out. According to the Department of Energy, diesel engines are 30% to 35% more efficient than their gasoline counterparts. They also have fewer moving parts which means less maintenance over the life of the engine.

Gasoline is cheaper than diesel, so if you were considering the cost for a car your gas is cheaper, somewhere around a dollar or more per gallon. There is something called off-road diesel, or farm diesel, or dyed diesel, and is produced for off-road use in tractors, power equipment, and diesel generators.

There is no difference in performance between off-road and regular diesel. In fact, they dye the diesel red so inspectors can spot illegal use in tractor-trailers. The only difference is the price because it isn’t taxed. Off-road diesel is as much as a dollar cheaper than regular diesel, so if you can find a source, your cost is about the same as gasoline.

What Size Diesel Generator Do I Need?

This all depends on what you want or need to power, and everything is measured in watts. Appliances like a stove or refrigerator use a lot of watts to run, whereas light bulbs and fans use next to nothing. Your heating and airconditioning units will use up the most, especially commercial-grade units.

You should download a wattage guide to help you calculate your particular needs.

Prioritize Your Needs

You’ll want to add up all the appliances you plan on running at the same time. Your refrigerator and freezer should be a priority as your food will spoil if they aren’t running for extended periods.

Next is your heater or air conditioner. If your power outages occur in the winter and the weather is extremely cold you’ll need your heat pump to work. If you live on the Gulf of Mexico and hurricanes knock out your power you’ll want the air conditioner on. These two major appliances can draw the most electricity so make sure you account for them.

After these, you might need to run things like a stove, microwave, and dishwasher. Wattages on these appliances vary so check your particular appliance for accurate numbers.

Lastly, you’ll want to power your lights, fans, televisions, radios, computers, and other small appliances. These generally draw very little wattage, but you should know those numbers.

Factor Startup Watts

Adding them all together you come up with a number, but there’s one last thing to consider. Most electrical appliances draw more power on startup. The bigger the appliance, the more power it will need to start. After startup, the appliances draw less electricity to operate.

Let’s say your air conditioner runs at 20,000 watts but draws 22,000 watts on startup. You don’t want to buy a generator that maxes out at 20,000 watts because it will never start your air conditioner. You’ll need one that exceeds your startup watts. Electric ovens and heat pumps for larger buildings also draw a lot of electricity on startup.


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Model specs:
Used MTU 200 kW standby (180 kW prime) rental grade portable diesel generator set, model DS250, SN-P070612001. John Deere 6068HF485 engine rated 314 HP at 1800 RPM, SN-PE6068L005067, EPA Tier 3 emissions. 3 position voltage selector switch 3/60/277/480V or 3/60/120/208V or 1/60/120/240V single phase. 800 Amp main-line circuit breaker. PMG – permanent magnet generator. Coolant heater. Deep Sea digital control panel. Sound attenuated enclosure. 260 gallon UL 142 listed sub base fuel tank with secondary containment. 549 Hrs. Year 2007

Quick link: https://csdieselgenerators.com/used-mtu-ds250-rental-grade-6068hf485-diesel-generator–549-hrs–epa-tier-3-3610.html

Hard-Wired Transfer Switch vs Plug-and-Play

Nearly all generators have plug-and-play capability. They come with a variety of outlets from which you can run different types of cords to power everything.

The keyword here is cord, and you’ll need one long enough to reach your appliance from the generator outside your building. This isn’t a big deal with most appliances. If they plug into your wall outlet, a regular extension cord will work. This isn’t ideal for large commercial properties.

Ovens and clothes dryers use special cords, so first make sure your generator has the outlet for the appliance. Next, you’ll need a cord long enough to reach, and these specialty cords can be very expensive the longer they get.

Cords get in the way too. They are a trip and fall hazard, and they present the potential for electrocution if the cord is old or damaged. That said, if you are only concerned with powering one or two appliances and they don’t require a long cord, you can get by fine with a plug-and-play setup if you’re careful.

The transfer switch offers the easiest distribution of power from your generator to your appliances because it routes the electricity generated through your building’s electrical fuse box.

