Get FinancingHow To Buy The Right Generator Set For The Job
Generator sets come in a truly vast array of form factors, from 5-kilowatt or smaller portable models to permanent power plant installations of 3 megawatts or more. A towable genset can power a construction site’s 480-volt tower crane, whereas a home standby or PTO generator can provide 120-V or 240-V emergency power to a domicile when a local outage occurs. Each generator set is designed for a purpose, such as backup power for a hospital or as prime power for an RV or outdoor event.
And yet, for all their variety, generator sets have a few core similarities: They all turn an engine’s mechanical energy into electricity to power lights, appliances, tools, computers, and more.
Types Of Power Ratings
Whether you’re looking for a large generator or a small one, you need to ascertain how much wattage your devices, appliances, machinery, and lights need. Then, in most cases, you will buy a unit that produces roughly a third more kilowatts (kW) or kilovolt-amperes (kVA) than that amount. (A kilowatt is 80% of a kilovolt-ampere, assuming a 0.8 power factor.)
The types of power ratings you’ll need to pay most attention to are the standby output for a backup generator, the prime output for temporary work or camp sites, or the continuous output rating for long-term use at 100% load.
Emergency standby generators are typically rated to run at 80% load for up to a couple hundred hours per year. They have no overload capacity and should only be run when no public power is available.
Generator sets rated for prime power can sustain a variable load of up to 70% or so for an unlimited period of time, or a non-variable load for a limited number of hours. Prime power gensets may have a 10% or greater overload capacity available for short durations.
Assessing Your Power Needs
Make a list of the devices you plan to run simultaneously on the generator’s power. For example, in an RV in the evening, you might typically need a portable prime power-rated genset to power the air conditioner, refrigerator, microwave, TV, and a few lights at the same time.
Next, check each device’s identification plate or owner's manual to determine its starting wattage as well as its running wattage. If you can’t find these figures, generator manufacturers and dealers often provide charts with typical wattage figures for common appliances and machinery. Tally up the wattage required to start and run the devices you need the generator to support.
Note that the amount of power necessary to start up a device will often be much greater than the wattage required to keep it going, especially if it has an electric motor that must overcome the inertia of its rotor in order to start up. The resulting voltage inrush can make the starting wattage six or more times higher than the running voltage.
Generator Sizing Tools
Because types of generators vary so much, it’s best to use a power calculator tailored to the role the generator will play. For instance, a marine generator for a charter boat will have a very different usage scenario than a backup unit for a data center.
Again, look to generator manufacturers’ websites for online wattage calculators. For example, Cummins’ Power Suite solution offers a generator sizing app called GenSize to help you ascertain your needs.
Select The Right Generator
As an example, we’ll look for a portable generator for a small bumper-pull camper. It has an air conditioner that draws 2,000 W to start up but only 700 W to run, and a small refrigerator requiring 500 W to start and 180 W to run. The camper’s other devices draw the same amount of power both at startup and while running. These include a 1,000-W microwave, a 150-W TV, and device chargers and lights totaling 40 W.
The RV Power Calculator Tool focuses on the major power draws, which are the refrigerator and AC unit. If they start up simultaneously, they would require 2,500 W from the generator and then 880 W to continue running. Using the combined startup figure as a minimum—and assuming that we follow the recommended procedure and turn on the biggest loads first, one after the other—the calculator recommends a 2,800-W generator. That’s about 35% more capacity than the 2,070-W total running figure for all of the camper’s devices.
Too Much Or Too Little?
When you’re selecting a generator, it’s better to err on the side of too much power than not enough. An overwhelmed (underdimensioned) generator can not only sustain damage, but also inflict electrical trauma on the appliances and electronics depending on it.
A modestly overdimensioned genset may cost more and weigh more, but it also may smooth out step voltage and frequency dips as high-inertia devices start up. Some modern generators mitigate this inrush effect using a digital automatic voltage regulator (D-AVR) or other means, so they don’t have to rely on overcapacity to tame these transient effects.
On the other hand, too much generator poses its own set of problems. Working a big generator with too light a load (less than its minimum set load, such as 30% of its capacity) can cause the engine to run inefficiently, accumulate carbon build-up and cylinder glazing, and produce more harmful emissions. Many gensets automatically throttle down their engine speeds to minimize issues with an undersized load.
In short, it’s best to choose a genset that meets your present needs while running at about 70 to 80% capacity (or near 100% for a model with a continuous rating). You can always add generator capacity later if you need to.
Fuel
Commercial and industrial generators usually run on diesel fuel, but more and more are compatible with natural gas, biodiesel, and other fuels. Residential standby units may use LP/propane or CNG, both of which have a long storage life, while portable generators may rely on propane, gasoline, or other fuels.
Be sure to note how long a generator will run before its tank must be refilled. Running under a heavy load will consume more fuel than a lighter load. Auxiliary fuel storage tanks can add run time as needed.
Features
Generators large and small may have useful features such as paralleling with other generators or with public power. Modular generators that can work in tandem can save on fuel costs. During periods of light demand, only one genset may need to run, and others can come online as needed to power larger loads.
Another potential money-saving feature is an automatic shut-off that stops the engine if its oil runs low. Newer diesel-powered generators comply with the latest emissions and efficiency standards such as the EPA’s Tier 4 Final, emitting less pollution and using less fuel than older models.
Many generators come with sound insulation for a relatively low noise rating in decibels (dBA). Some are equipped with an automatic start-up feature when utility power fails. Others may send remote alerts such as text messages to signal essential personnel when there’s a problem. A generator with an inverter can supply clean power that won’t damage your laptop, TV, or other sensitive electronic devices.
Larger Genset Considerations
The greater the wattage level, the more complex the generator becomes. Medium-to-large stationary backup models and power plant generators require proper control and switch gear. Odds are that you’ll need expert advice from a manufacturer representative or a generator specialist, not to mention professional installation by an experienced electrical contractor.
Larger gensets come in myriad variations to meet the needs of the specific application. Cummins recommends that you consider the minimum generator set capacity, maximum allowable step voltage dip, maximum allowable step frequency dip, duty cycle, altitude, temperature, fuel, phase, frequency, and voltage during the selection process.
Find The Right Generator Set
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