Sizing Generators

The key to properly sizing a generator is asking the right questions. Rental companies that capture the right information can quickly and easily recommend an optimally sized generator for the application and be confident the generator will perform to the customer’s expectation. It’s equally important to note that a properly sized generator also provides a benefit to the rental company in the form of reduced maintenance and downtime.

To properly size a generator, it’s necessary to understand how generators are rated. Most manufacturers give their products model names based on the machine’s kilovolt-ampere (kVA) rating. In the simplest terms, kVA is the generator’s voltage output multiplied by its amperage (amp) rating. However, most customers don’t specify their power needs in kVA, but rather refer in terms of kilowatts (kW) or amps. Fortunately, generator manufacturers list each of these values in their product data and on the generator’s nameplate as well. The table below shows an example for a 100 kVA generator:

Kilovolt-Amp Rating (kVA)	Kilowatt Rating (kW)	Voltage (V)	Amperage (A)
100 kVA	80 kW	480 V	120 A
100 kVA	80 kW	240 V	240 A
100 kVA	80 kW	208 V	278 A

In rental applications, the goal is not to calculate customer requirements down to the last amp. Often, the goal is to understand enough about the power needs and balance that against what generators are available in your fleet. Although generators are typically designed to operate at their published nameplate rating, it is good practice to size rental applications at 70 to 85 percent of this value. This will allow the generator to operate efficiently while also providing a reasonable amount of overhead to account for estimate errors in sizing calculations, or to provide extra power for unrealized needs.

It is also important, particularly with today’s advanced Tier 4 diesel engines, to avoid significantly oversizing a generator for an application. Not only is an oversized generator inefficient and more costly for the customer, it can also impact the rental company’s cost of ownership through increased maintenance and downtime. Operating a generator at less than 30 percent of rated output over an extended period of time can cause issues with the engine emission control systems, resulting in fault codes, shutdowns and potentially expensive repairs. For applications where periodic light load operation (less than 30 percent of rated output) cannot be avoided, rental companies should consider providing generators equipped with a load management solution.

In addition to capturing information about power requirements, properly sizing a generator also includes gathering general information about the application. Motor loads such as pumps or rock crushers are more complex and require more calculations than general utility loads such as heaters or small tools. Gathering general information about the application can help avoid situations where a customer requests a specific size generator but doesn’t have details about the application. Customers may ask for a certain generator because it’s what that customer has always used in the past, regardless of if the machine is the right size for the work being performed. Asking general application questions may uncover that a smaller generator will work for the application, saving the customer money on both the rental rate and the operating costs.

The next step in properly sizing a generator is getting a list of the equipment being powered, the voltage the equipment operates at and how much amperage each device draws. This information is typically found on dataplates on the equipment. Keep in mind that while most mobile generators are designed to produce different voltages, there are limitations on which voltages are available simultaneously. If an application requires several different voltages, you may need to quote multiple generators or a transformer to convert the generator output to the voltage that is needed. For utility loads, such as small tools, simply add up the amperage draw for each tool and confirm the total amperage is less than the machine rating (targeting 70 to 85 percent) at the desired voltage.

If the generator is powering an electric motor, which is a very common application, additional questions must be answered. The horsepower of the motor and the startup, or inrush current, have a significant impact on selecting the right generator. The motor dataplate will list the voltage, running amperage and horsepower, but the startup current must be calculated. This can be as much as six to 10 times greater than the amperage listed on the dataplate. The type of motor starter that the application is equipped with is also a factor. Full voltage or across-the-line starters result in the greatest amount of inrush current, while VFDs and other types of “soft starters” can lessen the inrush current and help reduce the size of the generator needed for the application. 

Becoming knowledgeable on generator sizing is not difficult. Most manufacturers offer product training that includes from how to utilize the features and benefits on the generator to how to provide a positive experience for customers and techniques for sizing generators for various applications. With many opportunities to receive training on generators, equipment rental suppliers should commit to training at least one sales representative on the product. The benefit to customer service and generator performance is well worth the investment in training.

Todd Howe is manager of global generator products for Doosan Portable Power.