Transfer switches are installed on the outside of your building on the other side of the wall to your fuse box. The transfer switch has a built-in safety feature. In order to turn the power on going from the generator into the fuse box, you must first turn off the power coming in from the electric company.

This safety feature uses a manual slide that physically blocks the switch from the generator from turning on while the switch from the power company is also on. This prevents electricity from back-feeding into the system or generator and causing damage.

Ready To Buy a Generator?

At Central States Diesel Generators, we have a large inventory of new and used portable diesel generators.

You can browse our inventory online or call us with any questions you might have. Our expert technicians will help you choose the right generator for your needs.

Frequently Asked Questions About Portable Diesel Generators 

  • What is the difference between an Inverter and a Portable Generator?

A portable diesel generator produces D/C voltage, which is then delivered to the rotor which then converts the D/C to A/C with the help of brushes. On the other hand, inverter generators change the D/C to A/C through electronic microprocessors. 

  • Can I use my Portable Generator indoors?

  1. It is strongly advised that you place your portable diesel generator outside. Since these generators release carbon monoxide, they can cause poisoning which is often fatal. Make sure the generator is placed far from all doors and windows, in an area with ample ventilation. 

  • Does my Portable Generator need to be grounded?

Yes. It is important that your portable diesel generator is grounded. This can save you from any instances of electrocution. Moreover, grounding the generator is also key to meeting the applicable federal, state, and local regulations. 

  • Why is it important to not overload my Portable Generator?

Overloading your portable diesel generator can have an adverse impact on your electronics and heavy appliances. Hence, make sure to use the portable generator only when absolutely necessary and only to power the most needed equipment.

How to Correctly Size Your Generator | Step-by-Step Guide

Choosing the Right Generator Size

Buying a generator is typically a significant investment for a home or business owner. It’s also something most consumers don’t do very often, so there’s a lot of confusion around which fuel type to use (diesel, natural gas, propane), which brand is best (Caterpillar, Cummins, MTU, Generac, Kohler, HIPOWER), and most especially, what size generator is appropriate.

To answer that last question, your first and best option is always to consult a certified electrician to assess your specific needs. However, if you opt to do it yourself, you can still make an informed generator purchase or rental by following a few basic guidelines. We’re laying those out in this guide to help you know how to determine what size generator you need.


Generator Classes: Residential vs. Industrial

You should already have an idea of what class of generator you need. If you’re a homeowner looking for backup or standby power, you need either a small, portable generator or a stationary standby generator. These sizes run from 2kW-2000 watts or less for a recreational unit, up to about 50kW for a whole-house standby generator. These generators typically use a single-phase current, which are sufficient to power smaller equipment that don’t require constant, high-voltage power.

Industrial generators are available in a range of sizes, from around 20kW to well over 3MW. Larger business and industrial applications obviously require more capacity and thus often utilize three-phase motors for higher power. Office buildings, manufacturing facilities, data centers, and building complexes such as shopping malls, educational institutions, and living centers all require larger-capacity generators. This is the case whether it’s regarding primary power supply or emergency back-up power generation.



How to Size a Generator

The basic generator sizing formula goes like this: 

  1. Form a list of all items that will be powered.

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  2. Determine the starting wattage (energy needed to turn it on) and running wattage (energy needed to operate it) for each piece of equipment. These figures are typically inscribed somewhere on the equipment itself, as well as recorded in the owner’s manual.

  3. Calculate your total power requirement by adding up these kW or KVA figures.

Click here to use our Power Calculator 

For equipment rated in amperes, you can convert amps to watts with the following formula:

  • For resistive loads (most common type): Wattage = amperes x volts

  • For reactive loads: Wattage = amperes x volts x load factor


Load factor is the ratio of your electric energy use in kilowatt-hours to your peak demand in kilowatts. You can calculate it by consulting your utility bill for data and using it in the following formula:

  • Total kWh for the previous month / (your peak demand for the period x 30 days x 24 hours)


Even if you lose the manual, you should be able to find wattage details online. However, if you can’t determine a piece of equipment’s power draw figures, you can use a ballpark number. For example, these are the starting and running figures for some common tools and electronics:

ApplianceStarting Load (W)Running Load (W)

Air compressor

40002000Disc grinder40002000Router1500600Electric chainsaw24001200Table saw45001800Water heater45004500Laptop250250Printer500500Air conditioner (10k BTU)22001500Dishwasher14501400LCD TV (55”)230230

Once you know the estimated wattage needed, the generator size is easier to choose. Whatever number you come up with, choose a generator with a capacity that’s 10-20% larger than your requirement. This will give you some leeway when and if you update your equipment and end up needing more power as a result. It also helps you manage “de-rating,” or under performance of the generator compared to its manufacturer-claimed abilities due to adverse operating conditions, such as extreme temperatures or high altitudes.


Buying a Single Generator vs. Paralleling

If your power needs are large enough, you may wonder whether it’s better to go with a single, huge diesel generator, or split the burden between two or more smaller generators. For example, instead of a 1200kW generator, you could install three 400kW generators. This is called paralleling, and it can be a great option in the right circumstances. Here’s why:


#1. Greater flexibility. It’s very likely the reason you’re buying a generator at all is the additional reliability, a hedge against an interrupted power supply causing a slowdown or halt to your business’s operations. With multiple generators, you don’t have to worry if one has to be shut down for maintenance. You simply shift the burden to the others and keep right on working. 

With just one generator, if it has to stop, so do you. 


#2. Can be more cost-effective. Obviously the economics will vary widely depending on brands, new vs. used generators, prices in your geographic location, etc. But all things being equal, once you breach a certain generator size, it becomes more economical to parallel than use a single unit. Specifically, once you go over the sizes of gas or diesel engines mass-produced for use in cars, as well, those engines are harder to come by and are thus more expensive. And the engine comprises more than half the cost of a generator. 

The threshold for diesel engines is 600kW and for gas engines it’s 150kW. 


#3. Can be more practical. Depending on your physical space, it may be easier to fit two or three parallel generators into a room than one oversized unit. Among large commercial generators, smaller capacities can mean the difference in 25 or more square feet per unit. 


#4. Easier on the generators. Load sharing among two or more generators can extend the life of each generator and avoid the risk of overburdening a single unit. For continuous usage, loads up to 80% generator capacity are ideal.

Need Help Sizing Your Generator?
WPP Is Here to Help!

  • WPP has the engineering and technical expertise to properly size and custom package the right power solution to fit you unique applications.
  • For Commercial, Industrial, Municipal, Marine and Oil & Gas applications.
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Get Help Sizing Your Generator

Generator Sizing and Fuel Consumption

In the same way a 3500-level truck uses more fuel than a 1500, larger generators consume fuel faster than smaller units. This is important to remember when “running the numbers” on what a new or used generator may cost. Not only do you have to refuel larger generators more often, it may be necessary to store extra fuel on-site, precipitating the need for additional storage tanks.  

For this reason, when it comes to residential generators, Consumer Reports recommends getting the smallest portable or home standby generator that meets your needs to limit the amount of fuel you have to store to run it. 

For industrial-strength generators, consult our approximate diesel fuel consumption chart to estimate how much fuel you’ll burn in a particular generator size operated at either a quarter-, half-, three-quarter, or full load. 


Why It’s Crucial to Choose the Right Generator Size

It may be tempting to skimp on the size of your generator, especially when doing so might save you a couple thousand dollars. However, selecting a generator that’s too small for your applications is a recipe for disaster. Not only can you damage the appliances and devices pulling power from the generator, you can harm the generator itself.

Here are some more benefits of erring on the side of a generator that’s too powerful, as opposed to too weak:

  • Increased usable life of the generator

  • Consistent production performance

  • Limited incidence of capacity overloads

  • Limited incidence of unexpected system failures

  • Reduced likelihood of asset damage

  • Increased personnel safety

  • Decreased risk of overheating and electrical fires

  • Reduction in frequency and cost of maintenance

  • Increased ease and speed of maintenance


Contact us to discuss your requirements of 500kw Diesel Generator. Our experienced sales team can help you identify the options that best suit your needs.


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